Question

Benzoquinone is prepared by reaction of phenol with _____________.
A. \[N{a_2}C{r_2}{O_7},{\text{ }}{H_2}S{O_4}\]
B. \[KMn{O_4},{\text{ }}{H_2}S{O_4}\]
C. \[N{a_2}Cr{O_4},{\text{ }}HCl\]
D. \[{K_2}Mn{O_4},{\text{ }}{H_2}S{O_4}\]

Answer 1

Hint:Phenol is an aromatic alcohol compound. Benzoquinone is quinone in a single ring. Two keto groups are in conjugation with double bonds inside the ring. The benzoquinone is represented as

Complete step by step answer: The given reaction is an example of oxidation reaction of phenol. Phenol is an aromatic alcohol compound in which a hydroxy is attached to the benzene ring. The hydroxyl group is known to contain free electrons.
The free electrons of the hydroxyl group of phenol take part in conjugation with the double bonds of the benzene ring. The process of delocalization of electrons in the ring is said to be a resonance phenomena. As a consequence the electron density of the benzene increases and it is ready for oxidation. The resonance of phenol is shown as

The oxidation process involves loss of electrons and loss of hydrogen in presence of a very strong oxidizing agent. The given sets of reagents are oxidizing agents. Out of the four given reagents the \[N{a_2}C{r_2}{O_7},{\text{ }}{H_2}S{O_4}\] reagent is the strongest oxidizing agent. It readily oxidizes the phenol to the benzoquinone by releasing electrons and protons. The mechanism of the reaction is as follows:

The use of other oxidizing agent’s mixtures like \[KMn{O_4},{\text{ }}{H_2}S{O_4}\] or \[N{a_2}Cr{O_4},{\text{ }}HCl\] or \[{K_2}Mn{O_4},{\text{ }}{H_2}S{O_4}\] does not oxidize the phenol into benzoquinone. These reagents are used to oxidize primary alcohols into aldehydes or carboxylic acids and secondary alcohols into ketones.

Note:
Oxidation and reduction is a very common reaction in organic synthesis. Both the reactions occurring simultaneously are called redox reactions. The desired product is obtained by the appropriate choice of the oxidizing agent.