Question

The $S_{N}2$ reactivity order for halides is:

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

Answer 1

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

$S_{N}2$ reactivity order for halide is $RI>RBr>RCl>RF$ beacause $I^{-}$ is a better leaving group as compared to others, so, it can be easily substituted. Also the rate determinig step involves breaking of $C-X$ bond, so, the weaker bond is easily broken and the reaction proceeds at a higher rate. We know that $C-X$ bond strength decreases as we move down the group in halogen family.
Hence, (b) is correct.