In the first reaction, dichlorocarbene adds to triethylamine to form an unstable adduct A.
In the second step, this unstable adduct loses a molecule of ethylene to form product B. The hydrolysis of product B gives product C. In this step, the dichloro methyl group is hydrolyzed to aldehyde group.
The structures of compounds A to C are as follows:
(C2H5)3N:+:CCl2→Et3N+CCl−2A (An unstable adduct)
Et3N+CCl−2A→Et2NCHCl2B+C2H4
Et2NCHCl2BH2O−−→Et2N−CHOC