Assertion (A) : In $S_N1$ reaction racemic mixture is formed and in $S_N2$ reaction inversion of configuration is observed.
Reason (R) : $S_N1$ reaction is concerted mechanism and $S_N2$ reaction involves carbocation intermediate.
In the light of the above statements, choose the most appropriate answer from the options given below :
SN1 and SN2 Reactions: Evaluating Assertion and Reason
This question asks us to evaluate the correctness of two statements regarding SN1 and SN2 reactions and determine if the Reason correctly explains the Assertion.
Assertion (A) Analysis
- SN1 Reaction: This reaction mechanism typically involves the formation of a carbocation intermediate. The nucleophile can attack this carbocation from either face (front or back), leading to a mixture of stereoisomers. Consequently, a racemic mixture (or near-racemic) is generally formed.
- SN2 Reaction: This reaction mechanism involves a concerted process where the nucleophile attacks the substrate from the backside as the leaving group departs. This results in a complete inversion of the stereochemical configuration at the carbon center.
- Conclusion for (A): Both parts of Assertion (A) are factually correct based on the established mechanisms of SN1 and SN2 reactions.
Reason (R) Analysis
- SN1 Reaction Mechanism: The statement claims SN1 reactions are 'concerted'. This is incorrect. SN1 reactions are stepwise, proceeding through a carbocation intermediate.
- SN2 Reaction Mechanism: The statement claims SN2 reactions involve a 'carbocation intermediate'. This is also incorrect. SN2 reactions occur via a single transition state, not a carbocation intermediate.
- Conclusion for (R): Reason (R) contains significant inaccuracies regarding both SN1 and SN2 mechanisms.
Final Determination
Assertion (A) accurately describes the stereochemical outcomes of SN1 (racemic mixture) and SN2 (inversion) reactions. However, Reason (R) incorrectly characterizes the mechanisms, stating SN1 is concerted and SN2 involves a carbocation. Since (A) is correct and (R) is incorrect, the most appropriate answer is that (A) is correct, but (R) is not correct.
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