Which of the two compounds shown in the figure (CH$_3$CH$_2$Br and (CH$_3$)$_3$CBr) would undergo the S$_\mathrm{N}$2 reaction faster, and why?

CH₃CH₂Br — a primary halide offers least steric hindrance to the backside attack of the nucleophile; the 3° (CH₃)₃CBr is heavily hindered.

CBSE Class 12 · 2015 · Delhi · Haloalkanes and HaloarenesVery Short Answer

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Which of the two compounds shown in the figure (CH

_{3}

_{2}

Br and (CH

_{3}

)

_{3}

CBr) would undergo the S

_{N}

2 reaction faster, and why?

Structure First compound of the pair — First compound of the pair

Structure Second compound of the pair — Second compound of the pair

Solution

_{N}

2 proceeds by a single-step backside attack, so its rate falls as crowding at the α-carbon rises: 1° > 2° > 3°.
Ethyl bromide (primary) lets the nucleophile approach freely; tert-butyl bromide (tertiary) blocks it with three methyl groups.

C H_{3} C H_{2} Br reacts faster in S_{N} 2

CBSE Class 12 Board · 2015 · Delhi · Subjective

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