Weekend Reading: Reagent - Lithium Aluminium Hydride

Reagent Name: Lithium Aluminium Hydride

Molecular Formula: LiAlH₄

Main Role in Organic Reactions: LiAlH₄ acts as a reducing agent in most of the organic reactions

Reaction Examples:

1. Reduction of carbonyl compounds to alcohols

2. Conversion of Alkyl Halides to Alkanes

3. Conversion of Alkyl Cyanides to Primary Amines

 Mechanism of Action:

 LiAlH₄ is a source of hydride (H^-) ions. Hydride ions produced by the reagent act as nulceophiles and attack the electrophillic centre in the reactant bringing about either nucleophilic addition or nucleophilic substitution depending on the nature of the reagent. In case of nucleophilic addition the salt formed is given acid treatment to release the alcohol.

Mechanism for its reaction bringing about Nucleophilic Addition

Mechanism for its reaction bringing about Nucleophilic Substitution

Related Reagents: NaBH₄ (Sodium Borohydride) is also a reducing agent which is similar in action but less reactive than LiAlH_­4.

Question: Can you explain why NaBH₄ is less reactive than LiAlH₄?

Weekend Reading

Hi Guys,

I’m starting a new section titled “Weekend Reading” from today on important reactions, reagents and concepts of Organic, Inorganic & Physical Chemistry. I’ll write (in fact I’ll do my best to write) about either a reaction or a reagent or a concept on Fridays. I hope this is useful. In case you have any suggestion to improve it leave your feedback as a comment.

Cheers!

Question on Functional Group Inter Conversion

If 3 steps are used to convert aniline to benzyl amine, what are they?

Answer:

The steps are

(1) diazotization reaction that converts aniline to benzene diazonium ion

(2) conversion of benzene diazonium ion to cyano benzene by reacting it with KCN or CuCN

(3) reduction of cyano benzene into benzyl amine using LiAlH₄

Question on Bleaching Action

Why is the bleaching action of SO₂ temporary while the bleaching action of CaOCl₂ (bleaching powder) permanent?

Answer:

The bleaching action of SO₂ is due to its reducing property while that of bleaching powder is due to is oxidizing property. The article bleached by SO₂ gets reduced while that getting bleached by bleaching powder gets oxidized. The article that gets reduced by the bleaching action of SO₂ can again be oxidized when it is exposed to air. So, the bleaching action of SO₂ is temporary.