Reaction of phenol with chloroform/sodium hydroxide to give o-hydroxy benzaldehyde involves the formation of

This is Reimer-Tiemann reaction where the electrophile is dichlorocarbene (: CCl₂) generated from chloroform by the action of a base.

${ }^{\mathrm{-}}\mathrm{OH}+{\mathrm{CHCl}}_{\mathrm{3}}\rightleftharpoons {\mathrm{HOH}+:{\mathrm{CCl}}_{\mathrm{3}}}^{\mathrm{-}}\to {\mathrm{Cl}}^{-}+:{\mathrm{CCl}}_{\mathrm{2}}$