Haloarenes – Definition, Classification, Uses

Haloarenes – Definition, Classification, Uses

Haloarenes are the compounds in which the halogen is directly attached to the benzene ring.

Haloarenes img 1

Nomenclature of Haloarenes

In the IUPAC nomenclature, the halo arenes are named by adding prefix halo before the name of the aromatic hydrocarbon. For naming disubstituted arenes, the relative position of the substituent 1, 2; 1, 3 and 1, 4 are indicated by the prefixes ortho, meta and para, respectively.

For poly haloarenes the numbering should be done in such a way that the lowest possible number should be given to the substituents and the name of the halogens are arranged in alphabetic order. Nomenclature can be well understood from the following examples.

Haloarenes img 2

Haloarenes

Nature of C – X Bond in Haloarenes

In halo arenes the carbon atom is sp2 hybridised. The sp2 hybridised orbitals are shorter and holds the electron pair of bond more tightly.

Halogen atom contains P-orbital with lone pair of electrons which interacts with π-orbitals of benzene ring to form extended conjugated system of π-orbitals. The delocalisation of these electrons give double bond character to C – X bond. The resonance structure of halobenzene is given as

Haloarenes img 3

Due to this double bond character of C – X bond in haloarenes, the C – X bond is shorter in length and stronger than in halo alkanes.

Example

Haloarenes img 4

1. Direct Halogenation

Chlorobenzene is prepared by the direct chlorination of benzene in the presence of lewis acid catalyst like FeCl3

Haloarenes img 5

Haloarenes

2. From Benzene Diazonium Chloride

Chloro benzene is prepared by Sandmeyer reaction or Gattermann reaction using benzene diazonium chloride.

(i) Sandmeyer Reaction

When aqueous solution of benzene diazonium chloride is warmed with Cu2Cl2 in HCl gives chloro benzene

Haloarenes img 6

3. Preparation of Iodobenzene

Iodobenzene is prepared by warming benzene diazonium chloride with aqueous KI solution.

Haloarenes img 7

4. Preparation of Fluorobenzene

Fluoro benzene is prepared by treating benzenediazonium chloride with fluoro boric acid. This reaction produces diazonium fluoroborate which on heating produces flourobenzene. This reaction is called Balz – schiemann reaction.

Haloarenes img 8

5. Commercial Preparation of Chloro Benzene (Raschig Process)

Chloro benzene is commercially prepared by passing a mixture of benzene vapour, air and HCl over heated cupric chloride. This reaction is called Raschig process.

Haloarenes img 9

Haloarenes

Physical Properties

1. Melting and boiling points

The boiling points of monohalo benzene which are all liquids follow the order

Iodo > Bromo > Chloro

The boiling points of isomeric dihalobenzene are nearly the same

The melting point of para isomer is generally higher than the melting points of ortho and meta isomers. The higher melting point of p-isomer is due to its symmetry which leads to more close packing of its molecules in the crystal lattice and consequently strong intermolecular attractive force which requires more energy for melting.

p – Dihalo benzene > o – Dichloro benzene > m – Dichloro benzene

2. Solubility

Haloarenes are insoluble in water because they cannot form hydrogen bonds with water, but are soluble in organic solvents

3. Density

Halo arenes are all heavier than water and their densities follow the order.

Iodo benzene > Bromo benzene > Chloro benzene

Haloarenes

Chemical Properties

A. Reactions invoving halogen atom

1. Aromatic nucleophilic substitution reaction

Halo arenes do not undergo nucleophilic substitution reaction readily. This is due to C-X bond in aryl halide is short and strong and also the aromatic ring is a centre of high electron density.

The halogen of haloarenes can be substituted by OH, NH2, or CN with appropriate nucleophilic reagents at high temperature and pressure.

For Example

Haloarenes img 10

This reaction is known as Dow’s Process

Haloarenes img 11

2. Reaction with Metals

(a) Wurtz Fittig Reaction

Halo arenes reacts with halo alkanes when heated with sodium in ether solution to form alkyl benzene. This reaction is called Wurtz Fittig reaction

Haloarenes img 12

(b) Fittig reaction

Haloarenes react with sodium metal in dry ether, two aryl groups combine to give biaryl products. This reaction is called Fittig reaction

Haloarenes img 13

B. Reaction involving aromatic ring

3. Electrophilic substitution reaction

Haloarenes undergo aromatic electrophilic substitution reactions. The rate of eleclophilic substitution of halobenzene is lower than that of benzene halogen is deactivating due to – I effect of halogen. The lone pair of electrons on the chlorine involves in resonance with the ring. It increases the electron density at ortho and para position (refer figure no 14.1).

Haloarenes img 14

The halogen attached to the benzine ring with draw electron and thereby and hence the halogen which is attached to the benzene directs the incoming, electrophile either to ortho or to para position in electrophilie substitution reaction.

Toluene

Haloarenes

4. Reduction

Haloarenes on reduction with NiAl alloy in the presence of NaOH gives corresponding arenes.

Haloarenes img 15

5. Formation of Grignard Reagent

Haloarenes reacts with magnesium to form Grignard reagent in tetra hydrofuran (THF).

Haloarenes img 16

Uses of Chloro Benzene

  1. Chloro benzene is used in the manufacture of pesticides like DDT
  2. It is used as high boiling solvent in organic synthesis
  3. It is used as fire – swelling agent in textile processing

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OrganoMetallic Compounds – Definition, Details, Properties, and Applications

OrganoMetallic Compounds – Definition, Details, Properties, and Applications

Organo metallic compounds are organic compounds in which there is a direct carbon – metal bond. For Example CH3Mg I – Methyl magnesium iodide CH3CH2MgBr – Ethyl magnesium bromide

The Carbon – Magnesium bond in Grignard reagent is covalent but highly polar. The carbon atom is more electro negative than magnesium. Hence, the carbon atom has partial negative charge and the magnesium atom has partial positive charge

Organo Metallic Compounds img 1

Organo Metallic Compounds

Preparation

When a solution of alkyl halide in ether is allowed to stand over pieces of magnesium metal, the metal gradually dissolves and alkyl magnesium halide (Grignard reagent) is formed. All the reagents used should be pure and dry

Example

Organo Metallic Compounds img 2

Uses of Grignard Reagent

Grignard reagents are synthetically very useful compounds. These reagents are converted to various organic compounds like alcohols, carboxylic acids, aldehydes and ketones. The alkyl group being electron rich acts as a carbanion or a nucleophile. They would attack polarized molecules at a point of low electron density. The following reactions illustrate the synthetic uses of Grignard reagent.

