Question

Consider thiol anion ($R{S^ - }$) and alkoxy anion ($R{O^ - }$). Which of the following statements is correct?
A. $R{S^ - }$is more basic but less nucleophilic than $R{O^ - }$.
B. $R{S^ - }$is less basic but less nucleophilic than $R{O^ - }$.
C. $R{S^ - }$is less basic but more nucleophilic than $R{O^ - }$.
D. $R{S^ - }$is more basic but more nucleophilic than $R{O^ - }$.

Answer 1

The nucleophilic strength increases on moving down in the group and the basic nature of the decreases as we move down in the group. They are the opposite of each other. The basic nature depends on the bond strength between the atoms.

Complete step by step answer:
The nucleophiles are those groups which have the ability to donate their pair of electrons to the other atom to form the chemical bond. The strength of the nucleophile to donate its lone pair depends on the electronegativity of the atom. Lower the value of electronegativity of atoms higher will be the nucleophilic strength of the anion and as a result the nucleophile can donate easily its electron pair to the electron deficient species.
The electronegativity of the atom decreases as we move down in the group and the nucleophilic strength increases. As oxygen atom is present above the sulphur atom in the group, therefore the thiol anion $R{S^ - }$is more nucleophilic in nature than alkoxy anion $R{O^ - }$.
The basic strength of the anion depends on the ability to lose the electron pair. As O-H bond is longer than S-H bond, therefore the O-H bond is broken easily and it can donate its lone pairs to neighbouring atoms. Thus, the alkoxy anion is more basic than the thiol anion.
Thus, $R{S^ - }$is less basic but more nucleophilic than $R{O^ - }$.

So, the correct answer is Option C.

Note: All the anions are nucleophilic in nature as well as basic in nature as they are electron rich species and can donate their lone pair to other atoms.