Rank the following in decreasing order of SN2 reactivity towards NaI in dry acetone: I CH3CH2-Br, II (CH3)2CHCH2-Br, III (CH3)3CCH2-Br, IV CH3CH2CH2-Br.

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JEE Main levelSingle correct optionDifficulty 2 of 3o3123

Rank the following in decreasing order of SN2 reactivity towards $NaI$ in dry acetone: I $CH X_{3} CH X_{2} - Br$ , II $(CH X_{3}) X_{2} CHCH X_{2} - Br$ , III $(CH X_{3}) X_{3} CCH X_{2} - Br$ , IV $CH X_{3} CH X_{2} CH X_{2} - Br$ .

A $III > II > IV > I$
B $I > IV > II > III$
C $I > IV > III > II$
D $IV > I > II > III$

Answer: B

All four are primary, so the difference is steric hindrance at the beta carbon. Ethyl is least hindered, n-propyl is slightly slower, isobutyl (branching at beta) is much slower, and neopentyl (three beta methyls) is extremely slow. The order is I > IV > II > III.

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Rank the following in decreasing order of SN2 reactivity towards NaI in dry acetone: I CHX3​CHX2​−Br, II (CHX3​)X2​CHCHX2​−Br, III (CHX3​)X3​CCHX2​−Br, IV CHX3​CHX2​CHX2​−Br.

Answer: B

More questions on SN1 vs SN2: substrate, nucleophile, solvent, leaving group

Rank the following in decreasing order of SN2 reactivity towards $NaI$ in dry acetone: I $CH X_{3} CH X_{2} - Br$ , II $(CH X_{3}) X_{2} CHCH X_{2} - Br$ , III $(CH X_{3}) X_{3} CCH X_{2} - Br$ , IV $CH X_{3} CH X_{2} CH X_{2} - Br$ .