1. Preparation of Primary Alcohol

Formaldehyde reacts with Grignard reagent to give addition products which on hydrolysis yields primary alcohol.

Organo Metallic Compounds img 3

2. Preparation of Secondary Alcohol

Aldehydes other than formaldehyde, react with Grignard reagent to give addition product which on hydrolysis yields secondary alcohol.

Organo Metallic Compounds img 4

Organo Metallic Compounds

3. Preparation of Tertiary Alcohol

Ketone reacts with Grignard reagent to give an addition product which on hydrolysis yields tertiary alcohols.

Example

Organo Metallic Compounds img 5

4. Preparation of Aldehyde

Ethyl formate reacts with Grignard reagent to form aldehyde. However, with excess of Grignard reagent it forms secondary alcohol

Example

Organo Metallic Compounds img 6

5. Preparation of Ketone

Acid chloride reacts with Grignard reagent to form ketones. However, with excess of Grignard reagent it forms tertiary alcohol.

Example

Organo Metallic Compounds img 7

6. Preparation of Carboxylic Acids

Solid carbon dioxide reacts with Grignard reagent to form addition product which on hydrolysis yields carboxylic acids.

For Example

Organo Metallic Compounds img 8

7. Preparation of Esters

Ethylchloroformate reacts with Grignard reagent to form esters.

Example

Organo Metallic Compounds img 9

Organo Metallic Compounds

8. Preparation of Higher Ethers

Lower halogenated ether reacts with Grignard reagent to form higher ethers.

Example

Organo Metallic Compounds img 10

9. Preparation of Alkyl Cyanide

Grignard reagent reacts with cyanogen chloride to from alkyl cyanide

Example

Organo Metallic Compounds img 11

Organo Metallic Compounds

10. Preparation of Alkanes

Compounds like water, alcohols and amines which contain active hydrogen atom react with Grignard reagents to form alkanes.

Example

Organo Metallic Compounds img 12

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VSEPR Theory – Postulates, Limitations, Predicting Shapes

VSEPR Theory – Postulates, Limitations, Predicting Shapes

Lewis concept of structure of molecules deals with the relative position of atoms in the molecules and sharing of electron pairs between them. However, we cannot predict the shape of the molecule using Lewis concept.

Lewis theory in combination with VSEPR theory will be useful in predicting the shape of molecules.

Valence Shell Electron Pair Repulsion (VSEPR) Theory

Important Principles of VSEPR Theory are as follows:

1. The shape of the molecules depends on the number of valence shell electron pair around the central atom.

2. There are two types of electron pairs namely bond pairs and lone pairs. The bond pair of electrons are those shared between two atoms, while the lone pairs are the valence electron pairs that are not involved in bonding.

3. Each pair of valence electrons around the central atom repels each other and hence, they are located as far away as possible in three dimensional space to minimize the repulsion between them.

4. The repulsive interaction between the different types of electron pairs is in the following order.

lp – lp > lp – bp > bp – bp
lp – lone pair; bp – bond pair

The lone pair of electrons are localised only on the central atom and interacts with only one nucleus whereas the bond pairs are shared between two atoms and they interact with two nuclei. Because of this the lone pairs occupy more space and have greater repulsive power than the bond pairs in a molecule.

The following Table illustrates the shapes of molecules predicted by VSEPR theory. Consider a molecule ABx where A is the central atom and x represents the number of atoms of B covalently bonded to the central atom A. The lone pairs present in the atoms are denoted as L.

Valence Shell Electron Pair Repulsion (VSEPR) Theory

Valence Shell Electron Pair Repulsion (VSEPR) Theory img 1

Valence Shell Electron Pair Repulsion (VSEPR) Theory img 2

Valence Shell Electron Pair Repulsion (VSEPR) Theory img 3

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Bond Parameters – Bond Order, Angle, Length, and Energy

Bond Parameters – Bond Order, Angle, Length, and Energy

A covalent bond is characterised by parameters such as bond length, bond angle, bond order etc. A brief description of some of the bond parameters is given below.

Bond Length

The distance between the nuclei of the two covalently bonded atoms is called bond length. Consider a covalent molecule A-B. The bond length is given by the sum of the radii of the bonded atoms (rA + rB). The length of a bond can be determined by spectroscopic, x-ray diffraction and electron-diffraction techniques. The bond length depends on the size of the atom and the number of bonds (multiplicity) between the combining atoms.

Bond Parameters img 1

Greater the size of the atom, greater will be the bond length. For example, carbon-carbon single bond length (1.54 Å) is longer than the carbon-nitrogen single bond length (1.43 Å). Increase in the number of bonds between the two atoms decreases the bond length. For example, the carbon-carbon single bond is longer than the carboncarbon double bond (1.33 Å) and the carbon-carbon triple bond (1.20 Å).

Bond Parameters

Bond Order

The number of bonds formed between the two bonded atoms in a molecule is called the bond order. In Lewis theory, the bond order is equal to the number of shared pair of electrons between the two bonded atoms. For example in hydrogen molecules, there is only one shared pair of electrons and hence, the bond order is one.  Similarly, in H2O, HCl, Methane, etc the central atom forms single bonds with bond order of one.

Bond Parameters img 2

Bond Angle

Covalent bonds are directional in nature and are oriented in specific directions in space. This directional nature creates a fixed angle between two covalent bonds in a molecule and this angle is termed as bond angle. It is usually expressed in degrees. The bond angle can be determined by spectroscopic methods and it can give some idea about the shape of the molecule.

Bond Parameters img 3

Bond Parameters

Bond Enthalpy

The bond enthalpy is defined as the minimum amount of energy required to break one mole of a particular bond in molecules in their gaseous state. The unit of bond enthalpy is kJ mol-1. Larger the bond enthalpy, stronger will be the bond.

The bond energy value depends on the size of the atoms and the number of bonds between the bonded atoms. Larger the size of the atom involved in the bond, lesser is the bond enthalpy.

In case of polyatomic molecules with, two or more same bond types, in the term average bond enthalpy is used. For such bonds, the arithmetic mean of the bond energy values of the same type of bonds is considered as average bond enthalpy. For example in water, there are two OH bonds present and the energy needed to break them are not same.

H2O(g) → H(g) + OH(g) ∆H1 = 502 kJ mol-1
OH(g) → H(g) + O(g) ∆H2 = 427 kJ mol-1

The average bond enthalpy of OH bond in water = \(\frac{502+427}{2}\) = 464.5 kJ mol-1

Bond Parameters img 4

Bond Parameters

Resonance

When we write Lewis structures for a molecule, more than one valid Lewis structures are possible in certain cases. For example let us consider the Lewis structure of carbonate ion [CO3]2-.

