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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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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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:

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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.

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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°.

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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

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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.

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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

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(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.

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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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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.

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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

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

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