Salicylaldehyde is formed as a main product from phenol by :

Reimer Tiemann reaction

The correct answer is option 2. Reimer Tiemann reaction.

Salicylaldehyde can be prepared from phenol using Reimer-Tiemann Reaction. So let us look into the reaction in detail:

Reimer Tiemann reaction is a type of substitution reaction named after chemists Karl Reimer and Ferdinand Tiemann. The reaction is used for the ortho-formylation of \(C_6H_5OH\) (phenols).

When phenol is treated with chloroform in the presence of sodium hydroxide, an aldehyde group \((-CHO)\) is introduced at the ortho position of the benzene ring leading to the formation of o-hydroxybenzaldehyde.

Since hydroxides are not readily soluble in chloroform, a biphasic solvent system is employed to carry out the reaction. This biphasic solvent system can consist of an aqueous hydroxide solution with an organic phase that contains chloroform.

Mechanism:

It involves electrophilic substitution of the aromatic nucleus, the electrophile being dichloro carbene \((:CCl_2)\) (carbon has only sextet of electrons), formed by the action of strong alkali on chloroform.