The skeletal structure of carbonate ion (The oxygen atoms are denoted as OA, OB & OC

Bond Parameters img 5

Total number of valence electrons = [1 × 4(carbon)] + [3 × 6 (oxygen)] + [2 (charge)]
= 24 electrons.
Distribution of these valence electrons gives us the following structure.

Bond Parameters img 6

Complete the octet for carbon by moving a lone pair from one of the oxygens (OA) and write the charge of the ion (2-) on the upper right side as shown in the figure.

Bond Parameters img 7

In this case, we can draw two additional Lewis structures by moving the lone pairs from the other two oxygens (OB and OC) thus creating three similar structures as shown below in which the relative position of the atoms are same.

They only differ in the position of bonding and lone pair of electrons. Such structures are called resonance structures (canonical structures) and this phenomenon is called resonance.

Bond Parameters img 8

It is evident from the experimental results that all carbon-oxygen bonds in carbonate ion are equivalent. The actual structure of the molecules is said to be the resonance hybrid, an average of these three resonance forms. It is important to note that carbonate ion does not change from one structure to another and vice versa.

It is not possible to picturise the resonance hybrid by drawing a single Lewis structure. However, the following structure gives a qualitative idea about the correct structure.

Bond Parameters img 9

It is found that the energy of the resonance hybrid (structure 4) is lower than that of all possible canonical structures (Structure 1, 2 & 3). The difference in energy between structure 1 or 2 or 3, (most stable canonical structure) and structure 4 (resonance hybrid) is called resonance energy.

Bond Parameters

Polarity of Bonds

Partial ionic character in covalent bond:

When a covalent bond is formed between two identical atoms (as in the case of H2, O2, Cl2 etc…) both atoms have equal tendency to attract the shared pair of electrons and hence the shared pair of electrons lies exactly in the middle of the nuclei of two atoms.

However, in the case of covalent bond formed between atoms having different electronegativities, the atom with higher electronegativity will have greater tendency to attract the shared pair of electrons more towards itself than the other atom. As a result the cloud of shared electron pair gets distorted.

Let us consider the covalent bond between hydrogen and fluorine in hydrogen fluoride. The electronegativities of hydrogen and fluorine on Pauling’s scale are 2.1 and 4 respective fluorine attracts the shared pair of electrons approximately twice as much as the hydrogen which leads to partial negative charge on fluorine and partial positive charge on hydrogen. Hence, the H-F bond is said to be polar covalent bond. Here, a very small, equal and opposite charges are separated by a small distance (91 pm) and is referred to as a dipole.

Dipole Moment:

The polarity of a covalent bond can be measured in terms of dipole moment which is defined as
μ = q × 2d

Where μ is the dipole moment, q is the charge and 2d is the distance between the two charges. The dipole moment is a vector and the direction of the dipole moment vector points from the negative charge to positive charge.

Bond Parameters img 10

The unit for dipole moment is columb meter (C m). It is usually expressed in Debye unit (D). The conversion factor is

1 Debye = 3.336 × 10-2

Diatomic molecules such as H2, O2 F2 etc have zero dipole moment and are called non polar molecules and molecules such as HF, HCl, CO, NO etc… have non zero dipole moments and are called polar molecules.

Molecules having polar bonds will not necessarily have a dipole moment. For example, the linear form of carbon dioxide has zero dipole moment, even though it has two polar bonds. In CO2, the dipole moments of two polar bonds (CO) are equal in magnitude but have opposite direction. Hence, the net dipole moment of the CO2 is, μ = μ1 + μ2 = μ1 + (-μ1) = 0.

Bond Parameters img 11

Incase of water net dipole moment is the vector sum of μ1 + μ2 as shown.

Bond Parameters img 12

Dipole moment in water is found to be 1.85D

Bond Parameters img 13

The extent of ionic character in a covalent bond can be related to the electro negativity difference to the bonded atoms. In a typical polar molecule, Aδ – Bδ+, the electronegativity difference (χA – xB) can be used to predict the percentage of ionic character as follows.

If the electronegativity difference (χA – χB), is equal to 1.7, then the bond A-B has 50% ionic character if it is greater than 1.7, then the bond A-B has more than 50% ionic character, and if it is lesser than 1.7, then the bond A-B has less than 50% ionic character.

Bond Parameters

Partial Covalent Character in Ionic Bonds:

Like the partial ionic character in covalent compounds, ionic compounds show partial covalent character. For example, the ionic compound, lithium chloride shows covalent character and is soluble in organic solvents such as ethanol.

The partial covalent character in ionic compounds can be explained on the basis of a phenomenon called polarisation. We know that in an ionic compound, there is an electrostatic attractive force between the cation and anion. The positively charged cation attracts the valence electrons of anion while repelling the nucleus.

This causes a distortion in the electron cloud of the anion and its electron density drifts towards the cation, which results in some sharing of the valence electrons between these ions. This, a partial covalent character is developed between them. This phenomenon is called polarisation.

The ability of a cation to polarise an anion is called its polarising ability and the tendency of an anion to get polarised is called its polarisability. The extent of polarisation in an ionic compound is given by the Fajans rules.

Fajans Rules

1. To show greater covalent character, both the cation and anion should have high charge on them. Higher the positive charge on the cation, greater will be the attraction on the electron cloud of the anion. Similarly higher the magnitude of negative charge on the anion, greater is its polarisability. Hence, the increase in charge on cation or in anion increases the covalent character.

Let us consider three ionic compounds aluminum chloride, magnesium chloride and sodium chloride. Since the charge of the cation increase in the order Na+ < Mg2+ < Al3+, the covalent character also follows the same order NaCl < MgCl2 < AlCl3.

2. The smaller cation and larger anion show greater covalent character due to the greater extent of polarisation. Lithium chloride is more covalent than sodium chloride. The size of Li+ is smaller than Na+ and hence the polarising power of Li+ is more. Lithium iodide is more covalent than lithium chloride as the size of I is larger than the Cl. Hence I will be more polarised than Cl by the cation, Li+.

3. Cations having ns2 np6 nd10 configuration show greater polarising power than the cations with ns2 np6 configuration. Hence, they show greater covalent character.

CuCl is more covalent than NaCl. Compared to Na+ (1.13 Å). Cu+ (0.6 Å) is small and have 3s2 3p6 3d10 configuration.

