Question

By aerial oxidation, which one of the following gives phthalic acid?
(A)Naphthalene
(B)Benzene
(C)Mesitylene
(D)Toluene

Answer 1

Hint: The reaction in which air or oxygen from air is used to oxidize other substances is called aerial oxidation. Phthalic acid is aromatic and it is also a dicarboxylic acid. Phthalic acid is an isomer of the acids: isophthalic acid and terephthalic acid. It is usually produced by the catalytic oxidation of naphthalene or ortho-xylene directly to phthalic anhydride and then a hydrolysis of the anhydride formed.

Complete answer:
Naphthalene on oxidation in the presence of potassium permanganate or potassium dichromate gives phthalic acid. When alkyl benzene compounds are heated with strong oxidizing agents, the alkyl group is oxidized to the carboxylic acid group. If two alkyl groups are oxidized, then two carboxylic acid groups are formed. Alkyl benzene is oxidized to phthalic acids by strong oxidizing agents like acidic potassium permanganate, acidic potassium dichromate and acidic potassium chromate. The aerial oxidation of naphthalene is given below:

Additional information: Exposure to phthalic acid is considered dangerous. It may include irritation of eyes, mucous membranes and respiratory passages. In high concentrations, it causes narcosis. The acid is sensitive to exposure to extreme heat.

Note:
Phthalic acid can be prepared from benzene. Benzene is first treated with methyl chloride in the presence of anhydrous aluminium chloride. It gives methylbenzene as a product. Benzene is treated with methyl chloride in excess in the presence of anhydrous aluminium chloride to give $1,2 - {\text{Dimethylbenzene}}$ and $1,4 - {\text{Dimethylbenzene}}$. By using the process of distillation we can separate these two compounds. $1,2 - {\text{Dimethylbenzene}}$ is oxidized by acidic potassium permanganate to produce required phthalic acid.