Chapter Summary And Reaction Map

Chapter 6 Summary & Reaction Map

Chapter 6 Summary & Reaction Map (Alkyl Halide Reactivity)

Nucleophilic substitution and β-elimination dominate alkyl halide chemistry. Use this map to predict SN1/SN2/E1/E2.

Mechanism snapshots

  • SN2 (concerted): Backside attack, inversion, favored by primary > secondary, strong nucleophile, polar aprotic solvent; no rearrangements.
  • SN1 (carbocation): Ionization then nucleophile attack, racemization/partial inversion, favored by tertiary > secondary, good leaving group, polar protic, weak nucleophile; rearrangements possible.
  • E2 (concerted elimination): Strong base removes β-H as L.G. leaves; anti-periplanar geometry required; favored by strong (often bulky) base, secondary/tertiary substrates; Zaitsev alkene unless bulky base → Hofmann; no rearrangements.
  • E1 (carbocation then deprotonation): Same ionization as SN1; base (often solvent) removes β-H → alkene; favors tertiary/secondary in protic solvent, heat; rearrangements possible; usually Zaitsev alkene.

Decision cues

  • Substrate: methyl → SN2 only; primary → SN2 (E2 if bulky/very strong base); secondary → SN2 vs E2 (strong base) vs SN1/E1 (weak/protic); tertiary → E2 (strong base) or SN1/E1 (weak/protic).
  • Nucleophile/base: strong, unhindered nucleophile → SN2 (unless steric pushes E2); strong bulky base → E2; weak nucleophile/base → SN1/E1 if carbocation feasible.
  • Solvent: polar aprotic boosts SN2; polar protic stabilizes carbocations → SN1/E1.
  • Temp: heat pushes elimination (E1/E2).
  • Leaving group: better L.G. accelerates all; SN1/E1 need good L.G.

Reaction map (mental flow)

  1. Identify substrate class (Me, 1°, 2°, 3°).
  2. Ask: strong base? If yes → E2 likely (especially 2°/3°, or 1° with bulky base).
  3. If strong nucleophile but not strongly basic and substrate unhindered → SN2.
  4. Weak nucleophile/base + carbocation-capable substrate + protic solvent → SN1/E1 mix (heat → more E1).
  5. Special: allylic/benzylic halides can go SN1 or SN2 faster (resonance stabilization). Vinyl/aryl halides do not do SN1/SN2.

Quick comparisons

  • Stereochemistry: SN2 inversion; SN1 racemization; E2 anti-periplanar requirement can control alkene geometry; E1 gives thermodynamic (often trans/Zaitsev) alkene.
  • Rearrangements: Only in carbocation paths (SN1/E1).
  • Regioselectivity: E2/E1 usually Zaitsev; bulky base can give Hofmann.