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Thursday, 6 December 2012

CONCEPT OF HYBRIDIZATION AND ISOMERISM : Two Most Important Chracteristics To Form Bondings between Elements .

                                                                        2     2     2     
Carbon (atomic number 6 and electronic configuration 1s 2s 2p ) forms a large number of compounds . From its electronic configuration , carbon is expected to be bivalent,but in all its compounds, it exibits tetravalency .The bonding between a carbon atom and another atom may involve pure p orbital or a hybrid orbital.The latter is obtained as a result of mixing of two or more pure atomic orbitals centered on the same atom.This process is known as hybridization. The number of the hybrid orbitals obtained is equal to the number of atomic orbitals mixed and these are completely identical in size, shape, orientations, and energies.

Carbon in its compound exibits three type of hybridization described in the following.

Acetylene : sp Bond

1) sp Hybridization:

This involves mixing of of 2s and one of the 2p orbitals to provide two sp hybrid orbitals oriented at an angle of 180 degree with respect to each other.The hybridization is also known as diagonal or linear hybridization.One of the example  is structure of Acetylene 


Ethylene : Sp2 Bond

2) sp Hybridization:

This involves mixing of 2s and two 2p orbital to provide three sp hybrid orbitals oriented at angle of 120 degree with respect to each other and point toward the three corners of an equlateral triangle.This hybridization is also known as trigonal hybridization . One of the common examples is Ethylene (CH2CH2)

Methane : Sp3 Bond

3) sp Hybridization:

This involves mixing of 2s and three 2p orbital to provide 4 sp hybrid orbitals oriented at angle of 109degree 28` with respect to each other and point towards the 4 corners of a regular tetrahedron .This hybridization is also known as tetrahedral hybridization .One of the common examples is Methane
                                 2          3
Comparison of sp, sp and sp hybrid orbitals of carbon:

The percent of s (or p) orbital in a hybrid orbital is given as

                                                               Number of s (or p)orbitals mixed
Percent of s (or p) contribution = ----------------------------------------------------     
                                                               Total number of orbitals mixed

Thus we have

sp orbitals: 50% contribution from each of s and p orbitals.
sp orbitals: 33.33% contribution from s orbital and 66.67% from p orbitals.
sp orbitals: 25% contribution from s orbital and 75% from p orbitals.

since s orbital in an atom is more near to the nucleus as compared to p orbital , we may expect the following relative characteristics of hybrid orbitals.

1) Size of orbitals: size of hybrid orbital increases with decrease of s (or p) contribution in it. Hence the relative sizes of hybrid orbitals follow the order.
                                   sp3 > sp2 > sp

2) Electronegetivity of orbitals: The electronegativity of an orbital increases with increase (or decrease) of s (or p) contribution in it. Hence, the electronegativity of hybrid orbitals follows the order
  sp > sp2 > sp3

Pi and Sigma Bonds

Sigma and Pi Bond:

The covalent bond between two atoms is formed by overlapping of two atomic orbitals-one from each atom. Head-to-head overlap of atomic orbitals results into the formation of pi(p) bond.The overlapping of two atomic orbitals between the two atoms produces a new orbital, known as localized molecular orbital, which embraces both atoms.The overlapping involving the same signof orbitals results into the formation of bonding molecular orbital as it creates more electron density between the atoms.The overlapping involving the different sign results into the formation of anti bonding molecular orbital as it creates a node (a point where electron density is zero) between the atoms.The bonding orbital has lower energy than the individual atomic orbitals whereas anti bonding molecular orbital has a higher energy.

The C-C in C2H6 is a s -bond ,  C-C bond in C2H4 involves one sigma  s -bond and one pi(p) bond, and C-C bond in C2H2 involves one sigma  s -bond and two pi(p) bond.

