IIT JEE Advanced 2007 Paper 2

The Reimer–Teimann reaction involves electrophilic substitution reaction. In this case, the electrophile is dichlorocarbene :CCl2.
This is generated by the reaction of chloroform (CHCl3) and NaOH as
CHCl3+NaOH → H2O+CCl3− → Cl−+:CCl2
Dichlorocarbene contains carbon atom with a sextet of electrons and hence is a strong electrophile