Which of the following compounds will show retention in the configuration on nucleophilic substitution by ion?
A
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B
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C
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D
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Solution
The correct option is A
The explanation for the correct answer
A.
Retention of configuration means that the symmetry of the reactant molecule remains the same even after the reaction.
This is an SN2 reaction as the reactant is a primary alkyl bromide.
The nucleophile will attack the carbon next to the carbon atom so the chirality will remain intact.
In this molecule a primary carbocation is formed after the removal of Bromine and as it is sterically hindered so it will attack only from one side only leading to the formation of one product with the retention of configuration.
Thus, it shows retention of configuration.
Thus, A is the correct option.
The explanation for the incorrect answers
B.
It is a secondary alkyl bromide and hence will undergo an SN1 reaction.
When Bromine is left from this molecule it forms secondary carbocation and hence the incoming nucleophile can attack either side.
Thus, it gives a racemic mixture not only retention.
Thus, it is an incorrect answer.
C.
It is a secondary alkyl bromide and hence will undergo an SN1 reaction.
When Bromine is left from this molecule it forms secondary carbocation and hence the incoming nucleophile can attack either side.
Thus, it gives a racemic mixture not only retention.
Thus, it is also an incorrect answer.
D.
It is a secondary alkyl bromide and hence will undergo an SN1 reaction.
When Bromine is left from this molecule it forms secondary carbocation and hence the incoming nucleophile can attack either side.
Thus, it also gives racemic mixture not only retention.
Thus, it is also an incorrect answer.
Out of the given options, only A shows retention of configuration as the carbon atom next to the chiral carbon is attacked by the nucleophile and as a result, the chirality of the carbon remains unchanged.
