Effects of Various Types of Environmental Pollution

Effects of Various Types of Environmental Pollution

Atmospheric pollution is generally studied as tropospheric pollution.

Different types of atmospheric pollutions are

  • Air pollution
  • Water pollution
  • Soil pollution

Air pollution

Any undesirable change in air which adversely affects living organisms is called air pollution. Air pollution is limited to troposphere and stratosphere. Air pollution is mainly due to the excessive discharge of undesirable foreign matter in to the atmospheric air.

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Types of Environmental Pollution

Types of Air Pollutants

Air pollutants may exist in two major forms namely, gases and particulates.

Gaseous Air Pollutants

Oxides of sulphur, oxides of nitrogen, oxides of carbon, and hydrocarbons are the gaseous air pollutants.

a. Oxides of Sulphur

Sulphur dioxide and sulphur trioxide are produced by burning sulphur containing fossil fuels and roasting sulphide ores. Sulphur dioxide is a poisonous gas to both animals and plants. Sulphur dioxide causes eye irritation, coughing and respiratory diseases like asthma, bronchitis, etc.

Sulphur dioxide is oxidised into more harmful sulphur trioxide in the presence of particulate matter present in polluted air.

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SO3 combines with atmospheric water vapour to form H2SO4, which comes down in the form of acid rain.

SO3 + H2O → H2SO4

Some harmful effects of acid rain will be discussed in section 15.3

b. Oxides of Nitrogen

Oxides of nitrogen are produced during high temperature combustion processes, oxidation of nitrogen in air and from the combustion of fuels (coal, diesel, petrol etc.).

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NO + O3 → NO2 + O2

The oxides of nitrogen are converted into nitric acid which comes down in the form of acid rain. They also form reddish brown haze in heavy traffic. Nitrogen dioxide potentially damages plant leaves and retards photosynthesis. NO2 is a respiratory irritant and it can cause asthma and lung injury. Nitrogen dioxide is also harmful to various textile fibres and metals.

Types of Environmental Pollution

c. Oxides of Carbon

The major pollutants of oxides of carbon are carbon monoxide and carbon dioxide.

(i) Carbon Monoxide

Carbon monoxide is a poisonous gas produced as a result of incomplete combustion of coal or firewood. It is released into the air mainly by automobile exhaust.

It binds with haemoglobin and forms carboxy haemoglobin which impairs normal oxygen transport by blood and hence the oxygen carrying capacity of blood is reduced. This oxygen deficiency results in headache, dizziness, tension, Loss of consciousness, blurring of eye sight and cardiac arrest.

(ii) Carbon Dioxide

Carbon dioxide is released into the atmosphere mainly by the process of respiration, burning of fossil fuels, forest fire, decomposition of limestone in cement industry etc.

Green plants can convert CO2 gas in the atmosphere into carbohydrate and oxygen through a process called photosynthesis. The increased CO2 level in the atmosphere is responsible for global warming. It causes headache and nausea.

(d) Hydrocarbon

The compounds composed of carbon and hydrogen only are called hydrocarbons. They are mainly produced naturally (marsh gas) and also by incomplete combustion of automobile fuel. They are potential cancer causing (carcinogenic) agents. For example, polynuclear aromatic hydrocarbons (PAH) are carcinogenic, they cause irritation in eyes and mucous membranes.

Types of Environmental Pollution

Greenhouse Effect and Global Warming:

In 1987, Jean Baptiste Fourier a French mathematician and scientist coined the term“Greenhouse Effect” for trapping of heat in the atmosphere by certain gases.

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The earth’s atmosphere allows most of the visible light from the Sun to pass through and reach Earth’s surface. As Earth’s surface is heated by sunlight, it radiates part of this energy back toward space as longer wavelengths (IR).

Some of the heat is trapped by CH4, CO2, CFCs and water vapour present in the atmosphere. They absorb IR radiation and effectively block a large portion of earth’s emitted radiation. The radiation thus absorbed is partly reemitted to earth’s surface. Therefore, the earth’s surface gets heated up by a phenomenon called greenhouse effect.

Thus Greenhouse effect may be defined as the heating up of the earth surface due to trapping of infrared radiations reflected by earth’s surface by CO2 layer in the atmosphere”. The heating up of earth through the greenhouse effect is called global warming.

Without the heating caused by the greenhouse effect, Earth’s average surface temperature would be only about – 18 °C (0 °F). Although the greenhouse effect is a naturally occurring phenomenon, it is intensified by the continuous emission of greenhouse gases into the atmosphere.

During the past 100 years, the amount of carbon dioxide in the atmosphere increased by roughly 30 percent and the amount of methane more than doubled. If these trends continue, the average global temperature will increase which can lead to melting of polar ice caps and flooding of low lying areas. This will increase incidence of infectious diseases like dengue, malaria etc.

Types of Environmental Pollution

Acid Rain

Rain water normally has a pH of 5.6 due to dissolution of atmospheric CO2 into it. Oxides of sulphur and nitrogen in the atmosphere may be absorbed by droplets of water that make up clouds and get chemically converted into sulphuric acid and nitric acid respectively. As a result, pH of rain water drops below the level 5.6, hence it is called acid rain.

Acid rain is a by-product of a variety of sulphur and nitrogen oxides in the atmosphere. Burning of fossil fuels (coal and oil) in power stations, furnaces and petrol, diesel in motor engines produce sulphur dioxide and nitrogen oxides. The main contributors of acid rain are SO2 and NO2. They are converted into sulphuric acid and nitric acid respectively by the reaction with oxygen and water.

2SO2 + O2 + 2H2O → 2H2SO4
4NO2 + O2 + 2H2O → 4HNO3

Harmful effects of acid rain:

Some harmful effects are discussed below.

(i) Acid rain causes extensive damage to buildings and structural materials of marbles. This attack on marble is termed as Stone leprosy.

CaCO3 + H2SO4 → CaSO4 + H2O + CO2

(ii) Acid rain affects plants and animal life in aquatic ecosystem.

(iii) It is harmful for agriculture, trees and plants as it dissolves and removes the nutrients needed for their growth.

(iv) It corrodes water pipes resulting in the leaching of heavy metals such as iron, lead and copper into drinking water which have toxic effects.

(v) It causes respiratory ailment in humans and animals.

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Particulate matter (Particulate Pollutants)

Particulate pollutants are small solid particles and liquid droplets suspended in air. Many of particulate pollutants are hazardous. Examples: dust, pollen, smoke, soot and liquid droplets (aerosols) etc,.

They are blown into the atmosphere by volcanic eruption, blowing of dust, incomplete combustion of fossil fuels induces soot. Combustion of high ash fossil fuels creates fly ash and finishing of metals throws metallic particles into the atmosphere.

Types of Environmental Pollution

Types of Particulates:

Particulate in the atmosphere may be of two types, viable or non-viable.

a. Viable Particulates

The viable particulates are the small size living organisms such as bacteria, fungi, moulds, algae, etc. which are dispersed in air. Some of the fungi cause allergy in human beings and diseases in plants.

b. Non-Viable Particulates

The non- viable particulates are small solid particles and liquid droplets suspended in air. They help in the transportation of viable particles. There are four types of nonviable particulates in the atmosphere. They are classified according to their nature and size as follows

(i) Smoke

Smoke particulate consists of solid particles (or) mixture of solid and liquid particles formed by combustion of organic matter. For example, cigarette smoke, oil smoke, smokes from burning of fossil fuel, garbage and dry leaves.

