Question

Arrange the following in their order of nucleophilicity in protic solvent.

A) i > ii > iii
B) iii > ii > i
C) ii > i > iii
D) None

Answer 1

Hint: We know that the ability of a nucleophile to react with an electron deficient compound is known as nucleophilicity. The electron withdrawing group decreases the nucleophilicity of an atom and the electron donating group increases the nucleophilicity of an atom.

Complete step-by-step answer:
We know that a species that donates a pair of electrons to form a chemical bond is known as a nucleophile. A solvent which contains hydrogen $\left( {{{\text{H}}^ + }} \right)$ ions is known as a protic solvent. In protic solvent, the positively charged hydrogen atom bonds with the negatively charged oxygen atom of the given compounds.

We are given three compounds.
Consider compound (i) – phenoxide ion: In the phenoxide ion, there is no conjugation for the pi-electrons of the benzene ring. Thus, the electron density on the negatively charged oxygen atom does not increase. The phenyl group is an electron withdrawing group which decreases the nucleophilicity of the negatively charged oxygen atom.

Consider compound (ii) – methoxide ion: In the methoxide ion, the methyl group is the electron donating group. The methyl group thus increases the nucleophilicity of the negatively charged oxygen atom. Thus, methoxide ion is more nucleophilic than phenoxide ion.

Consider compound (iii) – acetate ion: In the acetate ion, the methyl group is an electron donating group. The negatively charged oxygen atom increases the electron density around the carbonyl carbon Thus, the acetate ion is more nucleophilic than the phenoxide ion.

Thus, the order of nucleophilicity is as follows:
iii > ii > i

Thus, the correct option is (B) iii > ii > i.

Note: The hydrogen atom in protic solvents participates in hydrogen bonding. Thus, the nucleophiles easily attack the hydrogen atom. The factors affecting nucleophilicity are size, solvent and basicity.