Electronic confiuration of Cu+
[Ar] 3d10
Electronic Confiuration of Na+
[He] 2s2, 2p6

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Co-ordinate Bond – Definition, Examples, Formation

Co-ordinate Bond – Definition, Examples, Formation

In the formation of a covalent bond, both the combining atoms contribute one electron each and the these electrons are mutually shared among them. However, in certain bond formation, one of the combining atoms donates a pair of electrons i.e. two electrons which are necessary for the covalent bond formation, and these electrons are shared by both the combining atoms.

These type of bonds are called coordinate covalent bond or coordinate bond. The combining atom which donates the pair of electron is called a donor atom and the other atom an acceptor atom. This bond is denoted by an arrow starting from the donor atom pointing towards the acceptor atom. (Later in coordination compound, we will refer the donor atom as ligand and the acceptor atom as central-metal atom/ion.

Coordinate Covalent Bond

For Example, in ferrocyanide ion [Fe(CN)6]4-, each cyanide ion (CN) donates a pair of electrons to form a coordinate bond with iron (Fe2+) and these electrons are shared by Fe2+ and CN.

Coordinate Covalent Bond img 1

In certain cases, molecules having a lone pair of electrons such as ammonia donates its pair to an electron deficient molecules such as BF3 to form a coordinate.

Coordinate Covalent Bond img 2

In a coordinate covalent bond, one element transfer the electron pair to another element to make a bond. It is represented by the ‘→’ symbol. The head of the arrow represents the acceptor species and tail of the arrow represents the donor species. Example H3N : + H + → [H3N → H]+

Coordinate Covalent Bond

Coordinate Covalent Bond:

A covalent bond in which one of the atoms contributes both of the electrons in the shared pair.

A coordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom. A covalent bond is formed by two atoms sharing a pair of electrons. The atoms are held together because the electron pair is attracted by both of the nuclei.

Coordinate covalent bonds have one species donate both electrons to the forming the bond while usually covalent bonds have one electron come from each atom.

That is why Al is a good conductor of electricity. It transfers the three outermost electrons very easily to strongly electronegative elements like halogens. However, AlCl3 is not totally ionic, the bonds between Al and Cl are coordinate bond and it has trigonal planar structure (gas phase).

A coordinate covalent bond, also known as a dative bond, dipolar bond, or coordinate bond is a kind of two center, two-electron covalent bond in which the two electrons derive from the same atom. The bonding of metal ions to ligands involves this kind of interaction. This type of interaction is central to Lewis theory.

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Methods of Purification of Organic Compounds

Methods of Purification of Organic Compounds

Need for Purification:

In order to study the structure, physical properties, chemical properties and biological properties of organic compounds they must be in the pure state. There are several methods by which organic compounds can be purified. The methods employed for purification depend upon the nature of impurity and the nature of organic compound. The most widely used technique for the separation and purification of organic compounds are:

  • Crystallisation,
  • Sublimation
  • Distillation
  • Fractional Distillation
  • Steam Distillation
  • Azeotropic Distillation
  • Diffrential Extraction and
  • Chromatography

Purification of Organic Compounds img 1

Sublimation:

Few substances like benzoic acid, naphthalene and camphor when heated pass directly from solid to vapor without melting (ie liquid). On cooling the vapours will give back solids. Such phenomenon is called sublimation. It is a useful technique to separate volatile and non-volatile solid. It has limited application because only a few substance will sublime.

Substances to be purified is taken in a beaker. It is covered with a watch glass. The beaker is heated for a while and the resulting vapours condense on the bottom of the watch glass. Then the watch glass is removed and the crystals are collected. This method is applicable for organic substance which has high vapour pressure at temperature below their melting point.

Substances like naphthalene, benzoic acid can be sublimed quickly. Substance which has very small vapour pressure will decompose upon heating are purified by sublimation under reduced pressure. This apparatus consists of large heating and large cooling surface with small distance in between because the amount of the substance in the vapour phase is much too small in case of a substance with low vapour pressure.

Purification of Organic Compounds

Crystallization:

It is the most widely used method for the purification of solid organic compound. This process is carried out in by the following step

(i) Selection of Solvent:

Most of the organic substances being covalent do not dissolve in polar solvents like water, hence selection of solvent (suitable) becomes necessary. Hence the powdered organic substance is taken in a test tube and the solvent is added little by little with constant stirring and heating, till the amount added is just sufficient to dissolve the solute (ie) organic compound.

If the solid dissolves upon heating and throws out maximum crystals on cooling, then the solvent is suitable. This process is repeated with other solvents like benzene, ether, acetone and alcohol till the most suitably one is sorted out.

(ii) Preparation of Solution:

The organic substance is dissolved in a minimum quantity of suitable solvent. Small amount of animal charcoal can be added to decolorize any colored substance. The heating may be done over a wire gauze or water bath depending upon the nature of liquid (ie) whether the solvent is low boiling or high boiling.

Purification of Organic Compounds

(iii) Filtration of Hot Solution:

The hot solution so obtained is filtered through a fluted filter paper placed in a funnel.

(iv) Crystallization:

The hot filtrate is then allowed to cool. Most of the impurities are removed on the filter paper, the pure solid substance separate as crystal. When copious amount of crystal has been obtained, then the crystallization is complete. If the rate of crystallization is slow, it is induced either by scratching the walls of the beaker with a glass rod or by adding a few crystals of the pure compounds to the solution.

(v) Isolation and Drying of Crystals:

The crystals are separated from the mother liquor by fitration. Filtration is done under reduced pressure using a Bucher funnel. When the whole of the mother liquor has been drained into the filtration flask, the crystals are washed with small quantities of the pure cold solvent and then dried.

Distillation:

This method is to purify liquids from non-volatile impurities, and used for separating the constituents of a liquid mixture which differ in their boiling points. There are various methods of distillation depending upon the diffrence in the boiling points of the constituents. The methods are

  • Simple Distillation
  • Fractional Distillation and
  • Steam Distillation.

The process of distillation involves the impure liquid when boiled gives out vapour and the vapour so formed is collected and condensed to give back the pure liquid in the receiver. This method is called simple distillation. Liquids with large difference in boiling point (about 40k) and do not decompose under ordinary pressure can be purified by simply distillation eg. The mixture of C6H5NO2 (b.p 484k) & C6H6 (354k) and mixture of diethyl ether (b.p 308k) and ethyl alcohol (b.p 351k).

Purification of Organic Compounds

Fractional Distillation:

This is one method to purify and separate liquids present in the mixture having their boiling point close to each other. In the fractional distillation, a fractionating column is fitted with distillation flask and a condenser. A thermometer is fited in the fractionating column near the mouth of the condenser.