(ii) Dust

Dust composed of fine solid particles produced during crushing and grinding of solid materials. For example, sand from sand blasting, saw dust from wood works, cement dust from cement factories and fly ash from power generating units.

(iii) Mists

They are formed by particles of spray liquids and condensation of vapours in air. For example, sulphuric acid mist, herbicides and insecticides sprays can form mists.

(iv) Fumes

Fumes are obtained by condensation of vapours released during sublimation, distillation, boiling and calcination and by several other chemical reactions. For example, organic solvents, metals and metallic oxides form fume particles.

Types of Environmental Pollution

Health effects of particulate pollutants:

(i) Dust, mist, fumes,etc., are air borne particles which are dangerous for human health. Particulate pollutants bigger than 5 microns are likely to settle in the nasal passage whereas particles of about 10 micron enters the lungs easily and causes scaring or firosis of lung lining.

They irritate the lungs and causes cancer and asthma. This disease is also called pneumoconiosis. Coal miners may suffer from black lung disease. Textile workers may suffer from white lung disease.

(ii) Lead particulates affect children’s brain, interferes maturation of RBCs and even cause cancer.

(iii) Particulates in the atmosphere reduce visibility by scattering and absorption of sunlight. It is dangerous for aircraft and motor vehicles

(iv) Particulates provide nuclei for cloud formation and increase fog and rain.

(v) Particulates deposit on plant leaves and hinder the intake of CO2 from the air and affect photosynthesis.

Techniques to reduce particulate pollutants

The particulates from air can be removed by using electrostatic precipitators, gravity settling chambers, and wet scrubbers or by cyclone collectors. These techniques are based on washing away or settling of the particulates.

Smog

Smog is a combination of smoke and fog which forms droplets that remain suspended in the air.

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Smog is a chemical mixture of gases that forms a brownish yellow haze over urban cities. Smog mainly consists of ground level ozone, oxides of nitrogen, volatile organic compounds, SO2, acidic aerosols and gases, and particulate matter.

There are two types of smog. One is Classical smog caused by coal smoke and fog, second one is photo chemical smog caused by photo chemical oxidants. They are discussed below in detail.

Types of Environmental Pollution

(i) Classical smog or London smog

Classical smog was first observed in London in December 1952 and hence it is also known as London smog. It consists of coal smoke and fog.

It occurs in cool humid climate. This atmospheric smog found in many large cities. The chemical composition is the mixture of SO2, SO3 and humidity. It generally occurs in the morning and becomes worse when the sun rises. This is mainly due to the induced oxidation of SO2 to SO3, which reacts with water yielding sulphuric acid aerosol.

Chemically it is reducing in nature because of high concentration of SO2 and so it is also called as reducing smog.

Effects of classical smog:

  • Smog is primarily responsible for acid rain.
  • Smog results in poor visibility and it affects air and road transport.
  • It also causes bronchial irritation

(ii) Photo chemical smog or Los Angel Smog

Photo Chemical smog was first observed in Los Angels in 1950. It occurs in warm, dry and sunny climate. This type of smog is formed by the combination of smoke, dust and fog with air pollutants like oxides of nitrogen and hydrocarbons in the presence of sunlight.

It forms when the sun shines and becomes worse in the afternoon. Chemically it is oxidizing in nature because of high concentration of oxidizing agents NO2 and O3, so it is also called as oxidizing smog.

Photo chemical smog is formed through sequence of following reactions.

N2 + O2 → 2NO
2NO + O2 → 2NO2

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(O) + O2 → O3
O3 + NO → NO2 + O2

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NO and O3 are strong oxidizing agent and can react with unburnt hydrocarbons in polluted air to form formaldehyde, acrolein and peroxy acetyl nitrate (PAN).

Types of Environmental Pollution

Effcts of photo chemical smog

The three main components of photo chemical smog are nitrogen oxide, ozone and oxidised hydro carbon like formaldehyde (HCHO), Acrolein (CH2=CH-CHO), peroxy acetyl nitrate (PAN).

Photochemical smog causes irritation to eyes, skin and lungs, increase in chances of asthma.

High concentrations of ozone and NO can cause nose and throat irritation, chest pain, uncomfortable in breathing, etc.

PAN is toxic to plants, attacks younger leaves and cause bronzing and glazing of their surfaces. It causes corrosion of metals stones, building materials and painted surfaces.

Control of Photo Chemical Smog

The formation of photochemical smog can be suppressed by preventing the release of nitrogen oxides and hydrocarbons into the atmosphere from the motor vehicles by using catalytic convertors in engines. Plantation of certain trees like Pinus, Pyrus, Querus Vitus and juniparus can metabolise nitrogen oxide.

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Introduction of Haloalkens and Haloarenes – Definition, Classification, Uses

Introduction of Haloalkens and Haloarenes – Definition, Classification, Uses

In the previous unit we learnt about the chemistry of hydrocarbons. In this unit us learn about organic compounds containing halogens. When one or more hydrogen atoms of aliphatic or aromatic hydrocarbons are replaced by the corresponding number of halogens like fluorine, chlorine, bromine or iodine, the resultant compounds are called either haloalkanes or halo arenes. They serve as starting materials for many organic synthesis.

Halogen substituted organic compounds are widely spread in nature and find application in our day to day life as well as in industry. Certain compounds like chloramphenicol produced by soil microbes are used in the treatment of typhoid; chloroquine is used in the treatment of malaria, halothane is used as an anesthetic, and halogenated solvents like trichloroethylene are used for cleaning electronic equipments.

Haloalkanes are hydrocarbons containing aliphatic alkane with one or more hydrogen atoms replaced by halogens. Haloarenes are hydrocarbons containing aromatic alkane with one or more hydrogen atoms replaced by halogens.

Introduction of Haloalkens and Haloarenes

Classification of Haloalkanes

The haloalkanes, also known as alkyl halides, are a group of chemical compounds comprised of an alkane with one or more hydrogens replaced by a halogen atom (fluorine, chlorine, bromine, or iodine). The classification is determined by the number of carbons bonded to the carbon bearing the halide.

Name of Haloalkanes and Haloarenes

IUPAC name – The IUPAC (International Union of Pure and Applied Chemistry) naming system is the standard naming system that chemists generally use.

Reaction of Haloalkanes and Haloarenes

Haloarenes are more stable than the haloalkane that’s why haloarenes are less reactive than the haloalkane. The reasons for being stable are dependent on mainly three factors that are polarity of carbon-halogen bond, hybrid state of the carbon atom in the haloarenes and the presence of resonance.

Introduction of Haloalkens and Haloarenes

Halide Functional Group

An acyl halide (also known as an acid halide) is a chemical compound derived from an oxoacid by replacing a hydroxyl group with a halide group. If the acid is a carboxylic acid, the compound contains a – COX functional group, which consists of a carbonyl group singly bonded to a halogen atom.

Functional Group of Haloalkenes

Alkyl halides [haloalkanes] consist of an alkyl group attached to a halogen: F, Cl, Br, I. Chloro, bromo and iodo alkyl halides are often susceptible to elimination and/or nucleophilic substitution reactions. Aldehydes have a hydrogen and an alkyl (or aromatic) group attached to a carbonyl function.

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Orbital Overlap | Definition, Examples, Diagrams

Orbital Overlap | Definition, Examples, Diagrams

When atoms combines to form a covalent molecule, the atomic orbitals of the combining atoms overlap to form a covalent bond. The bond pair of electrons will occupy the overlapped region of the orbitals. Depending upon the nature of overlap we can classify the covalent bonding between the two atoms as sigma (σ) and pi (π) bonds.

