Which of the following order/statement (s) is/are correct?...

Which of the following order/statement (s) is/are correct? (a) In $S_{N} 1$ reactions, $\mathrm{AgNO}_{3}$ increases the rates of solvolysis (b) Soft bases have enhanced nucleophilicities towards $\mathrm{S}_{\mathrm{N}} 2$ reactions. (c) $\mathrm{PhSO}_{3}^{-}>\mathrm{Cl}_{3} \mathrm{CCOO}^{-}>\mathrm{CH}_{3} \mathrm{COO}^{-}>\mathrm{PhO}^{-}$(leav- ing group ability) (d) HCOOH may lead a tertiary halide towards $\mathrm{S}_{\mathrm{N}}^{2} .$

Which of the following order/statement (s) is/are correct?
(a) In $S_{N} 1$ reactions, $\mathrm{AgNO}_{3}$ increases the rates of solvolysis
(b) Soft bases have enhanced nucleophilicities towards $\mathrm{S}_{\mathrm{N}} 2$ reactions.
(c) $\mathrm{PhSO}_{3}^{-}>\mathrm{Cl}_{3} \mathrm{CCOO}^{-}>\mathrm{CH}_{3} \mathrm{COO}^{-}>\mathrm{PhO}^{-}$(leav-
ing group ability)
(d) HCOOH may lead a tertiary halide towards $\mathrm{S}_{\mathrm{N}}^{2} .$

Key Concepts

Activation of Leaving Groups via Protonation

Protonation is a technique used to convert a poor leaving group into a good one by forming a neutral species that is more easily dissociated from the substrate. In substitution reactions, especially when dealing with substrates that have less favorable leaving groups, the use of an acid can facilitate the reaction by increasing the leaving group ability and lowering the energy barrier for the reaction.

Leaving Group Ability in Substitution Reactions

The ability of a leaving group to depart from the substrate plays a critical role in both SN1 and SN2 reactions. Good leaving groups are typically those that can stabilize the negative charge once they have left the substrate, often through resonance or electronegativity. Trends in leaving group ability help predict reaction rates and mechanisms, where more stable anions equate to better leaving groups.

Nucleophilicity Trends in SN2 Reactions

SN2 reactions occur in a single concerted step, where a nucleophile attacks the electrophilic carbon from the opposite side of the leaving group. The rate and efficiency of these reactions depend critically on the nucleophilicity of the attacking species. Nucleophilicity can be influenced by factors such as charge, solvent, and polarizability. Generally, soft bases, which are more polarizable, can have enhanced nucleophilicities compared to hard bases in SN2 reactions, particularly in polar aprotic solvents.

SN1 Reaction Mechanism and Rate Acceleration

SN1 reactions involve a two-step mechanism where the rate-determining step is the formation of a carbocation intermediate, often called solvolysis when the solvent acts as the nucleophile. The rate of the reaction can be increased by agents that stabilize the leaving group or remove it from the reaction medium, thereby shifting the equilibrium toward carbocation formation. Additives such as silver salts can precipitate halide ions, driving the reaction forward.

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