Question

The correct decreasing order of reactivity for a given alkyl $(R)$ group in both $S_{N}1$ and $S_{N}2$ reaction mechanisms is

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

Answer 1

Answer: (A)

With increase in the size of halogen atom in $R-X,$ (where $X=$ halogen) bond length increases or $R-X$ bond strength decreases. Thus, rate of reaction in $S_{N}1$ as well as $S_N^2$ reaction increases. The order of size of halogen atom is

$I>Br>Cl>F$

Thus, the order of reactivity is

$R-I>R-Br>R-Cl>R-F$