Sigma and Pi bonds

When two atomic orbitals overlap linearly along the axis, the resultant bond is called a sigma (σ) bond. T is overlap is also called ‘head-on overlap’ or ‘axial overlap’. Overlap involves an s orbital (s-s and s-p overlaps) will always result in a sigma bond as the s orbital is spherical.

Overlap between two p orbitals along the molecular axis will also result in sigma bond formation. When we consider x-axis as molecular axis, the px-px overlap will result in σ-bond.

When two atomic orbitals overlaps sideways, the resultant covalent bond is called a pi(π)bond. When we consider x-axis as molecular axis, the py-py and pz-pz overlaps will result in the formation of a π-bond.

Following examples will be useful to understand the overlap:

Orbital Overlap

Formation of hydrogen (H2) Molecule

Electronic configuration of hydrogen atom is 1s1.

During the formation of H2 molecule, the 1s orbitals of two hydrogen atoms containing one unpaired electron with opposite spin overlap with each other along the internuclear axis. This overlap is called s-s overlap. Such axial overlap results in the formation of a σ-covalent bond.

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Formation of fluorine molecule (F2):

Valence shell electronic configuration of fluorine atom:
2s2 2px2, 2py2, 2pz1

When the half filled pz orbitals of two fluorine overlaps along the z-axis, a σ-covalent bond is formed between them.

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Formation of HF molecule:

Electronic configuration of hydrogen atom is 1s1
Valence shell electronic configuration of flourine atom: 2s2 2px2, 2py2, 2pz1
When half filled 1s orbital of hydrogen linearly overlaps with a half filled 2pz orbital flourine, as σ-covalent bond is formed between hydrogen and flourine.

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Orbital Overlap

Formation of oxygen molecule (O2):

Valence shell electronic configuration of oxygen atom:
2s2 2px2, 2py1, 2pz1

Orbital Overlap img 3a

When the half filled pz orbitals of two oxygen overlaps along the z-axis (considering molecular axis as z axis), a σ-covalent bond is formed between them. Other two half filled py orbitals of two oxygen atoms overlap laterally (sideways) to form a π-covalent bond between the oxygen atoms.

Thus, in oxygen molecule, two oxygen atoms are connected by two covalent bonds (double bond). The other two pair of electrons present in the 2s and 2px orbital do not involve in bonding and remains as lone pairs on the respective oxygen.

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Aromatic Hydrocarbons – Formula, Definition, Structure, Properties

Aromatic Hydrocarbons – Formula, Definition, Structure, Properties

Take a moment and think of substances that have a strong fragrance. What kind of things come to your mind?

Perfume, Vanila or cinnamon? They smell differently, they have something in common. These substances are made of aromatic compounds [Greek: AromaPleasant smelling]. However, some compounds are chemically aromatic but do not have distinct smell. The aromatic hydrocarbons are classified depending upon number of rings present in it.

(i) Monocyclic aromatic hydrocarbon (MAH) (Ex) Benzene (C6H6) and Toluene (C7H8)

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(ii) Polycyclic aromatic hydrocarbon (PAH) (Ex) Naphthalene (C10H8) and Anthracene (C14H10).

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Aromatic Hydrocarbons

Nomenclature and Isomerism

We have already discussed about nomenclature of aromatic hydrocarbons in Unit: 11. The first member of aromatic hydrocarbon is benzene (C6H6) represented by a regular hexagon with a circle inscribed in it.

Since, all the six hydrogen atom in benzene are equivalent, it can give only one monosubstituted compound (Ex) methyl benzene (C6H5-CH3) which named as toluene.

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When di substitution occurs either by a similar monovalent atom or two different atoms or groups in benzene, then three different position isomers are possible. Their relative positions are indicated as ortho (1, 2), meta (1, 3) and para (1, 4). For example, consider dimethyl benzene which is named as xylene.

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Aromaticity

Huckel proposed that aromaticity is a function of electronic structure. A compound may be aromatic, if it obeys the following rules

  • The molecule must be co-planar
  • Complete delocalization of π electron in the ring
  • Presence of (4n+2) π electrons in the ring where n is an integer (n=0, 1, 2….)

This is known as Huckel’s rule

Some of the examples for the Huckel rule

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Structure of benzene:

Aromatic Hydrocarbons

1. Molecular formula

Elemental Analysis and molecular weight determination have proved that the molecular formula of benzene is C6H6. This indicates that benzene is a highly unsaturated compound.

2. Straight chain structure not possible:

Benzene could be constructed as a straight chain or ring compound but it not feasible since it does not show the properties
of alkenes or alkynes. for example, it did not decolourise bromine in carbon tetrachloride or acidified KMnO4. It did not react with water in the presence of acid.

3. Evidence of cyclic structure:

I. Substitution of Benzene:

Benzene reacts with bromine in the presence of AlCl3 to form mono bromo benzene.

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Formation of only one monobromo compound indicates that all the six hydrogen atoms in benzene were identical. This is possible only if it has a cyclic structure of six carbons each containing one hydrogen.

II. Addition of Hydrogen:

Benzene can add on to three moles of hydrogen in the presence of nickel catalyst to give cyclohexane.

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This confirms cyclic structure of benzene and the presence of three carbon-carbon double bond.

Aromatic Hydrocarbons

4. Kekule’s Structure of Benzene:

In 1865, August Kekule suggested that benzene consists of a cyclic planar structure of six carbon with alternate single and double bonds. There were two objections:

(i) Benzene forms only one ortho disubstituted products whereas the Kekule’s structure predicts two o-di
substituted products as shown below.

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(ii) Kekule’s structure failed to explain why benzene with three double bonds did not give addition reactions like other alkenes. To overcome this objection, Kekule suggested that benzene was mixture of two forms (1 and 2)which are in rapid equilibrium.

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5. Resonance description of benzene:

The phenomenon in which two or more structures can be written for a substance which has identical position of atoms is called resonance. The actual structure of the molecule is said to be resonance hybrid of various possible alternative structures. In benzene, Kekule’s structures I & II represented the resonance structure, and structure III is the resonance hybrid of structure I & II.

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The structures 1 and 2 exist only in theory. The actual structure of benzene is the hybrid of two hypothetical resonance structures.

6. Spectrosscopic Measurments:

Spectroscopic measurements show that benzene is planar and all of its carboncarbon bonds are of equal length 1.40A°. This value lies between carbon-carbon single bond length 1.54A° and carboncarbon double bond length 1.34A°.

Aromatic Hydrocarbons

7. Molecular Orbital Structure:

The structure of benzene is best described in terms of the molecular orbital theory. All the six carbon atoms of benzene are sp2 hybridized. Six sp2 hybrid orbitals of carbon linearly overlap with six 1s orbitals of hydrogen atoms to form six C – H sigma bonds. Overlap between the remaining sp2 hybrid orbitals of carbon forms six C-C sigma bonds.

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All the σ bonds in benzene lie in one plane with bond angle 120°. Each carbon atom in benzene possess an un hybridized p-orbital containing one electron. The lateral overlap of their p-orbital produces 3 π- bond. The six electrons of the p-orbitals cover all the six carbon atoms and are said to be delocalised.

Due to delocalization, strong π-bond is formed which makes the molecule stable. Hence unlike alkenes and alkynes benzene undergoes substitution reactions rather addition reactions under normal conditions.

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Representation of Benzene:

Hence, there are three ways in which benzene can be represented.

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Benzene and its Homologous Series

Benzene and its homologous series are colorless liquids with pleasant odour. They are lighter than water and insoluble in it. Their vapours are highly flammable, and volatile and toxic in nature.

