Q.Explain the following (write only chemical equation): [1+1+1+1+1=5]

The Stephen Reaction: From Nitrile to Aldehyde

You already know that a nitrile ($R-C\equiv N$) has a triple bond between carbon and nitrogen. That triple bond is rich in electrons and can be reduced — but the trick is to stop the reduction at exactly the right stage. If you reduce too hard, you get a primary amine ($R-CH_2-NH_2$). The Stephen reaction is a clever way to stop halfway, giving you an aldehyde instead.

The Intuition

Think of the nitrile's triple bond as a tightly held rope. SnCl₂ in HCl is a mild reducing agent — it doesn't yank the rope all the way to the amine. Instead, it adds just enough hydrogen to break one of the three bonds, turning the triple bond into a double bond. That double-bonded product is an imine ($R-CH=NH$), which is essentially an aldehyde where the oxygen is replaced by NH.

But imines are unstable in water. As soon as you add water (during work-up), the NH group gets swapped for oxygen, and you get your aldehyde.

The Precise Statement

Stephen reaction: A nitrile ($R-C\equiv N$) is reduced by stannous chloride in concentrated hydrochloric acid (SnCl₂/HCl) to form an imine hydrochloride salt. This salt, upon hydrolysis with water, yields the corresponding aldehyde ($R-CHO$).

Step-by-Step Mechanism

1. Coordination: SnCl₂ donates electrons to the nitrile carbon, weakening the triple bond.
2. Protonation: HCl provides protons that add across the triple bond. The intermediate is an imine hydrochloride — a salt that precipitates out. This is important: the salt is stable and can be isolated if needed.
3. Hydrolysis: Adding water hydrolyses the imine. The C=N bond breaks, NH₂ leaves as ammonia, and C=O forms — you get the aldehyde. …