The process of separation of the components in a liquid mixture at their respective boiling points in the form of vapours and the subsequent condensation of those vapours is called fractional distillation. The process of fractional distillation is repeated, if necessary. This method finds a remarkable application in distillation of petroleum, coal-tar and crude oil.

Steam Distillation:

This method is applicable for solids and liquids. If the compound to be steam distilled the it should not decompose at the steam temperature, should have a fairly high vapour pressure at 373k, it should be insoluble in water and the impurities present should be non-volatile.

The impure liquid along with little water is taken in a round-bottom flask which is connected to a boiler on one side and water condenser on the other side, the flask is kept in a slanting position so that no droplets of the mixture will enter into the condenser on the brisk boiling and bubbling of steam.

The mixture in the flask is heated and then a current of steam passed in to it. The vapours of the compound mix up with steam and escape into the condenser. The condensate obtained is a mixture of water and organic compound which can be separated. This method is used to recover essential oils from plants and flowers, also in the manufacture of aniline and turpentine oil. (see Fig. 11.4)

Purification of Organic Compounds img 2

Azeotropic Distillation

These are the mixture of liquids that cannot be separated by fractional distillation. The mixtures that can be purified only by azeotropic distillation are called as azeotropes. These azeotropes are constant boiling mixtures, which distilled as a single component at a fixed temperature. For example ethanol and water in the ratio of 95.87:4.13.

In this method the presence of a third component like C6H6, CCl4, ether, glycerol, glycol which act as a dehydrating agent depress the partial pressure of one component of azeotropic mixture and raises the boiling point of that component and thus other component will distill over.

Subtances like C6H6, CCl4 have low boiling points and reduce the partial vapour pressure of alcohol more than that of water while subtances like glycerol & glycol etc have high boiling point and reduce the partial vapour pressure of water more than that of alcohol.

Purification of Organic Compounds

Differential Extraction:

The process of removing a substance from its aqueous solution by shaking with a suitable organic solvent is termed extraction. When an organic substance present as solution in water can be recovered from the solution by means of a separating funnel.

The aqueous solution is taken in a separating funnel with little quantity of ether or chloroform (CHCl3). The organic solvent immiscible with water will form a separate layer and the contents are shaken gently. The solute being more soluble in the organic solvent is transfered to it. The solvent layer is then separated by opening the tap of the separating funnel, and the substance is recovered.

Chromatography:

The most valuable method for the separation and purification of small quantity of mixtures. As name implies chroma-colour and graphed writing it was first applied to separation of different colored constituents of chlorophyll in 1906 by M.S Tswett, a Russian botanist.

He achieved it by passing a petroleum ether solution of chlorophyll present in leaves through a column of CaCO3 firmly packed into a narrow glass tube. Different components of the pigments got separated into land or zones of different colors and now this technique is equally well applied to separation of colorless substances.

The principle behind chromatography is selective distribution of the mixture of organic substances between two phases – a stationary phase and a moving phase. The stationary phase can be a solid or liquid, while the moving phase is a liquid or a gas.

When the stationary phase is a solid, the moving phase is a liquid or a gas. If the stationary phase is solid, the basis is adsorption, and when it is a liquid, the basis is partition. So the Chromatography is defined as a technique for the separation of a mixture brought about by differential movement of the individual compound through porous medium under the influence of moving solvent. The various methods of chromatography are

  • Column chromatography (CC)
  • Then layer chromatography (TLC)
  • Paper chromatography (PC)
  • Gas-liquid chromatography (GLC)
  • Ion-exchange chromatography

Purification of Organic Compounds

Adsorption Chromatography:

The principle involved is different compounds are adsorbed on an adsorbent to different degree. Silica gel and alumina are the commonly used adsorbent. The components of the mixture move by varying distances over the stationary phase Column chromatography and thin layer chromatography are the techniques based on the principle of differential adsorption.

Column Chromatography:

This is the simplest chromatographic method carried out in long glass column having a stop cock near the lower end. This method involves separation of a mixture over a column of adsorbent (Stationery phase) packed in a column. In the column a plug of cotton or glass wool is placed at the lower end of the column to support the adsorbent powder. The tube is uniformely packed with suitable absorbent constitute the stationary phase. (Activated aluminum oxides (alumina), Magnesium oxide, starch are also used as absorbents).

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The mixture to be separated is placed on the top of the adsorbent column. Eluent which is a liquid or a mixture of liquids is allowed to flow down the column slowly. Different components are eluted depending upon the degree to which the components are adsorbed and complete separation takes place. The most readily adsorbed substances are retained near the top and others come down to various distances in the column.

Purification of Organic Compounds

Thin layer Chromatography:

This method is an another type of adsorption chromatography with this method it is possible to separate even minute quantities of mixtures. A sheet of a glass is coated with a thin layer of adsorbent (cellulose, silica gel or alumina). This sheet of glass is called chromoplate or thin layer chromatography plate.

After drying the plate, a drop of the mixture is placed just above one edge and the plate is then placed in a closed jar containing eluent (solvent). The eluent is drawn up the adsorbent layer by capillary action. The components of the mixture move up along with the eluent to different distances depending upon their degree of adsorption of each component of the mixture. It is expressed in terms of its retention factor (ie) Rf value

Purification of Organic Compounds img 4

The spots of colored compounds are visible on TLC plate due to their original color. The colorless compounds are viewed under uv light or in another method using iodine crystals or by using appropriate reagent.

Partition Chromatography:

Paper chromatography (PC) is an example of partition chromatography. The same procedure is followed as in thin layer chromatography except that a strip of paper acts as an adsorbent. This method involves continues differential portioning of components of a mixture between stationary and mobile phase. In paper chromatography, a special quality paper known as chromatography paper is used. This paper act as a stationary phase.

A strip of chromatographic paper spotted at the base with the solution of the mixture is suspended in a suitable solvent which act as the mobile phase. The solvent rises up and flows over the spot. The paper selectively retains different components according to their different partition in the two phases where a chromatogram is developed.

The spots of the separated colored compounds are visible at different heights from the position of initial spots on the chromatogram. The spots of the separated colorless compounds may be observed either under ultraviolent light or by the use of an appropriate spray reagent.

