Which reaction in each of the following pairs would you...

Which reaction in each of the following pairs would you expect to be faster? (a) The $\mathrm{S}_{\mathrm{N}} 2$ displacement by $\mathrm{I}^{-}$ on $\mathrm{CH}_{3} \mathrm{Cl}$ or on $\mathrm{CH}_{3} \mathrm{OTos}$ (b) The $\mathrm{S}_{\mathrm{N}}^{2}$ displacement by $\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}$ on bromoethane or on bromocyclohexane (c) The $\mathrm{S}_{\mathrm{N}}$ 2 displacement on 2 -bromopropane by $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}$ or by CN $^{-}$ (d) The $\mathrm{S}_{\mathrm{N}} 2$ displacement by $\mathrm{HC}=\mathrm{C}^{-}$ on bromomethane in benzene or in acetonitrile

Which reaction in each of the following pairs would you expect to be faster?
(a) The $\mathrm{S}_{\mathrm{N}} 2$ displacement by $\mathrm{I}^{-}$ on $\mathrm{CH}_{3} \mathrm{Cl}$ or on $\mathrm{CH}_{3} \mathrm{OTos}$
(b) The $\mathrm{S}_{\mathrm{N}}^{2}$ displacement by $\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}$ on bromoethane or on bromocyclohexane
(c) The $\mathrm{S}_{\mathrm{N}}$ 2 displacement on 2 -bromopropane by $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}$ or by CN $^{-}$
(d) The $\mathrm{S}_{\mathrm{N}} 2$ displacement by $\mathrm{HC}=\mathrm{C}^{-}$ on bromomethane in benzene or in acetonitrile

Key Concepts

SN2 Reaction Mechanism

The SN2 mechanism is a one-step bimolecular nucleophilic substitution reaction where the nucleophile attacks the electrophilic carbon from the opposite side of the leaving group. This leads to an inversion of stereochemistry at the carbon center. The reaction rate depends on both the concentration of the nucleophile and the substrate, making it sensitive to factors like substrate structure, nucleophile strength, leaving group ability, and solvent environment.

Leaving Group Ability

Leaving group ability refers to how readily a group can depart with its bonding electrons during a reaction. A good leaving group stabilizes the negative charge it acquires when it leaves, facilitating a faster substitution reaction. In the context of SN2 reactions, substrates with superior leaving groups tend to react more rapidly.

Substrate Structure and Steric Hindrance

The structure of the substrate, particularly the degree of steric hindrance around the electrophilic carbon, plays a crucial role in determining the rate of SN2 reactions. Primary substrates typically react faster than secondary or tertiary ones because they offer less steric obstruction to the attacking nucleophile. The cyclic or branched nature of substrates can also influence accessibility, thereby affecting the reaction rate.

Nucleophilicity

Nucleophilicity is a measure of a species' ability to donate an electron pair and form a new chemical bond. In SN2 reactions, a more nucleophilic reagent will generally lead to a faster reaction. The strength of a nucleophile can be influenced by its charge, electronegativity, steric factors, and the solvent in which the reaction takes place.

Solvent Effects

Solvent effects are important in SN2 reactions because the solvent can stabilize either the nucleophile or the transition state, thus influencing the reaction rate. Polar aprotic solvents, for example, tend to enhance the reactivity of nucleophiles by not excessively stabilizing them, while polar protic solvents can hinder nucleophilicity by forming hydrogen bonds with the nucleophile.

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