Sources of aromatic compound:

  • Benzene and other aromatic compound are obtained from coal tar and petroleum
  • It can also be prepared in laboratory using some simple aliphatic compounds

Aromatic Hydrocarbons

1. Preparation of Benzene

(i) industrial preparation of benzene from coal tar:

Coal tar is a viscous liquid obtained by the pyrolysis of coal. During fractional distillation, coal tar is heated and distills away its volatile compounds namely benzene, toluene, xylene in the temperature range of 350 to 443 K. These vapours are collected at the upper part of the fractionating column (Table 13.5.)

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(ii) From Acetylene

Acetylene on passing through a red – hot tube trimerises to give benzene. We have already studied this concept in polymerization of alkynes.

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(iii) Laboratory Methods Of Preparing Benzene And Toluene

(a) Decarboxylaation Of Aromatic Acid. When sodium benzoate in heated with sodalime, benzene vapours distil over.

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(b) Preparation Of Benzene From Phenol

When phenol vapours are passed over zinc dust, then it is reduced to benzene.

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(c) Wurtz – Fitting Reaction:

When a solution of bromo benzene and iodo methane in dry ether is treated with metallic sodium, toluene is formed.

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(d) Friedel Craft’s Reaction:

When benzene is treated with methyl chloride in the presence of anhydrous aluminium chloride, toluene is formed.

Aromatic Hydrocarbons img 19

Aromatic Hydrocarbons

Physical Properties:

  1. Benzene is a colourless liquid, insoluble in water and solution alcohol, ether and chloroform.
  2. It burns with luminous sooty flme in contrast to alkanes and alkenes which usually burn with bluish flame.
  3. Their vapours are highly toxic which on inhalation produce loss of consciousness.

Chemical Properties:

  1. Benzene contains delocalized π-electrons which make the ring to act as an electron rich centre. So electrophilic substitution reaction occurs in benzene.
  2. Benzene ring is stabilized by delocalized π electrons. Though it is highly stable, it undergoes addition and oxidation reaction under specific conditions.

1. Electrophilic Substitution Reaction

(a) Nitration:

When benzene is heated at 330K with a nitrating mixture (Con. HNO3 + Con. H2SO4), nitro benzene is formed by replacing one hydrogen atom by nitronium ion NO+2 (electrophile)

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Concentrated H2SO4 is added to produce nitronium ion NO+2

Aromatic Hydrocarbons

(b) Halogenation:

Benzene reacts with halogens (X2=Cl2, Br2,) in the presence of Lewis acid such as FeCl3, FeBr3 or AlCl3 and give corresponding halo benzene. In the absence of catalyst, Fluorine reacts vigoursly with benzene even in the absence of catalyst. However iodine is very inactive even in the presence of catalyst.

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(c) Sulphonation:

Benzene reacts with fuming sulphuric acid (Con H24 + SO3) and gives benzene sulphonic acid. The electrophile SO3 is a molecule. Although it does not have positive charge, it is a strong electrophile. This is because the octet of electron around the sulphur atom is not reached. The reaction is reversible and desulphonation occurs readily in aqueous medium.

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(d) Friedel Craft’s Alkylation: (Methylation)

When benzene is treated with an alkyl halide in the presence of only AlCl3, alkyl benzene is formed.

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(e) Friedel Craft’s Acylation: Acetylation

When benzene is treated with acetyl chloride in the presence of AlCl3, acyl benzene is formed

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(f) Electrophilic Subistution Reactions: Mechanism

Benzene undergoes electrophilic substitution reaction because it is an electron-rich system due to delocalised π electron. So it is easily attacked by electrophilies and gives substituted products.

Aromatic Hydrocarbons img 25

Aromatic Hydrocarbons

Mechanism:

Step: 1
Formulation of the electrophile

E – Nu + Catalyst → E+ + Nu-Catalyst

Step: 2

The electrophile attacks the aromatic ring to form a carbocation intermediate which is stabilized by resonance.

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Step: 3

Loss of proton gives the substitution product.

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(ii) Addition Reaction:

a. Hydrogenation of benzene:

Benzene reacts with hydrogen in the presence of Platinum or Palladium to yield Cyclohexane. This is known as hydrogenation.

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b. Chlorination of Benzene:

Benzene reacts with three molecules of Cl2 in the presence of sun light or UV light to yield Benzene Hexa Chloride (BHC) C6H6Cl6. This is known as gammaxane or Lindane which is a powerful insecticide.

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(iii) Oxidation:
a. Vapour – phase oxidation:-

Although benzene is very stable to strong oxidizing agents, it quickly undergoes vapour phase oxidation by passing its vapour mixed with oxygen over V2O5 at 773k. The ring breaks to give maleic anhydride.

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b. Birch Reduction:

Benzene can be reduced to 1, 4-cyclohexadiene by treatment with Na or Li in a mixture of liquid ammonia and alcohol. It is the convenient method to prepare cyclic dienes.

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Aromatic Hydrocarbons

Directive Influence of a Functional Group in Monosubtituted Benzene:

When mono substituted benzene undergoes an electrophilic substitution reaction, the rate of the reaction and the site of attack of the incoming electrophile depends on the functional group already attached to it. Some groups increase the reactivity of benzene ring and are known as activating groups. While others which decrease the reactivity are known as deactivating groups.

We further divide these groups into two categories depending on the way they influence the orientation of attack by the incoming groups. These which increases electron density at ‘ortho’ and ‘para’ position are known as orthopara directors while those which increase electron density at ‘meta’ position is known as meta-directors. Some examples of directive influence of functional groups in mono-substituted benzene are explained below.

Ortho and Para Directing Groups

All the activating groups are ‘orthopara’ directors. Example – OH, – NH2, – NHR, – NHCOCH3, – OCH3 – CH3 – C2H5 etc. Let us consider the directive inflences of phenolic (-OH) group. Phenol is the resonance hybrid of following structures.

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In these resonance structures, the (-) charge residue is present on ortho and para position of ring structure. It is quite evident that the lone pair of electron on the atom which is attached to the ring involves in resonance and makes the ring more electron rich than benzene. The electron density at ortho and parapositions increases as compared to the meta position. Therefore phenolic group activates the benzene ring for electrophilic attack at ‘ortho’ and ‘para positions and hence -OH group is an orthopara director and activator.

In aryl halides, the strong -I effect of the halogens (electron withdrawing tendency) decreases the electron density of benzene ring, thereby deactivating for electrophilic attack. However the presence of lone pair on halogens involved in the resonance with pi electrons of benzene ring, increases electron density at ortho and para positions. Hence the halogen group is an ortho-para director and deactivator.

Aromatic Hydrocarbons

META DIRECTING GROUPS

Generally all deactivating groups are meta-directors. For example -NO2, -CN, -CHO, -COR, -COOH, -COOR, -SO3H etc. Let us consider the directive influence of aldehyde (-CHO) group. Benz aldehyde is the resonance hybrid of following structures.

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In these resonance structures, the (+) charge residues is present on the ring structure. It is quite evident that resonance delocalizes the positive charge on the atoms of the ring, making the ring less electron rich than benzene. Here overall density of benzene ring decreases due to -I effect of -CHO group there by deactivating the benzene for electrophilic attack. However resonating structure shows that electron density is more in meta position. Compared to o & p-position. Hence -CHO group is a meta-director and deactivator.

