Question: Which of the following is oxidation with alkaline \[{\text{KMn}}{{\text{O}}_{\text{4}}}\]followed by...

Question

Which of the following is oxidation with alkaline ${\text{KMn}}{{\text{O}}_{\text{4}}}$ followed by acidification with dil. HCl gives terephthalic acid?
A.p-Ethyl toluene
B.n-Butane
C.1,3-Di-isopropyl benzene
D.m-Xylene

Answer 1

Solution

Hint : We must know two things to address this question.
Structure of terephthalic acid
And, the role of ${\text{KMn}}{{\text{O}}_{\text{4}}}$ as an oxidizing agent. Then only we find the product of the reaction.

Complete step by step solution :
First we must remember that (i) Structure of terephthalic acid: - Terephthalic acid is a benzenedicarboxylic acid with carboxyl groups at positions 1 and 4.

(ii) Role of ${\text{KMn}}{{\text{O}}_{\text{4}}}$ as an oxidising agent in the presence of organic compounds: - The reaction of an alkyl benzene with potassium permanganate ( ${\text{KMn}}{{\text{O}}_{\text{4}}}$ ) results in oxidation to benzoic acid. Now, we must note that the position directly adjacent to an aromatic group is called the “benzylic” position, and should contain at least one hydrogen attached to the carbon. For example, in the reaction below you can find all the reactants get converted to carboxylic acid derivatives except for the last one due to the absence of hydrogen at the benzylic position.

Another important point, in order to get the product terephthalic acid, we must look for alkyl groups with at 1 and 4 positions on a benzene ring provided it has at least one hydrogen at benzylic carbon. Only option (A) satisfies these criteria. Let’s look at structure of option A given below:

We can analyse the other options for verification:
Option B: n-Butane is not aromatic so we cannot get an aromatic compound upon oxidation.

Therefore, it’s an incorrect option.
Option C: 1, 3-Di-isopropyl benzene has alkyl group at 1 and 3 positions, therefore we cannot get the desired product on oxidation.

Option D: m-Xylene also has methyl groups at 1 and 3 positions; therefore we cannot get the desired product on oxidation.

Therefore, we can conclude that Option A is the correct answer among the following.

Note : 1. ${\text{KMn}}{{\text{O}}_{\text{4}}}$ Oxidises all alkyl group on the benzene ring that has at least one hydrogen on the benzylic carbon.
2. Structure of terephthalic should not be confused with other derivatives of benzenedicarboxylic acid. Otherwise the student will end up opting for the wrong answer.