In this module, you will learn about the isomerism.
One of the most important characteristics of organic compounds is their ability to show isomerism.
What is isomerism?
Two or more compounds having the same molecular formula but different chemical and physical properties are called isomers and the phenomenon is known as isomerism.
Isomerism is of two main types. They are
1. Structural isomerism
2. Stero isomerism
Structural isomerism is a phenomenon in which the compounds have same molecular formula but possess different structures. That is, atoms or group of atoms within a molecule have different arrangement. These isomers are called as structural isomers.
Structural isomerism can be further classified as:
1. Skeletal or Chain or Nuclear isomerism,
2. Position isomerism,
3. Functional isomerism,
Let’s take a closer look at each type of structural isomerism.
In chain isomerism, the compounds having same molecular formula differ in the arrangement of carbon chain within the molecules. They are also called as skeletal isomerism or nuclear isomerism.
That is, if we observe the first three members such as methane, ethane, and propane of alkane family, we can find that they do exhibit chain isomerism as they hold only one structural formula.
Whereas if consider the next member of the alkane family, butane, we can observe that they have two structural isomers. They are Butane and 2-Methylpropane.
Similarly, if we consider, n-pentane, Isopentane, and Neopentane we can observe chain isomerism.
Some other examples of chain isomers are Butan-1-ol (n-Butyl alcohol), 2-Methylpropan-1-ol (Iso-butyl alcohol), But-1-ene (Butylene), and 2-Methylpropene (Iso-butylene).
Next, in position isomerism, the compounds having same molecular formula, differs in the position of functional group. That is, they vary in the carbon-carbon multiple bond or substituent group.
For example, if we consider this formula, C3H7OH, we can observe that it represents two position isomers such as Propan-1-ol (n-Propyl alcohol) and Propan-2-ol (Iso-propyl alcohol).
In certain aromatic hydrocarbons, we can observe the change in position of substituent groups attached to the benzene rings. That is, dichlorobenzene has three isomers namely, ortho-dicholorbenzene, meta-dichlorobenzene, and para-dicholorbenzene.
If we consider functional isomerism, we can observe that the compounds of same molecular formula differ in the functional groups and the phenomenon is known as functional isomerism.
We can observe that C2H6O having two isomers differ in the functional groups. That is, ethanol or ethyl alcohol falls under the OH functional group and methoxymethane or dimethyl ether falls under the function group–O-.
Similarly, C3H6O represents two isomers having the structural formulae, Propanal (Propionaldehyde) (Functional group -CHO) and Propanone (Acteone) (Functional group )
If we consider metamerism, the fourth classification of structural isomerism, we can observe that the compounds having the same molecular formula differ in the number of carbon atoms or alkyl groups on the either side of the functional group.
For example, the compound with the molecular formula of C5H10O has two metamers such as Pentan-3-one, Pentan-2-one and the compound with the molecular formula C4H10O has two metatmers such as 1-Methoxypropane (Methylpropyl ether) and Ethoxyethane (Diethyl ether).
The next classification of isomerism is stero isomerism.
Stero isomers are the isomers that have the same structural formula but differ in the relative arrangement of atoms or groups of atoms in the space. Stero isomers are classified as:
1. Geometrical isomerism and
2. Optical isomerism.
Geometrical isomerism are the most common stero isomers exhibited by alkanes. In this type of stero isomers, the position of the groups attached to the carbon is fixed in space due to restricted rotation around carbon-carbon double bond. The compounds containing double bond shows cis-trans isomerism.
We can observe, optical isomerism, when the compounds have the same molecular and structural formula, but one compound cannot be superimposed on the other compound. In simple terms, we can state that optical isomers are mirror images of each other.