Question

Why aldehydes cannot exhibit position isomerism?

Short answer

Answer: Aldehydes cannot exhibit position isomerism because their carbonyl functional group (C=O) is always present at the end of the carbon chain, and cannot have multiple positions within the structure.

Step-by-step solution

Definition of Position Isomerism

Position isomerism is a type of structural isomerism in which the compounds have the same molecular formula but differ in the position of their functional groups on the carbon chain.

Structure of Aldehydes

An aldehyde is an organic compound containing a carbonyl functional group (C=O) bonded to a hydrogen atom and an alkyl or aryl group. The general formula of an aldehyde is R-CHO, where R represents any carbon-containing chain. Aldehydes with one carbon atom are known as 'formaldehyde' (HCHO). For aldehydes with more carbon atoms, the carbonyl group is always bonded to the first carbon atom in the chain as aldehydes cannot have a functional group bonded to an internal carbon atom.

Functional Group Position in Aldehydes

In the case of aldehydes, the functional group (carbonyl group) is always present at the end of the carbon chain. That means the carbonyl group does not have two alkyl or aryl groups attached to it. This is the main reason why aldehydes cannot exhibit position isomerism; they cannot have multiple positions for the carbonyl group in the structure.

Comparing Aldehydes and Ketones

On the other hand, ketones, which have a similar structure to aldehydes, can exhibit position isomerism. Ketones have a carbonyl group (C=O) bonded to two alkyl or aryl groups. The general formula of a ketone is R-C(O)-R', where R and R' represent any carbon-containing chain. Because of the presence of two carbon chains on both sides of the carbonyl group in ketones, they can exhibit position isomerism by changing the position of the carbonyl group within the carbon chain. In conclusion, aldehydes cannot exhibit position isomerism due to their structural constraints and the location of their functional group at the end of the carbon chain.