Carcinogenity and Toxicity

Benzene and polycyclic aromatic hydrocarbons (PAH) are ubiquitous environmental pollutants generated during incomplete combustion of coal oil, petrol and wood. Some PAH originate from open burning, natural seepage of petroleum and coal deposits and volcanic activities. They are toxic, mutagenic and carcinogenic. It has hematological. immunological and neurological effect on humans. They are radiomimetic and prolonged exposure leads to genetic damage. Some of the examples of PAH are.

“L” shaped polynuclear hydrocarbons are much more toxic & carcinogenic

Aromatic Hydrocarbons img 35

Found in cigarette smoke Found in tobacco and cigarette and charcoal boiled food

Aromatic Hydrocarbons img 36

Aromatic Hydrocarbons

FLOWCHART AND REACTION SUMMARY OF HYDROCARBON

Aromatic Hydrocarbons img 37

Aromatic Hydrocarbons img 38

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Alkynes – Formula, Definition, Structure, Properties

Alkynes – Formula, Definition, Structure, Properties

Alkynes are unsaturated hydrocarbons that contain carbon-carbon triple bonds in their molecules. Their general formula is CnH2n-2. The first member of alkyne series is Ethyne popularly known as acetylene. Oxyacetylene torch is used in welding.

Nomencluture of alkynes:

Let us write the IUPAC name for the below mentioned alkynes by applying the general rules of nomenclature that we already discussed in unit No.11

Alkynes img 1

Alkynes

Preparation of alkynes from alkenes:

This process involves two steps:

(i) Halogenation of alkenes to form vicinal dihalides
(ii) Dehalogenation of vicinal dihalides to form alkynes.

Alkynes img 2

Preparation of alkene from gem dihalides:

A compound containing two halogen atoms on the same carbon atom is called gem dihalide (Latin word ‘Gemini’ means twins). On heating with alcoholic KOH, gem dihalides give alkynes.

Alkynes img 3

3. Preparation of alkynes from electrolysis of salts of unsaturated dicarboxylic acids. (Kolbe’s electrolytic method)

Electrolysis of sodium or potassium salt of maleic or fumaric acid yields alkynes.

Alkynes img 4

Alkynes

4. Industrial preparation of ethyne:

Ethyne can be manufactured in large scale by action of calcium carbide with water.

Alkynes img 5

Calcium carbide required for this reaction is prepared by heating quick lime and coke in an electric furance at 3273 K

Alkynes img 6

Physical properties of alkynes:

1. The first three members are gases, next eight are liquids and the higher alkynes are solids. They are all colourless and odourless except acetylene which has garlic odour.

2. They are slightly soluble in water but dissolve readily in organic solvents like benzene, acetone and ethyl alcohol.

Alkynes

Chemical properities of alkynes

Terminal Alkynes are acidic in nature. It undergoes polymerization and addition reaction.

1. Acidic nature of alkynes:

An alkyne shows acidic nature only if it contains terminal hydrogen. This can be explained by considering sp hybrid orbitals of carbon atom in alkynes.

The percentage of s-character of sp hybrid orbital (50%) is more than sp2 hybrid orbital of alkene (33%) and sp3 hybrid orbital of alkane (25%). Because of this, Carbon becomes more electronegative facilitating donation of H+ ion to bases. So hydrogen attached to triply bonded carbon atoms is acidic.

Alkynes img 7

2. Addition reactions of alkynes

(i) Addition of hydrogen

Alkynes img 8

(ii) Addition Of Halogens:

Alkynes img 9

(iii) Addition Of Hydrogen Halides:

Reaction of hydrogen halides to symmetrical alkynes is electrophilic addition reaction. This reaction also follows Markovnikof ’s rule.

Alkynes img 10

Addition of HBr to unsymmetrical alkene follows Markownikoff ’s rule.

Alkynes img 11

(iv) Addition of Water:

Alkynes undergo hydration on warming with mercuric sulphate and dilute H2SO4 at 333K to form carbonyl compounds.

Alkynes img 12

Alkynes

3. Ozonolysis:

Ozone adds to carbon-carbon triple bond of alkynes to form ozonides. The ozonides are hydrolyzed by water to form carbonyl compounds. The hydrogen peroxide (H2O2) formed in the reaction may oxidise the carbonyl compound to carboxylic acid.

Alkynes img 13

4. Polymerisation:

Alkyne undergoes two types of polymerisation reaction

(i) Linear Polymerisation:

Ethyne forms linear polymer, when passed into a solution of cuprous chloride and ammonium chloride.

Alkynes img 14

(ii) Cyclic Polymerisation:

Ethyne undergoes cyclic polymerization on passing through red hot iron tube. Three molecules of ethynepolymerises to
benzene.

Alkynes img 15

Alkynes

Uses of Alkynes

  1. Acetylene is used in oxy acetylene torch used for welding and cutting metals.
  2. It is used for manufacture of PVC, polyvinyl acetate, polyvinyl ether, orlon and neoprene rubbers.

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Alkanes – Formula, Definition, Structure, Properties

Alkanes – Formula, Definition, Structure, Properties

Alkanes are saturated hydrocarbons represented by the general formula CnH2n+2 where ‘n’ is the number of carbon atoms in the molecule. Methane CH4, is the first member of alkane family. The successive members are ethane C2H6, propane C3H8, butane C4H10, pentane C5H12 and so on. It is evident that each member differs from its proceeding or succeeding member by a – CH2 group.

Nomenclature and Isomerism:

We have already discussed the nomenclature of organic compound in Unit:11. Let us understand the nomenclature and isomerism in few examples. The first three members methane CH4, ethane C2H6 and propane C3H8 have only one structure.

Alkanes img 1

However, higher members can have more than one structure leading to constitutional isomers (differ in connectivity) or structural isomers. For example, an alkane with molecular formula C4H10 can have two structures. They are n-butane and iso-butane. In n-butane, all the four carbon atoms are arranged in a continuous chain. The ‘n’ in n-butane stand for ‘normal’ and means that the carbon chain is unbranched. The second isomer iso-butane has a branched carbon chain. The word iso indicates it is an isomer of butane.

Alkanes img 2

Though both the structures have same molecular formula but their carbon chains differ leading to chain isomerism. Let us understand the chain isomerism by writing the isomers of pentane C5H12

Solution:

Alkanes img 3

Alkanes:

IUPAC name for some branched alkanes

Let us write the IUPAC name for the below mentioned alkanes by applying the general rules of nomenclature that we already discussed in unit No.11

Alkanes img 4

How to draw structural formula for given IUPAC name:

After you learn the rules for naming alkanes, it is relatively easy to reverse the procedure and translate the name of an alkane into a structural formula. The example below show how this is done.

Let us draw the structural formula for

(a) 3-ethyl-2,3-dimethyl pentane

Solution:

Step 1:

The parent hydrocarbon is pentane. Draw the chain of fie carbon atoms and number it.

Alkanes img 5

Alkanes:

Step 2:

Complete the carbon skeleton by attaching the alkyl group as they are specified in the name. An ethyl group is attached to carbon 3 and two methyl groups are attached to carbon 2 and 3.

Alkanes img 6

Step 3:

Add hydrogen atoms to the carbon skeleton so that each carbon atoms has four bonds

Alkanes img 7

Preparation of Alkanes:

Alkanes are not laboratory curiosities but they are extremely important naturally occurring compounds. Natural gas and
petroleum (crude oil) are the most important natural sources. However, it can be prepared by the following methods.

