Sn1, sn2, e1, e1cb, e2 overview

Stereochemistry for Sn1

• Gives racemic mixtures due to planar intermediate

Stereochemistry SN2

Inversion of configuration

Substrate reactivity for E1cb reaction

• Poor leaving groups + electron-withdrawing group needed nearby.

• Electron-withdrawing group: e.g. carbonyl

Sn reaction vs E reaction in second reactant

For Sn reactions its a nucleophile

For Elimination reactions its a base.

Substitution type of nucleophile

• Sn1: Doesn’t matter (unimolecular)

• Sn2: Strong nucleophile (bimolecular)

  • See nucleophile strength in picture

Elimination reaction type of base

• E1: Weak base is fine (unimolecular)

• E1cb: Requires a base strong enough to deprotonate

• E2: Requires strong base (bimolecular)

  • Check picture for strong base.

Leaving group trend for elimination and substitution reactions

• For Sn1, Sn2, E1 and E2 a good leaving group is required. Good leaving groups are molecules that are bigger, weak bases (aka the conjugates of strong acids)

• E1cb will have bad leaving groups such as OH-

Solvents for substitution reactions

• Sn1: Polar protic (stabilizes carbocation intermediate)

• Sn2: Polar aprotic (enhances nucleophilicity)

Solvents for elimination reactions

• E1: Polar protic (stabilizes carbocation intermediate

• E1cb: Polar solvents preferred to stabilize carbonation

• E2: Polar aprotic preferred for strong base

Nucleophilic substitution between sn2 and e2

• Sn2 needs less substitution to be able to attack the carbon

• E2 needs a little more substitution because the resulting alkene is more stable that way.

Geometry E2 reaction

• Prefers anti-periplanar over syn-periplanar

• If the substrate of the E2 reaction has the hydrogen that is leaving cis to the halogen than another hydrogen has to be used.

E1 geometry

• Follows Zaitsev's rule

• More substituted alkene is better

When is a leaving group protonated?

In an Sn1 or E1 reaction