The reaction sequence shown is the Stephen reaction, used for the preparation of aldehydes from nitriles (cyanides).

1. Reduction: Methyl cyanide ($CH_3CN$) is partially reduced by stannous chloride ($SnCl_2$) in the presence of hydrochloric acid ($HCl$) to form an imine hydrochloride intermediate (X). $CH_3CN + 2[H] + HCl \xrightarrow{SnCl_2} CH_3CH=NH \cdot HCl \text{ (Acetaldimine hydrochloride)}$
2. Hydrolysis: The imine intermediate (X) undergoes hydrolysis with boiling water to yield acetaldehyde (Y). $CH_3CH=NH \cdot HCl + H_2O \xrightarrow{\Delta} CH_3CHO + NH_4Cl$ Thus, Y is Acetaldehyde ($CH_3CHO$).