1. Preparation of alkanes from catalytic reduction of unsaturated hydrocarbons:

When a mixture hydrogen gas with alkene or alkyne gas is passed over a catalysts such as platinum or palladium at room temperature, an alkane is produced. This process of addition of H2 to unsaturated compounds is known as hydrogenation. The above process can be catalysed by nickel at 298K. This reaction is known as Sabatier Sendersens reaction

For example:

Alkanes img 8

Alkanes:

2. Preparation of alkanes from carboxylic acids:

(i) Decarboxylation of sodium salt of carboxylic acid

When a mixture of sodium salt of carboxylic acid and soda lime (sodium hydroxide + calcium oxide) is heated, alkane is formed. The alkane formed has one carbon atom less than carboxylic acid. This process of eliminating carboxylic group
is known as decarboxylation.

For example:

Alkanes img 9

(ii) Kolbe’s Electrolytic Method

When sodium or potassium salt of carboxylic acid is electrolyzed, a higher alkane is formed. The decarboxylative dimerization of two carboxylic acid occurs. This method is suitable for preparing symmetrical alkanes(R-R).

Alkanes img 10

3. Preparation of alkanes using alkyl halides (or) halo alkanes

(i) By reduction with nascent hydrogen

Except alkyl florides, other alkyl halides can be converted to alkanes by reduction with nascent hydrogen. The hydrogen for reduction may be obtained by using any of the following reducing agents:
Zn+HCl, Zn+CH3COOH, Zn-Cu couple in ethanol, LiAlH4 etc.,

For Example:

Alkanes img 11

(ii) Wurtz Reaction

When a solution of halo alkanes in dry ether is treated with sodium metal, higher alkanes are produced. This reaction is used to prepare higher alkanes with even number of carbon atoms.

For Example:

Alkanes img 12

Alkanes:

(iii) Corey – House Mechanism

An alkyl halide and lithium di alkyl copper are reacted to give higher alkane.

For Example:

Alkanes img 13

4. Preparation of Alkanes from Grignard Reagents

Halo alkanes reacts with magnesium in the presence of dry ethers to give alkyl magnesium halide which is known as Grignard reagents. Here the alkyl group is directly attached to the magnesium metal make it to behave as carbanion. So, any compound with easily replaceable hydrogen reacts with Grignard reagent to give corresponding alkanes.

For Example:

Alkanes img 14

Physical Properties:

Alkanes img 15

1. Boiling Point and Physical State

The boiling point of continuous chain alkanes increases with increases in length of carbon chain roughly about 30°C for every added carbon atom to the chain. Being non polar, alkanes have weak Vanderwal’s force which depends upon molecular surface area and hence increases with increase molecular size.

We observe that with same number of carbon atoms, straight chain isomers have higher boiling point compared to branch chain isomers. The boiling point decreases with increase in branching as the molecule becomes compact and the area of the contact decreases.

Alkanes:

2. Solubility and Density

Water molecules are polar and alkanes are non-polar. The insolubility of alkanes in water makes them good water repellent for metals which protects the metal surface from corrosion. Because of their lower density than water, they form two layers and occupy top layer. The density difference between alkanes and water explains why oil spills in aqueous environment spread so quickly.

Conformations of Alkane:

Each carbon in alkanes is sp3 hybridized and the four groups or atoms around the carbon are tetrahedrally bonded. In alkanes having two or more carbons, there exists free rotation about C-C single bond. Such rotation leaves all the groups or atoms bonded to each carbon into an infinite number of readily interconvertible three dimensional arrangements. Such readily interconvertible three dimensional arrangement of a molecule is called conformations.

(i) Conformations of Ethane:

The two tetrahedral methyl groups can rotate about the carbon – carbon bond axis yielding several arrangements called conformers. The extreme conformations are staggered and eclipsed conformation. There can be number of other arrangements between staggered and eclipsed forms and their arrangements are known as skew forms.

Alkanes:

Eclipsed Conformation

Alkanes img 16

In this conformation, the hydrogen’s of one carbon are directly behind those of the other. The repulsion between the atoms is maximum and it is the least stable conformer.

Staggered Conformation:

Alkanes img 17

In this conformation, the hydrogens of both the carbon atoms are far apart from each other. The repulsion between the atoms is minimum and it is the most stable conformer.

Skew Conformation:

The infinite numbers of possible intermediate conformations between the two extreme conformations are referred as skew conformations.

The stabilities of various conformations of ethane are
Staggered > Skew > Eclipsed

The potential energy difference between the staggered and eclipsed conformation of ethane is around 12.5 KJmol-1. The various conformations can be represented by new man projection formula.

Alkanes img 18

Alkanes:

Newman Projection formula for Ethane

Conformations of n-Butane:

n-Butane may be considered as a derivative of ethane, as one hydrogen on each carbon is replaced by a methyl group

Eclipsed Conformation:

In this conformation, the distance between the two methyl group is minimum. So there is maximum repulsion between them and it is the least stable conformer.

Anti or Staggered Form

In this conformation, the distance between the two methyl groups is maximum and so there is minimum repulsion between them. And it is the most stable conformer. The following potential energy diagram shows the relative stabilities of various conformers of n-butane.

Alkanes img 19

Chemical Properties:

Alkanes are quite unreactive towards most reagents. However under favourable conditions, alkanes undergo the following type of reaction.

Combustion:

A combustion reaction is a chemical reaction between a substances and oxygen with evolution of heat and light (usually as a flame). In the presence of sufficient oxygen, alkanes undergoes combustion when ignited and produces carbondioxide and water.

For Example:

CH4 + 2O2 → CO2 + 2H2O ∆H° = – 890.4kJ

When alkanes burn in insufficient supply of oxygen, they form carbonmonoxide and carbon black.

Alkanes img 20

Alkanes:

Halogenation:

Halogenation reaction is the chemical reaction between an alkane and halogen in which one or more hydrogen atoms are substituted by the halogens.

Chlorination and Bromination are two widely used halogenation reactions. Fluorination is too quick and iodination is too slow. Methane reacts with chlorine in the presence of light or when heated as follows.

Alkanes img 21

Mechanism:

The reaction proceeds through the free radical chain mechanism. This mechanism is characterized by three steps initiation, propagation and termination.

(i) CHAIN INITITATION:

The chain is initiated by UV light leading to homolytic fission of chlorine molecules into free radicals (chlorine atoms).

Alkanes img 22

Here we choose Cl-Cl bond for fision because C-C & C-H bonds are stronger than Cl-Cl.

(ii) PROPAGATION:

It proceeds as follows,

(a) Chlorine free radial attacks the methane molecule and breaks the C-H bond resulting in the generation of methyl free radical.

Alkanes img 23

(b) The methyl free radical thus obtained attacks the second molecule of chlorine to give chloromethane (CH3Cl) and a chlorine free radical as follows.

Alkanes img 24

(c) This chlorine free radical then cycles back to step

(a) and both step (a) and (b) are repeated many times and thus chain of reaction is set up.

Alkanes:

(iii) Chain Termination:

After sometimes, the reactions stops due to consumption of reactant and the chain is terminated by the combination of free radicals.

Alkanes img 25

3. Aromatisation

Alkanes with six to ten carbon atoms are converted into homologous of benzene at high temperature and in the presence of catalyst. This process is known as aromatization. It occurs by simultaneous cyclisation followed by dehydrogenation of alkanes.

n-Hexane passed over Cr2O3 supported on alumina at 873 K gives benzene.

Alkanes img 26

4. Reaction With Steam:

Methane reacts with steam at 1273K in the presence of Nickel and decomposes to form carbon monoxide and hydrogen gas.

Alkanes img 27

Production of H2 gas from methane is known as steam reforming process and it is a well-established industrial process for the production of H2 gas from hydrocarbons.

