Question

Can Sodium borohydride (NaBH₄) reduce imine?

Answer 1

Hint: Sodium borohydride is a good reducing agent. It can reduce aldehydes and ketones into primary alcohols and secondary alcohols respectively.

Complete Step by Step Solution:
When ammonia is treated with aldehydes or ketones, the ammonia group gets added to the carbonyl group of aldehydes or ketones.
This is a reversible reaction and acid is used as a catalyst.
The product formed is called imine.

Image: Reaction of propanone with ammonia

Sodium borohydride is a good reducing agent that can reduce imines into secondary amines.
This reaction happens in the presence of alumina.
Here polar metal-hydrogen bond furnishes the hydride anion.
The mechanism of the reaction is of two steps.
Step-1
Nucleophilic attack by the hydride anion.

Image: Attack of the nucleophile

Step-2
Protonation leads to the formation of a secondary amine.

Image: Protonation in the presence of methanol.


Additional Information: Sodium borohydride is a lesser reducing agent than Lithium aluminium hydride. It can reduce aldehydes to primary alcohol and ketones, and acid chlorides to secondary alcohols. It cannot reduce esters, amides, acids, nitriles etc.

Note: Sodium borohydride is an inorganic compound and is a good reducing agent due to the existence of a polar metal-hydrogen bond. The hydrogen atom in the compound acts as a hydride and adds to the carbonyl carbon. The extremely familiar reducing agents in organic chemistry are lithium aluminium hydride and sodium borohydride. It is to be noted that the hydride anion does not directly exist during this reaction, rather a metal-hydrogen bond provides the hydride anion. This anion then acts as a nucleophile.