Question

The correct decreasing order of reactivity for a given alkyl $\left(R\right)$ 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

The correct option is A $R-I>R-Br>R-Cl>R-F$

With increase in the size of halogen atom in $R-X$, (where $X=halogen)$ bond length increase 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
$R-I>R-Br>R-Cl>R-F$.