The rate of beta-dehydrohalogenation in the f | vedclass

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(A) The $\beta$-dehydrohalogenation reaction proceeds via an $E2$ mechanism,which involves the abstraction of a $\beta$-hydrogen by a strong base.In t

Vedclass · Accepted Answer

$a > b > c$

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(A) The $\beta$-dehydrohalogenation reaction proceeds via an $E2$ mechanism,which involves the abstraction of a $\beta$-hydrogen by a strong base.In this reaction,the rate-determining step involves the breaking of the $C-H$ or $C-D$ bond at the $\beta$-position.Since the $C-D$ bond is stronger than the $C-H$ bond due to the primary kinetic isotope effect,the cleavage of the $C-H$ bond is faster than that of the $C-D$ bond.Compound $(a)$ has two $\beta$-hydrogens $(Ph-CH_2-CH_2-Br)$,making it the fastest.Compound $(b)$ has two $\beta$-deuteriums $(Ph-CD_2-CH_2-Br)$,making it slower than $(a)$ due to the isotope effect.Compound $(c)$ $(CD_3-CH_2-Br)$ also has $\beta$-deuteriums,but the electron-donating effect of the phenyl ring in $(b)$ stabilizes the transition state more effectively than the methyl group in $(c)$,making $(b)$ faster than $(c)$.Thus,the order of the rate of reaction is $a > b > c$.

The rate of $\beta$-dehydrohalogenation in the following halides under treatment with a strong base follows the order:

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