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Question 37

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Where Nu = Nucleophile
Β Find out the correct statement from the options given belowΒ for the above two reactions.

Reaction Mechanism & Kinetics Analysis

πŸ’‘ Core Organic Chemistry Concept: SN1 vs SN2 Substitution
  • SN1 Mechanism: Unimolecular (1st order). It relies entirely on the stability of the carbocation intermediate formed when the leaving group departs. Electron-donating groups stabilize carbocations via resonance or induction.
  • SN2 Mechanism: Bimolecular (2nd order). It occurs in a single concerted step via a transition state. Electron-withdrawing groups accelerate this path by making the alpha-carbon more electrophilic and stabilizing the transition state.

Step-by-Step Reaction Evaluation:

Reaction (I): 4-Methoxybenzyl Chloride System

The methoxy group (β€”OMe) at the para-position is a highly powerful electron-donating group (EDG) via resonance (+M effect). When the chloride leaving group departs, the resulting benzylic carbocation is strongly stabilized by the lone pairs on the oxygen atom. Because a highly stable carbocation forms effortlessly, this pathway strictly undergoes an SN1 mechanism, making it a 1st order reaction.

Reaction (II): 4-Nitrobenzyl Chloride System

The nitro group (β€”NO2) at the para-position is an exceptionally powerful electron-withdrawing group (EWG) via both resonance (βˆ’M) and induction (βˆ’I). An EWG strongly destabilizes any positive charge, making an SN1 carbocation path energetically unfavorable. Instead, the electron-withdrawing nature pulls electron density away from the benzylic carbon, making it highly partial-positive and susceptible to a direct, backside concerted attack by the nucleophile. Therefore, this pathway strictly proceeds through an SN2 mechanism, making it a 2nd order reaction.

Correct Option: C (Reaction I is of 1st order and reaction II is of 2nd order)

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