Question

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

Answer 1

A nucleophile is a chemical species that donates an electron pair to form a chemical bond in relation to a reaction. All molecules or ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases. A base is a chemical species which has the tendency to easily donate lone pair of electrons to the acids.

Complete step by step answer:
The difference between a nucleophile and a base is that a nucleophile is a nucleus/ proton loving species and will only release electrons if any electron deficient species is present nearby, whereas the base can only lose the electrons.
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 (electron deficient) species.
Since, electronegativity decreases down the group, thus nucleophilic strength increases down the group. Hence,$R{S^ - }$ is more nucleophilic than $R{O^ - }$ due to less electronegativity of $S$ than $O$ .
On the other hand, basic strength of an atom depends upon the tendency to accept a proton more readily or to simply lose the electrons. Since, $O - H$ bond is longer than $S - H$ bond, thus,$R{O^ - }$ has a greater tendency to accept a proton than $R{S^ - }$ . Hence, $R{O^ - }$ is a stronger base than $R{S^ - }$. Thus, the basic strength of the central atom decreases down the group.
Hence,$R{S^ - }$ is more nucleophilic but less basic than $R{O^ - }$.

So, the correct answer is Option A.

Note:
Nucleophilicity, sometimes referred to as nucleophile strength, refers to a substance's nucleophilic character and is often used to compare the affinity of atoms. Neutral nucleophilic reactions with solvents such as alcohols and water are named solvolysis. Nucleophiles may take part in nucleophilic substitution, whereby a nucleophile becomes attracted to a full or partial positive charge.