Question

Consider thiol anion ($R{S}^{-}$) and alkoxy anion ($R{O}^{-}$). Which of the following statements is correct?

• A. $R{S}^{-}$ is less basic and less nucleophilic than $R{O}^{-}$
• B. $R{S}^{-}$ is less basic and more nucleophilic than $R{O}^{-}$
• C. $R{S}^{-}$ is more basic and more nucleophilic than $R{O}^{-}$
• D. $R{S}^{-}$ is more basic and less nucleophilic than $R{O}^{-}$

Answer 1

The correct option is B $R{S}^{-}$ is less basic and more nucleophilic than $R{O}^{-}$

Nucleophilic strength depends on the electronegativity of the atom. Lower the electronegativity of an atom, larger the nucleophilic strength of an atom and more readily it will donate electrons to the electrophilic ($e^{-}$ deficient) species.
On the other hand, basic strength of an atom depends upon the tendency to accept a proton more readily. Since, the size of $S$ is more than that of $O$ hence electron pair is more dispersed on it and hence less available for donation. Hence, $R{O}^{-}$ is a stronger base than $R{S}^{-}$.
Hence, $R{S}^{-}$ is less basic but more nucleophilic than $R{O}^{-}$.