Reimer–Tiemann reaction _ an electrophillic substitution or carbene attack.

Reimer Tiemann's reaction

On treating phenol with chloroform in presence of sodium hydroxide at 340 K, a - CHO group is introduced at ortho position of benzene ring. 

The reaction is an example of carbene based reactions where

it is also considerd to be an electrophillic attack on benzene  .

This reaction is known as Reimer - Tiemenn reaction. 

This results in the formation of O - hydroxybenzaldehyde (salicylaldehyde) and para-hydroxybenzaldehyde, the ortho isomer being the major product.

The intermediate substituted benzal chloride is hydrolysed in presence of alkali to produce salicylaldehyde.

Reimer Tiemenn reaction is an electrophilic substitution reaction. The first step is generation of electrophile.

                               

Dichlorocarbene contains a sextet of electrons and thus is a strong electrophile.