Alkanes:

5. Pyrolysis

Pyrolysis is defined as the thermal decomposition of organic compound into smaller fragments in the absence of air through the application of heat. ‘Pyro’ means ‘fie’ and ‘lysis’ means ‘separating’. Pyrolysis of alkanes also named as cracking. In the absence of air, when alkane vapours are passed through red-hot metal it breaks down into simpler hydrocarbons.

Alkanes img 28

The products depends upon the nature of alkane, temperature, pressure and presence or absence of catalyst. The ease of cracking in alkanes increases with increase in molecular weight and branching in alkanes. Cracking plays an important role in petroleum industry.

6. Isomerisation:

Isomerisation is a chemical process by which a compound is transformed into any of its isomeric forms. Normal alkanes can be converted into branched alkanes in the presence of AlCl3 and HCl at 298 k.

Alkanes img 29

This process is of great industrial importance. The quality of gasoline is improved by isomerising its components.

Alkanes:

Uses

The exothermic nature of alkane combustion reaction explains the extensive use of alkanes as fuels. Methane present in natural gas is used in home heating. Mixture of propane and butane are known as LPG gas which is used for domestic cooking purpose. GASOLINE is a complex mixture of many hydrocarbons used as a fuel for internalcombustion engines.

Carbon black is used in the manufacture of ink, printer ink and black pigments. It is also used as fillers.

Alkanes img 30

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Classification of Organic Halogen Compounds – Definition, Examples, Uses, & Facts

Classification of Organic Halogen Compounds – Definition, Examples, Uses, & Facts

A General Survey. Organic halogen compounds are derivatives of organic compounds in which one or more hydrogen atoms have been replaced by an equal number of halogen atoms (F, Cl, Br, or I). Almost any class of organic compounds (e.g., alcohols, ketones, carboxylic acids) can contain halogen atoms.

The haloalkanes, also known as alkyl halides, are a group of chemical compounds comprised of an alkane with one or more hydrogens replaced by a halogen atom (fluorine, chlorine, bromine, or iodine). The classification is determined by the number of carbons bonded to the carbon bearing the halide.

These are the compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom next to carbon-carbon double bond (C=C) i.e. to an allylic carbon. These are the compounds in which the halogen atom is bonded to an sp3 hybridised carbon atom next to an aromatic ring.

Classification of Organic Halogen Compounds

Both chlorine and bromine are used as disinfectants for drinking water, swimming pools, fresh wounds, spas, dishes, and surfaces. They kill bacteria and other potentially harmful microorganisms through a process known as sterilization. Chlorine and bromine are also used in bleaching.

The halogens all form binary compounds with hydrogen, and these compounds are known as the hydrogen halides: hydrogen fluoride (HF), hydrogen chloride (HCl), hydrogen bromide (HBr), hydrogen iodide (HI), and hydrogen astatide (HAT). When in aqueous solution, the hydrogen halides are known as hydrohalic acids.

Classification of Organic Halogen Compounds img 1

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Environmental Pollution – Definition, History, Types, & Facts

Environmental Pollution – Definition, History, Types, & Facts

Any undesirable change in our environment that has harmful effects on plants, animals and human beings is called environmental pollution.

Environmental pollution is usually caused by the addition of waste products of human activity to the environment. The substances which cause pollution of environment are called pollutants. The pollutants may be solids, liquids or gaseous substances present in significant concentration in the environment.

Environmental Pollution

Our environment becomes polluted day by day, by the increased addition of industrial and domestic wastes to it. The air we breathe, the water we drink and the place where we live in, are highly contaminated.

The pollutants are classified as bio-degradable and non-biodegradable pollutants.

(i) Bio-Degradable Pollutants:

The pollutants which can be easily decomposed by the natural biological processes are called bio-degradable pollutants. Examples: plant wastes, animal wastes etc.

Environmental Pollution

(ii) Non Bio-Degradable Pollutants:

The pollutants which cannot be decomposed by the natural biological processes are called Non bio degradable pollutants. Examples: metal wastes (mainly Hg and Pb), D.D.T, plastics, nuclear wastes etc., These pollutants are harmful to living organisms even in low concentration. As they are not degraded naturally, it is difficult to eliminate them from our environment.

Environmental Pollution img 1

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Chemistry | Definition, Topics & History Concepts, Free Resources

Chemistry | Definition, Topics & History Concepts, Free Resources

Physical and Chemical Equilibrium 

Solutions

Chemical Bonding

Fundamentals of Organic Chemistry 

Basic Concepts of Organic Reactions  

Hydrocarbons

Haloalkanes and Haloarenes 

Environmental Chemistry 

Alkenes – Formula, Definition, Structure, Properties

Alkenes – Formula, Definition, Structure, Properties

Alkenes are unsaturated hydrocarbons that contain carbon-carbon double bond. They are represented by the general formulae CnH2n where ‘n’ stands for number of carbon atoms in the molecule. Alkenes are also known as olefins (in Latin oil maker) because the first member ethene combines with chlorine gas to form an oily liquid as a product.

(i) Nomenclature of Alkenes:

Let us write the IUPAC name for the below mentioned alkanes by applying the general rules of nomenclature that we already discussed in unit No.11

Alkenes:

IUPAC NAMES FOR ALKENES

Alkenes img 1

(ii) Isomerism:

Presence of double bond in Alkene provides the possibility of both structural and geometrical isomerism.

Structural Isomerism:

The first two member’s ethene C2H4 and propene C3H6 do not have isomers because the carbon atoms in the molecules can be arranged only one distinct way. However from the third member of alkene family butene C4H8, structural isomerism exists.

Alkenes img 2

structures (i) & (ii) are position isomers. structures (i) & (iii), (ii) & (iii) are chain isomers

Alkenes:

Geometrical Isomerism:

It is a type of stereoisomerism and it is also called cis-trans isomerism. Such type of isomerism results due to the restricted rotation of doubly bounded carbon atoms. If the similar groups lie on the same side, then the geometrical isomers are called Cis-isomers. When the similar groups lie on the opposite side, it is called a Trans isomer.

For Example:
The geometrical isomers of 2-Butene is expressed as follows

Alkenes img 3

General methods of preparation of alkenes:

(1) Preparation of alkene by dehydration of alcohol:

When an alcohol is heated at 430-440 K with excess of concentrated sulphuric acid, a molecule of water from alcohol is removed and an alkene is formed. This reaction is called elimination reaction.

Alkenes img 4

Ethene can also be prepared in laboratory by catalytic dehydration of alcohol.

Alkenes img 5

Alkenes:

(2) Preparation of alkenes from alkynes:

Alkynes can be reduced to cisalkenes using Lindlar’s catalyst (CaCO3 supported in palladuium partially deactivated with sulphur (or) gasoline). This reaction is stero specific giving only the cis-alkene.

Alkenes img 6

Alkynes can also be reduced to transalkenes using sodium in liquid ammonia. This reaction is stereospecific giving only the trans-alkene.

Alkenes img 7

(3) Preparation of alkenes by dehydrohalogenaton of halo alkanes.

Halo alkanes react with alcoholic KOH and eliminate hydrohalide resulting in the formation of alkene.

Alkenes img 8

(4) Preparation of alkenes from vicinal dihalogen derivative of alkanes or vicinal dihalides

The compound in which two halogen atoms are attached to adjacent carbon-atoms are called as vicinal dihalides. When vicinal dihalides are warmed with granulated zinc in methanol, they lose a molecule of ZnX2 to form an alkene.

Alkenes img 9

(5) Preparation of ethene by kolbe’s electrolytic method:

When an aqueous solution of potassium succinate is electrolyzed between two platinum electrodes, ethene is produced at the anode.

