Reactions at sp3 Carbon: Leaving Groups Other Than Halogen

In this lesson we will reason by analogy to predict the outcome of new reactions (e.g., predictions of leaving group ability)

Breaking C–O bonds: –OH is a poor leaving group and does NOT directly participate in [SN1], [SN2], [E1] or [E2] reactions.

Why is –OH a poor leaving group?

Why –OH is a Poor LG

-10.0

-9.0

15.7

Protonation of an Alcohol Improves the Leaving Group Ability

___ _____________ ____________ LG Conjugate Acid pKa of

Conjugate Acid

H2O H3O+ -1.7

Protonated Alcohol as a Leaving Group

Reactions of Protonated Ethers [SN2] or [SN1] pathway depending on: •  steric accessibility of nucleophile •  stability of carbocation

If a stable carbocation can form, the reaction will follow the [SN1] pathway (otherwise [SN2])

[SN2]

[SN1]

Examples - Nucleophilic Substitution

Examples - Water Elimination (dehydration)