Question

Consider thiol anion $\left(R{S}^{\Theta }\right)$ and alkoxy anion $\left(R{O}^{\Theta }\right)$. Which of the following statement is correct?

• A. $\left(R{S}^{\Theta }\right)$ is less basic but more nucleophilic than $\left(R{O}^{\Theta }\right)$.
• B. $\left(R{S}^{\Theta }\right)$ is more basic but more nucleophilic than $\left(R{O}^{\Theta }\right)$.
• C. $\left(R{S}^{\Theta }\right)$ is more basic but less nucleophilic than $\left(R{O}^{\Theta }\right)$.
• D. $\left(R{S}^{\Theta }\right)$ is less basic but less nucleophilic than $\left(R{O}^{\Theta }\right)$.

Answer 1

The correct option is A $\left(R{S}^{\Theta }\right)$ is less basic but more nucleophilic than $\left(R{O}^{\Theta }\right)$.

On moving down the group, the basicity and nucleophilicity are inversly related i.e nucleophilicity increases while basicity decreases i.e $R{S}^{-}$ is more nucleoplhilic but less basic than $R{O}^{-}$. This opposite behaviour is because of the fact that basicity and nucleophilicity depends upon different factors. Basicity is directly related to the strength of H-bond. While nucleophilicity is indirectly related to the electronegativity of the atom to which proton is attached.