In the presence of air, phenols are slowly oxidised to dark coloured mixture containing-

benzoquinone

The correct answer is Option (4) → benzoquinone

In the presence of air (oxygen), phenols undergo slow oxidation, forming various products. One of the primary products formed during the oxidation of phenol is benzoquinone.

Oxidation of Phenols:

Phenols are susceptible to oxidation due to the presence of the hydroxyl group \((-OH)\) attached to the aromatic ring. The oxidation of phenol typically produces benzoquinones, which are conjugated cyclic diketones.

In the presence of air, phenol is oxidized to p-benzoquinone (para-benzoquinone) as a major product. This oxidation process often causes the solution to turn a dark color, which is characteristic of the formation of quinones.

Reaction:

This reaction occurs in a slow process when phenol is exposed to air, as oxygen acts as the oxidizing agent.

The product formed during the slow oxidation of phenol in air is primarily benzoquinone.

Thus, the correct option is: benzoquinone.