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Preparation of Alcohols (iii)
from Haloalkanes, Aldehydes and Ketones

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From haloalkanes

Haloalkanes are heated with aqueous sodium or potassium hydroxide to form alcohols.

Preparation of alcohols from haloalkanes general formula
Preparation of alcohols from haloalkanes

Identify ?A and ?B

Convert ethane to ethanol

From aldehydes and ketones

By reduction

Aldehydes and ketones are reduced to corresponding alcohols in a number of ways like :

By addition of Grignard reagents

Grignard reagents (R−MgX) are polar in nature where carbon is electronegative and Mg is electropositive. Due to the polar nature of Grignard reagents, they react with aldehydes and ketones to form addition products which upon decomposition gives alcohols with water or dil. H2SO4.

Mechanism of how alcohols are prepared from carbonyl compounds using grignard reagents
Preparation of alcohols from grignard reagents

Important points regarding this reaction

Identify ?A and ?B

Convert bromoethane to 2-methylpropan-2-ol

Questions

Question : Write structures of the products of the following compounds when reduced with NaBH4.

  1. Reduction of an aldehyde with NaBH4
  2. Reduction of an ester with NaBH4

Answer :

  1. The −CHO group is reduced to CH2OH. The reaction is given below:
  2. Reduction of an aldehyde with NaBH4 gives an alcohol
  3. NaBH4, being a weak reducing agent, reduces aldehydes to alcohols, but fails to reduce ester.
  4. NaBH4 fails to reduce esters
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