Question

The order of rate of hydrolysis of alkyl halide $1^{\circ },2^{\circ },3^{\circ }$ and ${CH}_{3}X$ by ${SN}^2$ pathway

• A. $1^{\circ }>2^{\circ }>3^{\circ }>{CH}_{3}X$
• B. ${CH}_{3}X>3^{\circ }>2^{\circ }>1^{\circ }$
• C. ${CH}_{3}X>1^{\circ }>2^{\circ }>3^{\circ }$
• D. $3^{\circ }>2^{\circ }>1^{\circ }>{CH}_{3}X$

Answer 1

Answer: (C)

${CH}_{3}X>1^{\circ }>2^{\circ }>3^{\circ }$

The order of rate of hydrolysis depends on the steric hindrance:

$C{H}_{3}X>1°>2°>3°$ alkyl halides

Large groups on the electrophile hinder the approach of the nucleophile.

For an $S{N}_2$ reaction, the nucleophile must approach the small backside of the C-X $s{p}^3$ orbital. Generally, one alkyl group slows the reaction, two alkyl groups make it difficult, three alkyl groups close to impossible.

Option C is correct.