LESSON 6: ISOMERISM

What is isomerism?

This is the occurrence of two or organic compound having the same molecular formula but different but different structural formula. E.g. C4H10 is the molecular formula of butane. But I can have two different structural formula for the same compound.

CH3-CH2-CH2-CH3 this is butane.

CH3-CH-(CH3)-CH3 this 2-methlybutane.

But compound are of the same formula (C4H10) but their structure is different. So they are isomers of each other and their existence is referred to as isomerism.

Types of isomerism

There are basically two types of isomerism which are:

1.      Structural or constitutional isomer

2.      Stereo isomers

   

1. Structural isomers: these are isomers that are differ from one another due to their due to their form of attachment. E.g. CH3CH2CH2CH3 can also be written as 

And if you look at the two structure, it is still C4H10.

Structural isomers can be further divided into four (4) which are:

a.       Branched chain isomers: they are isomers having the same number and kind of atoms but different structures due to the presence of branched chain. e.g. 2-methlypropane and butane. CH3-CH-(CH3)-CH3 and CH3-CH2-CH2-CH3, 2-methylbutane and pentane. CH3-CH-(CH3)-CH2-CH3 and CH3-CH2-CH2-CH2-CH3.

b.      Positional isomers: they are isomers with the same substituent on different carbon chain. e.g. 3-methylpentane and 2-methylpentane. CH3-CH2-CH(CH3)-CH2-CH3 and CH3-CH(CH3)-CH2-CH2-CH3

c.       Functional group isomers: the functional group isomers are different from one another as a result of placement of the functional on different carbon atom of compounds having the same number of carbon. E.g. propan-1-ol and propan-2-ol are both alcohol but the location of the hydroxyl group is on carbon 1and 2 respectively. CH2(OH)-CH2-CH3 and CH3-CH(OH)-CH3.

d.      Tautomerism: this is a state of equilibrium between two functional groups on the same compound. There are some compounds like aldehyde and ketone that when they undergo certain reaction, they bear two functional group at the same time, the equilibrium point of these functional group is called tautomerism. ENOL-KETO tautomerism. It will be discussed later under the chemistry of carbonyl compounds.

2.      Stereo isomerism or stereo isomers: this type of isomers occurs as a result of different spatial orientation of the compounds despite the fact that they are having the same molecular formula, their structure differs as a result of how the atoms are being orientated. E.g. cis and trans isomers (it will discussed below.)

There are two types of stereo isomers, which are

a.       Geometric isomers: this occurs in compounds with double bond. The double bond restricts free rotation within a molecule and fixes the positions in space of those atoms or group link to it.

The geometric isomers can generally be represented as follows.

The Cis – isomer contains like groups on the same side of the double bond while the trans – isomer has like groups on opposite sides of the double bond.  

b.      Optical isomers: these are isomers that just like mirror image of each other on a plane polarized light. E.g. see the picture below.