Question

Which is the more reactive nucleophile in polar protic solvent?
(A)${{F}^{-}}$
(B)$C{{l}^{-}}$
(C)$B{{r}^{-}}$
 (D)${{I}^{-}}$

Answer 1

Hint: Polar protic solvents are used in $S{{N}_{1}}$ reactions, which involves carbocation as an intermediate. Also, the ability of nucleophiles to participate in hydrogen bonding decreases down the group.

Complete answer:
Polar protic solvents are those which have polarity and high dielectric constant. They release acidic hydrogen and shows hydrogen bonding. They are generally used in dissolving salts. Polar protic solvent includes water, hydrogen fluoride, ammonia and formic acid. A polar protic solvent can take part in hydrogen bonding with a nucleophile, because of this reason, the lone pair of nucleophiles are interacting with the electron poor hydrogen of the solvent, which decreases the reactivity of the nucleophile. The ability of nucleophiles to participate in hydrogen bonding decreases down the group. This makes fluoride, the strongest hydrogen acceptor, and iodine, the weakest hydrogen acceptor. This explains that the lone pairs of iodide ion will be considerably more “free” than those of fluoride, resulting in higher rates and therefore greater nucleophilicity. According to this , we have the order of nucleophilicity of ions in polar protic solvent as follows:
     \[{{F}^{-}} < C{{l}^{-}} < B{{r}^{-}} < {{I}^{-}}\]
Polar protic solvents are favourable for $S{{N}_{1}}$reactions.In reaction mechanism, We all know that , $S{{N}_{1}}$ reaction involves formation carbocation. The stability of carbocation increases from primary to secondary to tertiary. So when tertiary carbocation undergoes $S{{N}_{1}}$, it requires a nucleophile or bigger size ion, which can polarize and approach towards the more hindered site. More the size of an anion, more is the polarizing power. All of the given options, iodide ions, have the biggest size.

Therefore, the correct answer is option (D).

Note:
Order of nucleophilicity is reversed for polar aprotic solvent(e.g. DMSO,Acetone). The most basic nucleophile is the most effective nucleophile in polar aprotic solvent. Polar aprotic solvents are best used for $S{{N}_{2}}$ reactions.