SN2 Effect of Solvent

The Role of Solvent in SN2 Reactions

I. Polar Aprotic Solvents Favor SN2 Reactions

SN2 reactions proceed most efficiently in polar aprotic solvents. These solvents have a significant dipole moment (they are polar), but lack hydrogen atoms bonded directly to highly electronegative atoms (like O or N), so they do not participate in hydrogen bonding.

Polar aprotic solvents solvate cations well but do not effectively solvate anions. As a result, the nucleophile remains largely "naked"—unsolvated and highly reactive—which significantly enhances the SN2 reaction rate.

In acetonitrile (ACN), Br⁻ is poorly solvated, making it a stronger nucleophile for SN2 reactions.

In water, Br⁻ is stabilized by hydrogen bonding, reducing its nucleophilicity and slowing SN2 reactivity

II. Polar Protic Solvents Disfavor SN2 Reactions

Polar protic solvents contain hydrogen atoms bonded to oxygen or nitrogen, enabling them to form strong hydrogen bonds with nucleophiles.

These hydrogen bonds solvate and stabilize anionic nucleophiles, effectively "shielding" them and decreasing their reactivity in SN2 reactions. As a result, SN2 reactions proceed much more slowly in protic solvents like water or alcohols.

III. Common Polar Aprotic Solvents

Solvents that are especially effective for promoting SN2 reactions include:

DMSO (dimethyl sulfoxide)
DMF (dimethylformamide)
Acetone
Acetonitrile (CH₃CN)