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Question: Which one of the following alcohols are oxidised by $MnO_2$?...

Which one of the following alcohols are oxidised by MnO2MnO_2?

A

C6H5−CH2−CH2−OHC_6H_5-CH_2-CH_2-OH

B

CH2=CH−CH2−CH(OH)−CH3CH_2=CH-CH_2-CH(OH)-CH_3

C

CH3−CH=CH−CH(OH)−CH3CH_3-CH=CH-CH(OH)-CH_3

D

CH3−CH2−CH2−OHCH_3-CH_2-CH_2-OH

Answer

The correct options are (B) and (C).

Explanation

Solution

MnO2MnO_2 is a mild oxidizing agent that selectively oxidizes allylic and benzylic alcohols to carbonyl compounds (aldehydes or ketones). It does not oxidize saturated primary or secondary alcohols.

  • (A) C6H5−CH2−CH2−OHC_6H_5-CH_2-CH_2-OH: This is 2-phenylethanol. The hydroxyl group is on a primary carbon, but this carbon is not directly attached to the phenyl ring. Therefore, it is not a benzylic alcohol. MnO2MnO_2 does not oxidize this compound.
  • (B) CH2=CH−CH2−CH(OH)−CH3CH_2=CH-CH_2-CH(OH)-CH_3: This molecule contains a hydroxyl group on a secondary carbon that is adjacent to a CH2CH_2 group, which is part of an alkene (CH2=CH−CH_2=CH-). Thus, it is a secondary allylic alcohol. MnO2MnO_2 oxidizes allylic alcohols to ketones.
  • (C) CH3−CH=CH−CH(OH)−CH3CH_3-CH=CH-CH(OH)-CH_3: This molecule contains a hydroxyl group on a secondary carbon that is adjacent to a CHCH group, which is part of an alkene (CH3−CH=CH−CH_3-CH=CH-). Thus, it is a secondary allylic alcohol. MnO2MnO_2 oxidizes allylic alcohols to ketones.
  • (D) CH3−CH2−CH2−OHCH_3-CH_2-CH_2-OH: This is propan-1-ol, a saturated primary alcohol. MnO2MnO_2 does not oxidize saturated alcohols.