Different Types of Isomerism


In organic chemistry, two or more compounds may have same molecular formula but different structural formulae. Such compounds are known as isomers and this phenomenon is known as isomerism. Since the compounds are known as isomers and this phenomenon is known as isomerism.Since the compounds have different structural formulae, they differ in both physical and chemical properties.

There are two main types of isomerism.These are described in the following.

Constitutional isomerism. Both compounds have the molecular formula C4H10.

1) Structural Isomerism  This is due to the different arrangement atoms within the molecule.

2) Stereoisomerism: This is due to the different spatial arrangement of atoms or groups around a particular atom in the molecule.

The structural isomerism may be classified into five categories. These are:

a) Chain Isomerism: Differ in the order in which carbon atoms are linked in the molecule.

b) Position isomerism: Differ in placement of a functional group on  the carbon chain or placement of multiple bond.

c) Functional isomerism: Differ in functional groups.

d) Metamerism: Differ in the groups attached to a functional group.

e) Tautomerism: Migration of a proton from carbon atom to the adjacent atom; The two forma are in dynamic equlibrium with each other .One of the examples exibiting tautomerism is Ethyl acetoacetate:
                       O               O                                        OH           O
                       ||                ||                                           |              ||      
           CH3---C---CH2---C---OC2H5 <-------> CH3---C=CH---C---OC2H5
                           Keto Form                                                                    Enol Form

This compound shows the properties of both keto and enol groups. For example, it forms oxine with hydrooxylamine and phenylhydrazone with phenylhydrazone which are characteristics of a keto group; it also decolorises bromine water indicating unsaturation and also give violet colour with natural ferric chloride --- a test for enol group.

The Stereoisomerism has been classified into two main categories these are :


a) Geometric Isomerism: Due to the restricted motion within the molecule. For examples, 2-butene .
              H3C    CH3                                     H3C    H
                   |      |                                              |      |
                  C = C                                            C = C
                   |      |                                              |      |
                  H    H                                            H    CH3
             Cis-2-Butene                                                   Trans-2-butene

In cis form, similar groups are on the same side whereas in trans from, they are on the opposite side.

cis-1,2-dichloroethene (left); trans-1,2-dichloroethene (right)

b) Optical Isomerism:
Differ in the capacity to rotate the plane polarized light in the opposite directions.

The following terms are used to describe isomers.

1) Optically active molecule: A molecule that cannot be superimposed on its mirror image. It is also called chiral molecule.

2) Asymmetric or chiral carbon: A carbon atom that is bonded to four different groups.


3) Enantiomers: The optical isomers which are mirror images of each other.

4) Dextroratatory isomer or (+) isomer: An isomer which rotates the plane of polarized light to the right (clockwise direction).

5) Laevorotatory isomer
or (-) isomer: An isomer which rotates plane of polaeized light to the left (anticlockwise direction)

6) Racemic Mixture: An equimolar mixture of two isomers. Since the optical rotatory powers of two isomers are equal in magnitude but opposite in sign, theracemic mixture does not rotate the plane of polarized light.The two isomers in a racemic mixture cannot be separated by ordinary laboratory methods as the two isomers resemble each other closely in all properties except optical. They can however, be separated if they are made to combine another optically active compound.

7) Diastereomers: The optical isomers that are not mirror images.Diastereomers are observed if the molecule contains more than one chiral carbon  atom. The two isomers have the same configuration with respect to one chiral carbon atom and will have different configurations with respect to the rest of molecules. Taking the example of tartaric acid we have. 

8) Meso-Compound: A compound which has more than one chiral carbon atom and it is superimposable on its mirror image.In this compound, optical rotation caused by one chiral carbon is cancelled by that of the other, consequently the meso-compounds are optically inactive.The molecule of a meso-compound has a plane of symmetry dividing it midway between the two chiral atoms.

9) Racemisation: The phenomenon of conversion of one enantiomer into the other such that a mixture containing 50% of each is formed is known as racemisation.

10) Resolution: The separation of a racemic mixture into its two optically active components is known as resolution.


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