Question

Aldehydes are isomeric with
(A) Ketones
(B) Ethers
(C) Alcohols
(D) Fatty acids

Answer 1

Hint: All the given compounds in the question are organic compounds which differ in functional group. So, for these types of questions, the isomerism should be functional constitutional isomers i.e., the molecular formula is the same but the compounds differ in the functional group attached within the compound.

Complete Step by Step Solution:
Aldehydes: These are the organic compounds which consist of a carbon chain that may be aliphatic, aromatic or hydrogen atom bonded to a terminal carbon consisting of double bond oxygen and single bond hydrogen.
The structure of an aldehyde can be diagrammatically represented as follows:

Where R can be a hydrogen atom, aliphatic or aromatic ring.

Ketones: These are the organic compounds in which the carbon atom is bonded to oxygen via double bond and both the terminal bonds of the carbon atom are attached to a carbon chain that may be aromatic or aliphatic.
The structure of a ketone can be diagrammatically represented as follows:


Ethers: These compounds belong to the class of organic compounds in which two carbon chains are connected to each other via oxygen atoms. An ether is generally represented as ${\rm{R}} - {\rm{O}} - {\rm{R}}$

Alcohols: These compounds belong to the class of organic compounds in which a carbon chain (aliphatic or aromatic) is bonded to the hydroxide group. The general structure of an alcohol is ${\rm{R}} - {\rm{OH}}$.

Fatty acids: These compounds are the type of organic compounds which consist of a long aliphatic chain of carbon bonded to a carboxylic group. The general structure of a fatty acid is $C{H_3}{(C{H_2})_n}COOH$, where n is usually an even number and lies between $2 - 28$.

Among the given options, it is clear that alcohols and ether have one carbon less as compared to aldehyde and thus cannot be isomeric while in fatty acid, there is an extra oxygen atom present in the structure as compared to aldehyde, so this option is eliminated as well.

Thus, the aldehydes are isomeric in nature with ketones showing functional isomerism. An example showing the functional isomerism in aldehydes and ketones is as follows:

Therefore, option (A) is the correct answer.

Note: It is important to note that in organic chemistry, the functional groups which are isomeric to each other i.e., they have similar molecular structure but the functional groups attached to the compound are not same and differ in physical and chemical properties. The following are the functional groups which are generally isomeric to each other:
1. Alcohols and ethers
2. Aldehydes and ketones
3. Carboxylic acids and esters.