The alkyl bromides $CH X_{3} - Br$ , $CH X_{3} CH X_{2} - Br$ , $(CH X_{3}) X_{2} CH - Br$ and $(CH X_{3}) X_{3} C - Br$ are each treated with $NaI$ in dry acetone at room temperature. Which one reacts fastest by the SN2 pathway?

A $(CH X_{3}) X_{3} C - Br$
B $CH X_{3} - Br$
C $(CH X_{3}) X_{2} CH - Br$
D $CH X_{3} CH X_{2} - Br$

Answer: B

SN2 rate falls as alkyl substitution at the reacting carbon increases, because the backside approach of the nucleophile becomes more hindered. Methyl is least hindered, so

CH X_{3} - Br

reacts fastest and the tertiary bromide is slowest.

Practice Organic Reaction Mechanisms III: Nucleophilic Substitution More on SN1 vs SN2: substrate, nucleophile, solvent, leaving group

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The alkyl bromides CHX3​−Br, CHX3​CHX2​−Br, (CHX3​)X2​CH−Br and (CHX3​)X3​C−Br are each treated with NaI in dry acetone at room temperature. Which one reacts fastest by the SN2 pathway?

Answer: B

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

The alkyl bromides $CH X_{3} - Br$ , $CH X_{3} CH X_{2} - Br$ , $(CH X_{3}) X_{2} CH - Br$ and $(CH X_{3}) X_{3} C - Br$ are each treated with $NaI$ in dry acetone at room temperature. Which one reacts fastest by the SN2 pathway?