Question

The decreasing order of reactivity of alkyl halides with zinc and dilute $HCl$ is :

• A. $R-Cl>R-I>R-Br$
• B. $R-Cl>R-Br>R-I$
• C. $R-I>R-Br>R-Cl$
• D. $R-Br>R-I>R-Cl$

Answer 1

The correct option is C $R-I>R-Br>R-Cl$

In this reaction alkyl halide get reduced to alkane by cleavage of $C-X$.
$C{H}_{3}X+H_2\stackrel{Zn,dil.HCl}{⟶}RH+HX$
As the size of the halogen increases down the group, the strength of $C-X$ bond decreases. Also the stability of $X^{-}$ formed in the reaction is higher for $I^{-}$ due to its larger size. Thus, the reactivity of alkyl halide increases down the group. Therefore, the decreasing order of reactivity of alkyl halides with zinc and dil. $HCl$ is $R-I>R-Br>R-Cl$