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IUPAC Nomenclature Of Organic Compounds

IUPAC Nomenclature Of Organic Compounds

The International Union of Pure and Applied Chemistry (IUPAC) is the world authority on chemical nomenclature and terminology, naming of new elements in the periodic table standardized methods for measurement; atomic weights, and many other critically-evaluated data. According to IUPAC recommendations to name any organic compound, it is considered as a derivative of its parent saturated hydrocarbon. The IUPAC name of an organic compound consists of three parts.

prefix + root word + suffix

Root word denotes the number of carbon atoms in the longest continiuous chain in molecules. Prefix denotes the group(s) attached to the main chain which is placed before the root. Suffix denotes the funtional group and is placed after the root word.

Number of carbons in parent chain and the corresponding root words

Nomenclature of Organic Compounds img 1
Nomenclature of Organic Compounds img 2

Nomenclature of Organic Compounds:

Suffix:

There are two types of suffix. They are primary suffix and secondary suffix

Primary Suffix:

It denotes the saturation/unsaturation of organic compounds. It is added immediately after the root word. Primary suffix for various saturated and unsaturated carbon chains are as follows:

Nomenclature of Organic Compounds img 3

Secondary Suffix:

It is used to denote the nature of functional group present in the organic compound. It is added to the primary suffix by removig its terminal ‘e’. Secondary suffix names for some functional groups is listed below in table 11.4

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Nomenclature of Organic Compounds:

Prefix:

Substituents that are attached to the parent carbon chain are denoted by adding prefix names before the root word. The prefix names for some common substituents are listed below. If the functional groups are not part of the parent chain, they are considered as substituents. In such cases its prefix name is added before the root word. Prefix names for some functional groups mentioned along with their secondary prefix are listed in table 11.4

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IUPAC rules for nomenclature of organic compounds

The following steps should be followed for naming an organic compound as per IUPAC nomenclature.

  1. Choose the longest carbon chain. (Root word). Consider all the other groups attached to this chain as substitutents.
  2. Numbering of the longest carbon chain
  3. Naming of the substituents (prefies or suffixs)
  4. Arrange the substitutents in the alphabetical order
  5. Write the name of the compound as below

“prefix + root word + primary suffix + secondary suffix

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The following are guide lines for writing IUPAC of the organic compound.

1. The IUPAC names are always written as single word, with notable exception of organic salts, acids and acid derivatives.

2. Commas are used between two adjacent number or letter symbols, and hypens are used to separate numbers and letter symbol in names Eg: 2, 2-Dimethyl-3-hexene N, N-Dimethyl methanamide

3. Structural prefix such as, meso-, cis-, trans-, are italicised and joined to the name by a hypen. These prefixes are omitted in alphabetising compound names or in capitalising names at the beginning of a sentence.Eg: trans-2-Butene

4 .Structural prefixes such as di, tri, tetra are treated as a part of the basic name and therefore are neither italicised nor separated by a hypen. These prefixes are not taken into account in alphabetising compound names eg: 4-Ethyl -2,2-dimethyl hexane.

5. To name alicyclic compounds , the additional rules should be followed as illustrated in the table 11.6

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Nomenclature of Organic Compounds:

NOMENCLATURE OF AROMATIC COMPOUNDS:

An aromatic compound consists of two parts nucleus and side chain

(A) Nucleus:
The benzene ring present in aromatic compound is called nucleus. It is represented as follows

Nomenclature of Organic Compounds img 13

(B) Side chain:
Alkyl or any other aliphatic group attached to the benzene nucleus by replacing one or more hydrogen atom is called the side chain.

Nomenclature of Organic Compounds img 14

If one hydrogen atom, (or) two hydrogen atoms or three hydrogen atoms are replaced in the benzene ring by some other groups, they are termed as mono substituted, di substituted or tri substituted derivative respectively.

Example

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If more than one hydrogen atom of benzene ring is replaced by some other atom or group, then their position is mentioned by Arabic numerals 1,2,3 ….. In case of disubstitution, respective position of two groups can also be mentioned as follows.

ortho – adjacent; represented as – o
meta – alternate; represented as – m
Para – opposite; represented as – p

Aromatic compounds are basically of two types:

Nomenclature of Organic Compounds:

1. Nuclear substituted aromatic compounds:

These are the compounds in which the functional group is directly attached to the benzene ring. They are named as derivatives of benzene.

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Nuclear Substituaed Aramatic Halogen Derivatives Compounds.

Nomenclature of Organic Compounds img 17

2. Side chain substituted aromatic compounds:

These are the compounds in which the functional group is present in the side chain of the benzene ring. These are named as phenyl derivatives of the corresponding aliphatic compounds.

Nomenclature of Organic Compounds:

Side Chain Substituted

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Aryl Groups

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Selection of parent hydrocarbon out of side chain and benzene ring is based on (more or less) some rule as for the alicyclic compounds.

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Definition, Examples & Classification Of Organic Compounds

Definition, Examples & Classification Of Organic Compounds

The existing large number of organic compounds and ever-increasing number have made it necessary to classify them. They may be classified based on their structure or the functional group.

Classification of Organic Compounds

Classification based on the structure:

Classification of Organic Compounds img 1

Based on the above classification let us classify the following compounds.

1. Classify the following compounds based on the structure

Classification of Organic Compounds img 2

Solutions:

(i) Unsaturated open chain compound
(ii) Saturated open chain compound
(iii) Aromatic benzenoid compound
(iv) Alicyclic compound

Classification of Organic Compounds

Classification based on functional groups:

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Classification of Organic Compounds img 3a

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Molecular Orbital Theory – Detailed Explanation with Illustrations

Molecular Orbital Theory – Detailed Explanation with Illustrations

Lewis concept and valence bond theory qualitatively explains the chemical bonding and molecular structure. Both approaches are inadequate to describe some of the observed properties of molecules. For example, these theories predict that oxygen is diamagnetic.

However, it was observed that oxygen in liquid form was attracted towards the poles of strong magnet, indicating that oxygen is paramagnetic. As both these theories treated the bond formation in terms of electron pairs and hence they fail to explain the bonding nature of paramagnetic molecules. F. Hund and Robert. S. Mulliken developed a bonding theory called molecular orbital theory which explains the magnetic behaviour of molecules.

Molecular Orbital Theory

The salient features of Molecular orbital Theory (MOT):

1. When atoms combines to form molecules, their individual atomic orbitals lose their identity and forms new orbitals called molecular orbitals.

2. The shapes of molecular orbitals depend upon the shapes of combining atomic orbitals.

3. The number of molecular orbitals formed is the same as the number of combining atomic orbitals. Half the number of molecular orbitals formed will have lower energy than the corresponding atomic orbital, while the remaining molecular orbitals will have higher energy.

