Question

The decreasing order of relative nucleophilicity of the following nucleophiles in a protic solvent is SH^-, AcO^-, PhO^-, OH^-, H₂O.
(A) SH^- > OH^- > H₂O > AcO^- > PhO^-
(B) SH^- > OH^- > PhO^- > AcO^- > H₂O
(C) SH^- > PhO^- > OH^- > H₂O > AcO^-
(D) OH^- > SH^- > PhO^- > AcO^- > H₂O

Answer 1

Hint: The capacity of the nucleophiles to denote their lone pairs to a positive centre is known as nucleophilicity. It refers to the rate at which the nucleophile assaults the substrates (R - LG) and is a kinetic term. There are numerous ways to compare the nucleophilicity of various nucleophiles.

Complete Step by Step Solution:
One nucleophile substitutes another in a class of organic reactions known as nucleophilic substitution reactions. It closely resembles the typical displacement reactions we observe in chemistry, in which a more reactive element displaces a less reactive element from its salt solution. The molecule on which substitution occurs is known as the "substrate," and the group that accepts an electron pair and is moved from the carbon is known as the "leaving group." In its final state, the leaving group is a neutral molecule or anion.

The action of hydration (solvation), in the case of polar solvents as alcohol acids, water, etc., plays a part in determining the nucleophilicities of nucleophiles. A very hydrated ion's ionic mobility is drastically reduced, which lowers its nucleophilicity. An ion is surrounded by several water molecules, a process known as hydration.
$Nucleophilicity \ \alpha \ \dfrac{1}{Delocalisation}$

Because they have a higher electron density than neutral substances, negative charged ions are more nucleophilic.
SH^-, OH^-, PhO^-, AcO^- > H₂O

In the case of protic solvents, the nucleophilicity of the ions with larger sizes (along the group) will be higher.
SH^- > OH^-, PhO^-, AcO^- > H₂O

Because there is less electron density available, the nucleophilicity decreases as delocalization increases. It is a weaker nucleophile than PhO^- because there is an analogous resonance to that in AcO^-.
SH^- > OH^- > PhO^- > AcO^- > H₂O

Hence option B is correct.

Note: The order of the aforementioned nucleophiles' nucleophilicities with respect to their fundamental strengths has already been established, but when these nucleophiles function in a protic solvent like water, the degree of hydration that occurs to the nucleophile is also crucial.