Preparation of Alcohols MCQ - Practice Questions with Answers

All three types of monohydric alcohols can be prepared by the use of Grignard reagents. Grignard reagents form addition compounds by nucleophile attack with aldehydes and ketones which on hydrolysis with dilute acid yields alcohol.

Mechanism

For example:

Alcohols are produced by the reaction of Grignard reagents with aldehydes and ketones. The first step of the reaction is the nucleophilic addition of Grignard reagent to the carbonyl group to form an adduct. Hydrolysis of the adduct yields alcohol.

Mechanism

Anhydrides are formed by heating of two (-COOH) groups to remove (H₂O) molecule. Aldehydes, ketones, carboxylic acids and derivatives on reduction yield alcohols. A number of reducing agents link Zn/HCl, Na/C₂H₅OH, LiAlH₄ or NaBH₄ can be used for this purpose. These derivatives are reduced by nascent hydrogen into corresponding alcohols. 

Some examples include, 

NaBH₄ can only reduce keto groups. But LiAlH₄ can reduce even anhydrides and esters. LiAlH₄ is a very good reducing agent because (Al) atom present in ir is more covalent than (B) atom in NaBH₄. Therefore, Al has more tendency to gain the electrons, thus, it will try to keep the electrons to itself and hence H^- will go in a particular manner. Thus, LiAlH₄ is better reducing agent than NaBH₄.

Mechanism
The mechanism for LiAlH₄ occurs in the following steps:

1. Deprotonation
   
    
2. Nucleophilic attack by the hydride ion
   
    
3. Nucleophilic attack by the hydride ion
   
    
4. Leaving group removal
   
    
5. Alkoxide is protonated
   

Some examples include: