Nucleophilic Substitution and Elimination Reactions

Product ratio in nucleophilic substitution vs elimination

The relative rates of substitution vs elimination reactions.

Factors affecting reaction rates of substitution vs elimination

Reaction conditions and the specific structure of the reagents.

Requirements for elimination reactions

Brønsted bases to accept protons.

Alpha carbons and alpha protons

Alpha carbons are attached directly to the leaving group; alpha protons are hydrogen atoms on these carbons.

Beta protons

Hydrogen atoms on carbons not directly attached to the leaving group.

E2 elimination reaction

The proton is removed at the same time as the leaving group departs.

E1 elimination reaction

The leaving group departs first, then the proton is removed.

E1cb elimination reaction

The proton is removed first, then the leaving group departs.

Mechanism of an E2 reaction

A 1-step mechanism where proton removal and leaving group departure occur simultaneously.

E2 reaction rate dependence

Both the [Brønsted base] and the [alkyl halide].

E2 rate-determining step

Bimolecular.

E2 reactions forming multiple products

Yes, if there are different types of beta-hydrogens.

Stereoselectivity in E2 eliminations

The eliminated groups must be anti-periplanar (180° dihedral angle).

E2 reactions and staggered conformations

Because it's more stable and maximizes distance between base and leaving group.

Mechanism of an E1 reaction

A 2-step mechanism: leaving group departs first, then proton is removed.

Rate-determining step in E1 reactions

Step 1; depends only on the concentration of the alkyl halide.

E1 rate-determining step

Unimolecular.

Intermediate in E1 reactions

A carbocation.

E1 mechanism favorability

When the carbocation intermediate is stable.

Mechanism of an E1cb reaction

A 2-step mechanism: proton is removed first forming a carbanion, then the leaving group departs.

Rate-determining step in E1cb reactions

Step 2; depends on the concentration of the carbanion intermediate.

Formation when a molecule loses a proton in E1cb

A conjugate base (anion).

Elimination in E1cb reactions

Via the conjugate base of the alkyl halide.

E1cb mechanism favorability

When the intermediate carbanion is stable due to lone pair delocalization.