Question

Arrange the following according to their leaving group ability in a $S_{N}2$ reaction :
S$S_N2$ reaction.

$H{O}^{-},H_{2}O,{H_{3}O}^{+}$

• A. $H{O}^{-}<H_{2}O<{H_{3}O}^{+}$
• B. $H{O}^{-}<{H_{3}O}^{+}<H_{2}O$
• C. $H_{2}O$
• D. None of these

Answer 1

Answer: (A)

$H{O}^{-}<H_{2}O<{H_{3}O}^{+}$

The conjugate acid $\left(H_3{O}^{+}\right)$ is always a better leaving group, and acid can be helpful for getting substitution reactions to proceed at a much faster rate. Because the leaving group $\left(H{O}^{-}\right)$ is a strong base and thus a poor leaving group.

Thus the leaving group ability is in rank $O{H}^{-}<H_{2}O<H_3{O}^{+}$