The molecular orbital with lower energy is called bonding molecular orbital and the one with higher energy is called anti-bonding molecular orbital. The bonding molecular orbitals are represented as σ (Sigma), π (pi), δ (delta) and the corresponding antibonding orbitals are denoted as σ*, π* and δ*.

4. The electrons in a molecule are accommodated in the newly formed molecular orbitals. The filling of electrons in these orbitals follows Aufbu’s principle, Pauli’s exclusion principle and Hund’s rule as in the case of filling of electrons in atomic orbitals.

5. Bond order gives the number of covalent bonds between the two combining atoms. The bond order of a molecule can be calculated using the following equation

Bond Order = \(\frac{\mathrm{N}_{\mathrm{b}}-\mathrm{N}_{\mathrm{a}}}{2}\)

Where,
Nb = Total number of electrons present in the bonding molecular orbitals
Na = Total number of electrons present in the antibonding molecular orbitals and

A bond order of zero value indicates that the molecule doesn’t exist.

Molecular Orbital Theory

Linear Combination of Atomic Orbitals

The wave functions for the molecular orbitals can be obtained by solving Schrodinger wave equation for the molecule. Since solving the Schrodinger equation is too complex, approximation methods are used to obtain the wave function for molecular orbitals. The most common method is the linear combination of atomic orbitals (LCAO).

We know that the atomic orbitals are represented by the wave function Ψ. Let us consider two atomic orbitals represented by the wave function ψA and ψB with comparable energy, combines to form two molecular orbitals. One is bonding molecular orbital(ψbonding) and the other is antibonding molecular orbital (ψantibonding).

The wave functions for these two molecular orbitals can be obtained by the linear combination of the atomic orbitals ψA and ψB as below.

ψbonding = ψA + ψB
ψantibonding = ψA – ψB

The formation of bonding molecular orbital can be considered as the result of constructive interference of the atomic orbitals and the formation of anti-bonding molecular orbital can be the result of the destructive interference of the atomic orbitals. The formation of the two molecular orbitals from two 1s orbitals is shown below.

Molecular Orbital Theory

Constructive Interaction:

The two 1s orbitals are in phase and have the same sign.

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Molecular Orbital Theory

Metallic Bonding

Metals have some special properties of lustre, high density, high electrical and thermal conductivity, malleability and ductility, and high melting and boiling points. The forces that keep the atoms of the metal so closely in a metallic crystal constitute what is generally known as the metallic bond. The metallic bond is not just an electrovalent bond(ionic bond), as the latter is formed between atoms of different electro negativities.

Similarly, the metallic bond is not a covalent bond, as the metal atoms do not have sufficient number of valence electrons for mutual sharing with 8 or 12 neighboring metal atoms in a crystal. So, we have to search for a new theory to explain metallic bond. The first successful theory is due to Drude and Lorentz, which regards metallic crystal as an assemblage of positive ions immersed in a gas of free electrons.

The free electrons are due to ionization of the valence electrons of the atoms of the metal. As the valence electrons of the atoms are freely shared by all the ions in the crystal, the metallic bonding is also referred to as electronic bonding. As the free electrons repel each other, they are uniformely distributed around the metal ions. Many physical properties of the metals can be explained by this theory, nevertheless there are exceptions.

The electrostatic attraction between the metal ions and the free electrons yields a three-dimensional close packed crystal with a large number of nearest metal ions. So, metals have high density. As the close packed structure contains many slip planes along which movement can occur during mechanical loading, the metal acquires ductility. Pure metals can undergo 40 to 60% elongation prior to rupturing under mechanical loading.

Molecular Orbital Theory

As each metal ion is surrounded by electron cloud in all directions, the metallic bonding has no directional properties. As the electrons are free to move around the positive ions, the metals exhibit high electrical and thermal conductivity. The metallic luster is due to reflection of light by the electron cloud. As the metallic bond is strong enough, the metal atoms are reluctant to break apart into a liquid or gas, so the metals have high melting and boiling points.

The bonding in metal is better treated by Molecular orbital theory. As per this theory, the atomic orbitals of large number of atoms in a crystal overlap to form numerous bonding and antibonding molecular orbitals without any band gap. The bonding molecular orbitals are completely filled with an electron pair in each, and the antibonding molecular orbitals are empty. Absence of band gap accounts for high electrical conductivity of metals.

High thermal conductivity is due to thermal excitation of many electrons from the valence band to the conductance band. With an increase in temperature, the electrical conductivity decreases due to vigorous thermal motion of lattice ions that disrupts the uniform lattice structure, that is required for free motion of electrons within the crystal. Most metals are black except copper, silver and gold. It is due to absorption of light of all wavelengths. Absorption of light of all wavelengths is due to absence of bandgap in metals.

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Hybridisation – Definition, Types, Rules, Examples

Hybridisation – Definition, Types, Rules, Examples

Bonding in simple molecules such as hydrogen and fluorine can easily be explainedon the basis of overlap of the respective atomic orbitals of the combining atoms. But the observed properties of polyatomic molecules such as methane, ammonia, berylliumchloride etc cannot be explained on the basis of simple overlap of atomic orbitals.

For example, it was experimentally proved that methane has a tetrahedral structure and therefore C-H bonds are equivalent. This fact cannot be explained on the basis of overlap of atomic orbitals of hydrogen (1s) and the atomic orbitals of carbon with different energies (2s2 2px2 2py2pz).

In order to explain these observed facts, Linus Pauling proposed that the valence atomic orbitals in the molecules are different from those in isolated atom and he introduced the concept of hybridisation.

Hybridisation is the process of mixing of atomic orbitals of the same atom with comparable energy to form equal number of new equivalent orbitals with same energy. The resultant orbitals are called hybridised orbitals and they posses maximum symmetry and definite orientation in space so as to minimize the force of repulsion between their electrons.

Hybridisation

Types of Hybridisation and Geometry of Molecules

sp Hybridisation:

Consider the bond formation in beryllium chloride. The ground state valence shell electronic confiuration of Beryllium atom is [He]2s2 2p0

Hybridisation img 1

In BeCl2 both the Be-Cl bonds are equivalent and it was observed that the molecule is linear. VB theory explain this observed behaviour by sp hybridisation. One of the paired electrons in the 2s orbital gets excited to 2p orbital and the electronic configuration at the excited state is shown.

Now, the 2s and 2p orbitals hybridise and produce two equivalent sp hybridised orbitals which have 50% s-character and 50% p-character. These sp hybridised orbitals are oriented in opposite direction as shown in the figure.