Alkenes img 10

At anode

Alkenes img 11

Alkenes:

Physical properities of alkenes:

The first three members (Ethene, Propene and Butene) are gases, next fourteen members are liquids and the higher alkenes are waxy solids. They are all colourless and odourless except ethene which has a sweet smell.

1. The melting and boiling point of alkenes increases along the homologous series. Like alkanes, straight chain alkenes have high boiling point compared to its isomeric branched alkenes.

2. Alkenes are slightly soluble in water but readily in organic solvents.

Chemical Properties of Alkenes:

Alkenes are more reactive than alkanes due to the presence of a double bond. The σ- bond is strong but the π- bond is weak. The typical reactions of alkenes involve addition of an electrophile across the double bonds proceeding through ionic mechanism. However addition reactions proceed through free-radical mechanism also. Ozonolysis and polymerization are some of the characteristic reactions of alkenes.

(i) Addition Reactions

(ii) Addition of hydrogen: (Hydrogenation of alkenes)

Hydrogen adds on to alkenes in the presence of a metal catalyst (Ni, Pd (or) Pt) to yield corresponding alkanes. This is known as catalytic hydrogenation. This process is of great importance in the manufacture of vanaspathi from vegetable oil. This helps to prevent rancidity of vegetable oils.

(ii) Addition of halogens: (Halogenation of alkenes)

When alkene is treated with halogens like chlorine or bromine, addition takes place rapidly and forms 1, 2 – dihalo alkane (or) vicinal dihalide

Alkenes img 12

Iodine reacts very slowly to form 1, 2 – diiodo alkane which are unstable and regenerate the original alkene by elimination of iodine.

Alkenes img 13

Alkenes:

TEST FOR ALKENE:

Alkenes img 14

Bromine in water is reddish brown colour. When small amount of bromine water is added to an alkene, the solution is decolourised as it forms dibromo compound. So, this is the characteristic test for unsaturated compounds.

Markovnikoff ’s Rule:

“When an unsymmetrical alkene reacts with hydrogen halide, the hydrogen adds to the carbon that has more number of hydrogen and halogen add to the carbon having fewer hydrogen”. This rule can also be stated as in the addition reaction of alkene / alkyne, the most electro negative part of the reagent adds on to the least hydrogen attached doubly bonded carbon.

(iii) Addition of water:- (Hydration of alkenes)

Normally, water does not react with alkenes. In the presence of concentrated sulphuric acid, alkenes react with water to form alcohols. This reaction follows carbocation mechanism and Markovnikof ’s rule.

Alkenes img 15

(iv) Addition of hydrohalides: (Hydrohalogenation of Alkenes)

Hydrogen halides (HCl, HBr and HI) add to alkene to yield alkyl halides. The order of reactivity of different hydrogen halides is HI>HBr>HCl. It is an example for electrophilic addition.

Alkenes:

(a) Addition of HBr to symmetrical alkene:

Addition of HBr to symmetrical alkene (similar groups are attached to double bond) yields alkyl halides (haloalkanes)

Alkenes img 16

(b) Addition HBr to unsymmetrical alkene:

In the addition of hydrogen halide to an unsymmetrical alkene, two products are obtained.

Alkenes img 17

Mechanism:

Consider addition of HBr to propene

Step: 1

Formation of electrophile:

In H-Br, Br is more electronegative than H. When bonded electron moves toward Br, polarity is developed and creates an electrophile H+ which attacks the double bond to form carbocation, as shown below.

Alkenes img 18

Step: 2

Secondary carbocation is more stable than primary carbocation and it predominates over a the primary carbocation.

Step: 3

The Brθ ion attacks the 2° carbocation to form 2-Bromo propane, as the major product.

Consider addition of HBr to 3-methyl-1-butene. Here the expected product according to markovnikoff ’s rule is 2-bromo-3-methyl butane but the actual major product is 2-Bromo-2-methyl butane. This is because, the secondary carbocation formed during the reaction rearranged to more stable tertiary carbocation. Attack of Br on this tertiary carbocation gives the major product 2-bromo-2-methyl butane.

Alkenes img 19

Alkenes:

Carbocation Rearrangement

Alkenes img 20

The addition of HBr to an alkene in the presence of organic peroxide, gives the anti Markovnikof ’s product. This effect is called peroxide effect.

Alkenes img 21

Mechanism:

The reaction proceeds via free radical mechanism.

Step: 1

The weak O-O single bond linkages of peroxides undergoes homolytic cleavage to generate free radical.

Alkenes img 22

Step: 2

The radicals abstracts a hydrogen from HBr thus generating bromine radical.

Alkenes img 23

Step: 3

The Bromine radical adds to the double bond in the way to form more stable alkyl free radical.

Alkenes img 24

Step: 4

Addition of HBr to secondary free radical

Alkenes img 25

Alkenes:

Addition of HBr to secondary free radical

The H-Cl bond is stronger (430.5 kJmol-1) than H-Br bond (363.7 kJmol-1), thus H-Cl is not cleaved by the free radical. Thus H-I bond is weaker (296.8 kJ mol-1), than H-Cl bond. Thus H-I bond breaks easily but iodine free radicals combine to form iodine molecules instead of adding to the double bond and hence peroxide effect is not observed in HCl & HI.

Kharasch Addition

Metal catalysed free radical addition of CXCl5 Compounds to alkene is called Kharash addition reaction.

(v) Addition of sulphuric acid to alkenes

Alkenes react with cold and concentrated sulphuric acid to form alkyl hydrogen sulphate accordance with Markownikof ‘s rule. Further hydrolysis yields alcohol.

Alkenes img 26

(2) Oxidation:

(i) With cold dilute alkaline KMnO4 solution (Baeyer’s Reagent)

Alkenes react with Baeyer’s reagent to form vicinal diols. The purple solution (Mn7+) becomes dark green (Mn6+), and then produces a dark brown precipitate (Mn4+).

Alkenes img 27

(ii) With acidified KMnO4 Solution:

Alkenes react with acidified KMnO4 solution and are oxidised to ketones or carboxylic acid depends on the substituent at the olefinic carbon atom. The purple solution becomes colourless. This is one of the test for unsaturation.

Alkenes img 28

Alkenes:

(iii) Ozonolysis:

Ozonolysis is a method of oxidative cleavage of alkenes or alkynes using ozone and forms two carbonyl compounds. Alkenes react with ozone to form Ozonide and it is cleaved by Zn/H2O to form smaller molecules. This reaction is often used to identify the structure of unknown alkene or alkyne by detecting the position of double or triple bond.

Alkenes img 29

(iv) Polymerisation:

A polymer is a large molecule formed by the combination of larger number of small molecules. The process in known as polymerisation. Alkenes undergo polymerisation at high temperature and pressure, in the presence of a catalyst.

For Example

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Recycling plastics

Extensive use of polymers clogs up landfills and polute the environment. Because of diversity of polymers in consumer products, recycling requires sorting the polymers into various sub-types, labels with codes and symbols, which are then recycled separately.

Table shows the codes and symbols used in recycling of ethene-based additionpolymers.

(Lower the number, greater the ease of recycling the material)

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Alkenes:

Uses of Alkenes

1. Alkenes find many diverse applications in industry. They are used as starting materials in the synthesis of alcohols, plastics, liquors, detergents and fuels.

2. Ethene is the most important organic feed stock in the polymer industry. E.g. PVC, Sarans and polyethylene. These polymer are used in the manufacture of floor tiles, shoe soles, synthetic fires, raincoats, pipes etc.,

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