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MnO2 selectively oxidises which one of the following alcohols?

1. C6H5 - CH2 - CH2 - OH
2. CH2 = CH - CH2 - CH(OH) - CH3
3. CH3 - CH = CH - CH(OH) - CH3
4. CH3 - CH2 - CH2 - OH

Correct answer: CH3 - CH = CH - CH(OH) - CH3

Solution

MnO2 is a mild, selective oxidant that works only on allylic alcohols (OH carbon adjacent to C=C) and benzylic alcohols (OH carbon on the benzylic position). Option A: OH is two carbons away from the ring (homobenzylic) => not oxidised. Option B: double bond is C1-C2; OH is on C4, which is not adjacent to C3 allylic position directly to the double bond (C4 is beta to the double bond) => not efficiently oxidised by MnO2. Option C: double bond C2=C3; OH on C4, which is directly adjacent to C3 => allylic alcohol => oxidised by MnO2. Option D: simple primary aliphatic alcohol => not oxidised by MnO2.

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