91Ó°ÊÓ

Species that leave a particular molecule taking an electron pair are known as leaving groups. There are several leaving groups, and some of them include halides like ${\mathrm{Cl}}^{-}$and${\mathrm{Br}}^{-}$and tosylate $\left({\mathrm{TsO}}^{-}\right)$.

Good leaving groups are the ones that possess a weak base and whose conjugate acids have a low $p{K}_a$value.

a. ${\mathrm{Br}}^{-}$ is a good leaving group as it is a weak base, and the conjugate acid of ${\mathrm{Br}}^{-}$, which is HBr, possesses a low ${\mathrm{pK}}_{\mathrm{a}}$value.

b.${\mathrm{OH}}^{-}$ is not a good leaving group as it is a strong base, and its conjugate acid has a very high ${\mathrm{pK}}_{\mathrm{a}}$value.

c.${\mathrm{H}}_2\mathrm{O}$ is a good leaving group as it is a weak base, and the conjugate acid of ${\mathrm{H}}_2\mathrm{O}$, which is ${\mathrm{H}}_3{\mathrm{O}}^{+}$, possesses a low ${\mathrm{pK}}_{\mathrm{a}}$value.

d. ${\mathrm{H}}^{-}$is not a good leaving group as it is a strong base, and its conjugate acid possesses a high ${\mathrm{pK}}_{\mathrm{a}}$value.