Which would you expect to be the stronger nucleophile in a...

Which would you expect to be the stronger nucleophile in a polar aprotic solvent? (a) $\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}$ or $\mathrm{CH}_{3} \mathrm{O}^{-}$ (b) $\mathrm{H}_{2} \mathrm{O}$ or $\mathrm{H}_{2} \mathrm{S}$ (c) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{P}$ or $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N}$

Which would you expect to be the stronger nucleophile in a polar aprotic solvent?
(a) $\mathrm{CH}_{3} \mathrm{CO}_{2}^{-}$ or $\mathrm{CH}_{3} \mathrm{O}^{-}$
(b) $\mathrm{H}_{2} \mathrm{O}$ or $\mathrm{H}_{2} \mathrm{S}$
(c) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{P}$ or $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N}$

Key Concepts

Solvent Effects on Nucleophilicity

This concept explains how the nature of a solvent influences the reactivity of nucleophiles. In polar aprotic solvents, anions are poorly solvated because the solvent molecules do not form strong hydrogen bonds with them. This leads to a nucleophilicity more closely correlated with the intrinsic properties of the nucleophile, such as basicity and polarizability, rather than being heavily influenced by solvation.

Basicity vs. Nucleophilicity

While basicity measures the ability of a species to accept a proton, nucleophilicity refers to its tendency to donate an electron pair to an electrophile in a chemical reaction. These two properties are related but not identical; factors like steric hindrance and solvent interactions can cause deviations between a species’ basicity and its reactivity as a nucleophile.

Steric Hindrance

Steric hindrance describes how the size and spatial arrangement of groups around the reactive center of a nucleophile can impede its approach to an electrophile. Bulky groups near the nucleophilic center can lower the reactivity by creating physical barriers, reducing the rate at which the nucleophile can attack the electrophile.

Polarizability

Polarizability is the ease with which the electron cloud of an atom or molecule can be distorted. More polarizable nucleophiles, which often contain larger and less electronegative atoms, can stabilize transition states better and react more readily with electrophiles, enhancing their nucleophilic strength.

Electronegativity and Charge Distribution

The intrinsic electronegativity of the atoms involved plays a key role in nucleophilicity. Atoms with lower electronegativity tend to have more diffuse electron clouds, making their electrons more available for donation to electrophiles. This characteristic often leads to increased nucleophilic activity when other factors remain comparable.

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