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NEET CHEMISTRYHaloalkanes and HaloarenesMedium

Question

Consider the reactions: (i) (CH3)2CHCH2BrC2H5OH(CH3)2CHCH2OC2H5+HBr(\text{CH}_3)_2\text{CHCH}_2\text{Br} \xrightarrow{\text{C}_2\text{H}_5\text{OH}} (\text{CH}_3)_2\text{CHCH}_2\text{OC}_2\text{H}_5 + \text{HBr} (ii) (CH3)2CHCH2BrC2H5O(CH3)2CHCH2OC2H5+Br(\text{CH}_3)_2\text{CHCH}_2\text{Br} \xrightarrow{\text{C}_2\text{H}_5\text{O}^-} (\text{CH}_3)_2\text{CHCH}_2\text{OC}_2\text{H}_5 + \text{Br}^- The mechanisms of reactions (i) and (ii) are, respectively:

A

SN2S_N2 and SN1S_N1

B

SN1S_N1 and SN2S_N2

C

SN1S_N1 and SN1S_N1

D

SN2S_N2 and SN2S_N2

Step-by-Step Solution

The substrate in both reactions is isobutyl bromide, (CH3)2CHCH2Br(\text{CH}_3)_2\text{CHCH}_2\text{Br}, which is a primary (11^\circ) alkyl halide. Primary alkyl halides undergo nucleophilic substitution predominantly via the SN2S_N2 mechanism because the corresponding primary carbocation required for an SN1S_N1 reaction is highly unstable . Furthermore, if an SN1S_N1 mechanism were to occur, the primary carbocation would undergo a hydride shift to form a more stable tertiary carbocation, leading to a rearranged product. Since the given product in both cases is the unrearranged ether (isobutyl ethyl ether), it confirms that no carbocation intermediate is formed. Therefore, both reactions proceed via the SN2S_N2 mechanism, irrespective of whether the attacking nucleophile is neutral (C2H5OH\text{C}_2\text{H}_5\text{OH}) or a strong anionic nucleophile (C2H5O\text{C}_2\text{H}_5\text{O}^-).

Exam Context & Concepts Covered

This question aligns with the NEET CHEMISTRY syllabus, specifically targeting concepts from Haloalkanes and Haloarenes. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

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