The Stephen's reaction is used to prepare aldehydes from nitriles or cyanides. The reduction of nitriles with stannous chloride and hydrogen chloride in ethyl acetate solvent produces imine intermediate. The hydrolysis of this intermediate with watergives the corresponding aldehyde.
HI∕ Red P is a strong reducing agent. It reduces alcohols, aldehydes, ketones to alkanes. CH3CHO+HI+RedP⟶CH3−CH3+H2O+2I2 So, finally the CN gets converted to methyl (−CH3) group. CN contains triple bond between carbon and nitrogen so it is sp -hybridised,whereas ' C ' in - CH3 is sp3 hybridised.