The reaction implied is the Reimer-Tiemann reaction, used to prepare salicylaldehyde (2-hydroxybenzaldehyde) from phenol.

Mechanism of Electrophile Generation:

1. The hydroxide ion ($OH^-$) from the alkali removes the acidic proton from chloroform ($CHCl_3$) to form the trichloromethyl carbanion ($^-CCl_3$). $CHCl_3 + OH^- \rightleftharpoons H_2O + ^-CCl_3$
2. The carbanion loses a chloride ion ($Cl^-$) to form dichlorocarbene ($:CCl_2$). $^-CCl_3 \rightarrow :CCl_2 + Cl^-$

Dichlorocarbene contains a sextet of electrons (electron-deficient) and acts as the electrophile that attacks the electron-rich benzene ring of the phenoxide ion.