Question 20 of 26intermediate🔍 AnalyzeShort Answer3 marks

Arrange the compounds of each set in order of reactivity towards S_N2 displacement:

(i)

2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane

Answer

1-Bromopentane > 2-Bromopentane > 2-Bromo-2-methylbutane

(ii)

1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-Bromo-3-methylbutane

Answer

1-Bromo-3-methylbutane > 2-Bromo-3-methylbutane > 2-Bromo-2-methylbutane

(iii)

1-Bromobutane, 1-Bromo-2,2-dimethylpropane, 1-Bromo-2-methylbutane, 1-Bromo-3-methylbutane

Answer

1-Bromobutane > 1-Bromo-3-methylbutane > 1-Bromo-2-methylbutane > 1-Bromo-2,2-dimethylpropane

Explanation

The answer is based on the fundamental principle stated in the NCERT textbook: 'Primary halide > Secondary halide > Tertiary halide' is the order of reactivity for S_N2 reactions. This is because S_N2 reactions involve backside attack by the nucleophile, and steric hindrance around the electrophilic carbon increases from primary to tertiary halides, making tertiary halides least reactive. For part (iii), all compounds are primary halides, so the order depends on steric hindrance around the electrophilic carbon - more branching near the reactive site decreases reactivity.

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