Overview
This lecture reviews the key features, mechanisms, and distinguishing clues for SN1, SN2, E1, and E2 reactions in organic chemistry.
SN1 Reaction Overview
- SN1 stands for unimolecular nucleophilic substitution.
- Competes with E1 reaction; both involve a carbocation intermediate.
- Mechanism starts as the leaving group departs, leaving a positively charged carbocation.
- Solvent nucleophile attacks the carbocation, forming a new bond.
- Deprotonation restores neutrality.
- Clues: carbocation intermediate, rate-determining step, nucleophile replaces the leaving group.
SN2 Reaction Overview
- SN2 stands for bimolecular nucleophilic substitution.
- One-step mechanism, no intermediate formed.
- Good nucleophile attacks carbon, displacing the leaving group directly.
- Results in inversion of configuration if starting material is chiral.
- Clues: no carbocation intermediate, fast reaction, nucleophile replaces leaving group in one step.
E1 Reaction Overview
- E1 stands for unimolecular beta elimination.
- Competes with SN1 and shares the carbocation intermediate.
- Leaving group departs, forming carbocation.
- Base removes beta hydrogen, electrons form a pi-bond (double bond).
- Clues: carbocation intermediate, double bond between alpha and beta carbon.
E2 Reaction Overview
- E2 stands for bimolecular beta elimination.
- One-step, no intermediate; requires strong base.
- Base removes beta hydrogen while leaving group departs simultaneously.
- Forms a double bond (Zaitsev product favored).
- Clues: no carbocation, fast one-step reaction, double bond at more substituted beta carbon.
Reaction Determination Criteria
- Evaluate the alkyl chain structure.
- Assess strength and type of nucleophile or base.
- Consider the solvent (protic/aprotic).
- Analyze the leaving group quality.
Key Terms & Definitions
- Carbocation — Positively charged carbon intermediate.
- Leaving group — Atom/group that departs with a pair of electrons.
- Beta hydrogen — Hydrogen on carbon adjacent to the site bearing the leaving group.
- Pi-bond — Double bond formed between two carbons.
- Solvolysis — Reaction where the solvent acts as the nucleophile.
- Zaitsev product — More substituted alkene formed preferentially in elimination.
Action Items / Next Steps
- Review and practice the four reaction mechanism criteria.
- Watch the full substitution-elimination tutorial series for detailed mechanisms and examples.
- Complete the practice quiz and review the cheat sheet at leah4sci.com/substitution-elimination.