Hybridisation

Overlap with Orbital of Chlorine

Each of the sp hybridized orbitals linearly overlap with 3pz orbital of the chlorine to form a covalent bond between Be and Cl as shown in the Figure.

Hybridisation img 2

sp2 Hybridisation:

Consider the bond formation in boron triflouride. The ground state valence shell electronic confiuration of Boron atom is [He]2s2 2p1.

Hybridisation img 3

In the ground state boron has only one unpaired electron in the valence shell. In order to form three covalent bonds with flourine atoms, three unpaired electrons are required. To achieve this, one of the paired electrons in the 2s orbital is promoted to the 2py orbital in the excite state.

In boron, the s orbital and two p orbitals (px and py) in the valence shell hybridses, to generate three equivalent sp2 orbitals as shown in the Figure. These three orbitals lie in the same xy plane and the angle between any two orbitals is equal to 120°.

Hybridisation

Overlap with 2pz Orbitals of Flourine:

The three sp2 hybridised orbitals of boron now overlap with the 2pz orbitals of flourine (3 atoms). This overlap takes place along the axis as shown below.

Hybridisation img 4

sp3 Hybridisation:

sp3 hybridisation can be explained by considering methane as an example. In methane molecule the central carbon atom bound to four hydrogen atoms. The ground state valence shell electronic configuration of carbon is [He]2s2 2px1 2py12pz0.

In order to form four covalent bonds with the four hydrogen atoms, one of the paired electrons in the 2s orbital of carbon is promoted to its 2pz orbital in the excite state.

The one 2s orbital and the three 2p orbitals of carbon mixes to give four equivalent sp3 hybridised orbitals. The angle between any two sp3 hybridised orbitals is 109°28′.

Hybridisation

Overlap with 1s Orbitals of Hydrogen:

The 1s orbitals of the four hydrogen atoms overlap linearly with the four sp3 hybridised orbitals of carbon to form four C-H σ-bonds in the methane molecule, as shown below.

Hybridisation img 5

Hybridisation img 6

sp3d Hybridisation:

In the molecules such as PCl3, the central atom phosphorus is covalently bound to five chlorine atoms. Here the atomic orbitals of phosphorous undergoes sp3d hybridisation which involves its one 3s orbital, three 3p orbitals and one vacant 3d orbital (dz2). The ground state electronic configuration of phosphorous is [Ne]3s2 3px1 3py13pz1 as shown below.

Hybridisation img 7

One of the paired electrons in the 3s orbital of phosphorous is promoted to one of its vacant 3d orbital (dz2) in the excite state.

One 3s orbital, three 3p orbitals and one 3dz2 orbital of phosphorus atom mixes to give five equivalent sp3d hybridised orbitals. The orbital geometry of sp3d hybridised orbitals is trigonal bi-pyramidal as shown in the figure 10.25.

Hybridisation img 8

Hybridisation

Overlap with 3pz Orbitals of Chlorine:

The 3pz orbitals of the five chlorine atoms linearly overlap along the axis with the five sp3d hybridised orbitals of phosphorous to form the five P-Cl σ-bonds, as shown below.

sp3d2 Hybridisation:

In sulphur hexafloride (SF6) the central atom sulphur extend its octet to undergo sp3d2 hybridisation to generate six sp3d2 hybridised orbitals which accounts for six equivalent S-F bonds. The ground state electronic configuration of sulphur is [Ne]3s2 3px23py13pz1.

Hybridisation img 9

One electron each from 3s orbital and 3p orbital of sulphur is promoted to its two vacant 3d orbitals (dz2 and dx2-y2) in the excite state. A total of six valence orbitals from sulphur (one 3s orbital, three 3p orbitals and two 3d orbitals) mixes to give six equivalent sp3d2 hybridised orbitals. The orbital geometry is octahedral as shown in the figure.

Overlap with 2pz Orbitals of Flourine:

The six sp3d2 hybridised orbitals of sulphur overlaps linearly with 2pz orbitals of six flourine atoms to form the six S-F bonds in the sulphur hexaflouride molecule.

Hybridisation img 10

Hybridisation

Bonding in Ethylene:

The bonding in ethylene can be explained using hybridisation concept. The molecular formula of ethylene is C2H4. The valency of carbon is 4. The electronic configuration of valence shell of carbon in ground state is [He]2s22px12py12pz0. To satisfy the valency of carbon promote an electron from 2s orbital to 2pz orbital in the excited state.

Hybridisation img 11

In ethylene both the carbon atoms undergoes sp2 hybridisation involving 2s, 2px and 2py orbitals, resulting in three equivalent sp2 hybridised orbitals lying in the xy plane at an angle of 120° to each other. The unhybridised 2pz orbital lies perpendicular to the xy plane.

Formation of Sigma Bond:

One of the sp2 hybridised orbitals of each carbon lying on the molecular axis (x-axis) linearly overlaps with each other resulting in the formation a C-C sigma bond. Other two sp2 hybridised orbitals of both carbons linearly overlap with the four 1s orbitals of four hydrogen atoms leading to the formation of two C-H sigma bonds on each carbon.

Hybridisation

Formation of Pi (π) bond:

The unhybridised 2pz orbital of both carbon atoms can overlap only sideways as they are not in the molecular axis. This lateral overlap results in the formation a pi(π) bond between the two carbon atoms as shown in the figure.

Hybridisation img 12

Bonding in Acetylene:

Similar to ethylene, the bonding in acetylene can also be explained using hybridisation concept. The molecular formula of acetylene is C2H2. The electronic configuration of valence shell of carbon in ground state is [He]2s22px12py12pz0. To satisfy the valency of carbon promote an electron from 2s orbital to 2pz orbital in the excited state.

In acetylene molecule, both the carbon atoms are in sp hybridised state. The 2s and 2px orbitals, resulting in two equivalent sp hybridised orbitals lying in a straight line along the molecular axis (x-axis). The unhybridised 2py and 2pz orbitals lie perpendicular to the molecular axis.

Hybridisation

Formation of Sigma Bond:

One of the two sp hybridised orbitals of each carbon linearly overlaps with each other resulting in the formation a C-C sigma bond. The other sp hybridised orbital of both carbons linearly overlap with the two 1s orbitals of two hydrogen atoms leading to the formation of one C-H sigma bonds on each carbon.

Formation of pi Bond:

The unhybridised 2py and 2pz orbitals of each carbon overlap sideways. This lateral overlap results in the formation of two pi bonds (py-py) between the two carbon atoms as shown in the figure.

Hybridisation img 13

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