Question

Which one of the following reducing agents is likely to be most effective in bringing about the following change?

(a) \[{H_2} - Ni\]
(b) \[NaB{H_4}\]
(c) \[LiAl{H_4}\]
(d) \[Na - Alcohol\]

Answer 1

Hint: The reducing agents are the compounds or elements which have the tendency to lose their electrons or hydrogen in a chemical reaction and get oxidised.
Properties of a reducing agent:
(a) They have the tendency to lose their electrons.
(b) They have the tendency to get self-oxidised by reducing other species.
(c) They have the tendency to increase their oxidation state.

Complete Step by Step Solution:
The \[{H_2} - Ni\]or Raney nickel is a strong reducing agent. It can reduce:
- Aldehydes (\[ - CHO\]), and carboxylic acids (\[ - COOH\]) to primary alcohols.
- Ketones (\[ - CO - \]) to secondary alcohols.

The \[NaB{H_4}\] or sodium borohydride is a weaker reducing agent than \[LiAl{H_4}\] . It can reduce:
- Aldehydes, ketones, and acid chlorides (\[RCOCl\]) to alcohols.

The \[LiAl{H_4}\] i.e Lithium Aluminium Hydride or LAH is a very strong and unselective reducing agent. It can reduce:
- Aldehydes, ketones, esters, carboxylic acid chlorides and carboxylic acid to alcohols.
- It can reduce the amides (\[RCON{H_2}\]), and nitriles (\[RCN\])to amines (\[RC{H_2}N{H_2}\]).

Na-Alcohol is also a strong reducing agent. It can reduce:
- The alkyne to trans-alkene.
The above points indicate that \[LiAl{H_4}\]is the best reducing agent for the conversion of amides (\[RCON{H_2}\]) to amines (\[RC{H_2}N{H_2}\]). Therefore, option (c) will be the correct answer (Image 1).

Image: Reduction of amide to amine by \[LiAl{H_4}\] reagent.

Note: The \[NaB{H_4}\] and \[LiAl{H_4}\] both are hydride donors (\[{H^ - }\]). The \[NaB{H_4}\] cannot reduce ester, amide, carboxylic acid, and nitriles. The \[NaB{H_4}\] and \[LiAl{H_4}\] must be handled in inert condition. A little bit of moisture can reduce the reaction yield of the reaction and also the reaction with water is violent in nature.