What factors affect SN2 reactions?

Question

Nolan Allen · Accepted Answer

The nucleophile, electrophile, solvent, and physical conditions (temperature, pressure, etc.) must all be considered in any reaction.

As long as the solvent and reaction conditions don't interfere, most problems are broad enough for us to concentrate primarily on the electrophile and nucleophile.

For #SN_2# reactions, we check the nucleophile for its nucleophilicity, basicity, and steric hindrance. If the nucleophile is part of a long molecule, we should also consider potential functional groups that it can interact with

A good nucleophile is less bulky so that it can approach and interact with the electrophile, and it has an unstable negative charge but isn't so basic that it would rather abstract a hydrogen instead of attack the electrophile.

A good electrophile as a net polar positive moment or charge and is not sterically hindered. For tertiary electrophiles, #SN_2# is virtually impossible.

Regarding solvents, we would prefer an aprotic, nonpolar solvent; the reason for this is that we don't want any solvent molecules to lessen the electrophilicity or nucleophilicity of the reactants; additionally, we want our nucleophile to target the electrophilic reactant rather than a hydrogen atom.

Isaac Aronson · Answer

undefined The factors that affect SN2 (substitution nucleophilic bimolecular) reactions include:

1. **Nucleophile Strength**: Strong nucleophiles favor SN2 reactions.
2. **Substrate Structure**: Primary substrates are most reactive, followed by secondary, and then tertiary substrates.
3. **Solvent**: Polar aprotic solvents (e.g., acetone, DMSO) favor SN2 reactions by stabilizing the transition state.
4. **Leaving Group Ability**: Good leaving groups promote SN2 reactions.
5. **Steric Hindrance**: Bulky groups near the reaction site hinder SN2 reactions.
6. **Temperature**: Higher temperatures can increase the rate of SN2 reactions, but too high temperatures can favor other reaction pathways.
7. **Concentration**: Higher concentrations of reactants generally increase the rate of SN2 reactions.