Which substance in each of the following pairs is more...

Which substance in each of the following pairs is more reactive as a nucleophile? Explain. (a) $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{~N}^{-}$ or $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}$ (b) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~B}$ or $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}$ (c) $\mathrm{H}_{2} \mathrm{O}$ or $\mathrm{H}_{2} \mathrm{~S}$

Which substance in each of the following pairs is more reactive as a nucleophile? Explain.
(a) $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{~N}^{-}$ or $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}$
(b) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~B}$ or $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}$
(c) $\mathrm{H}_{2} \mathrm{O}$ or $\mathrm{H}_{2} \mathrm{~S}$

Key Concepts

Nucleophilicity

Nucleophilicity is the ability of a species to donate an electron pair to an electrophile. It depends on several factors including charge, electronegativity, steric hindrance, and the solvent used. Generally, species with higher electron density and less steric bulk are more nucleophilic.

Basicity

Basicity refers to the tendency of a species to accept a proton. There is often a parallel between basicity and nucleophilicity: stronger bases (especially those with a negative charge) are usually better nucleophiles because they have more available electron density for donation.

Charge Effect

The presence of a negative charge generally enhances nucleophilicity compared to neutral species. A negatively charged nucleophile has a greater electron density that is more readily available for bonding with an electrophile, increasing its reactivity.

Steric Effects

Steric hindrance occurs when bulky groups around the reactive center interfere with the approach of the nucleophile to the electrophile. Less sterically hindered nucleophiles can more easily attack electrophilic sites, making them more reactive.

Periodic Trends and Polarizability

As you move down a group in the periodic table, atoms become larger and more polarizable, which can increase nucleophilicity despite lower basicity. This often explains why a nucleophile with a larger, more polarizable atom (such as sulfur compared to oxygen) can be more reactive under certain conditions.

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