Chapter 12 — Key Takeaways

The 12 most important things to leave Chapter 12 knowing.

1. $E2$ is concerted: one-step elimination. Three arrows: base attacks β-H, C-H bond shifts to form $\pi$ bond, C-X bond breaks. No intermediate. One TS.

2. $E2$ requires anti-periplanar geometry. The β-H and the leaving group must be 180° apart on opposite faces of the C-C bond. This is forced by orbital overlap (the C-H electrons must align with C-X $\sigma^*$).

3. $E2$ in cyclohexane requires axial-axial geometry. In the chair conformation, both the β-H and the leaving group must be axial for E2. Substrates locked in the equatorial-LG chair (e.g., by a bulky t-butyl) cannot undergo E2.

4. Second-order kinetics: rate = k[substrate][base]. Both reactant concentrations affect rate.

5. Zaitsev's rule: more-substituted alkene preferred. Comes from greater alkene stability and lower TS energy (Hammond postulate).

6. Hofmann's rule (with bulky base): less-substituted alkene preferred. The bulky base can't reach the more-hindered β-H; it goes for the methyl C-H instead.

7. $E1$: two-step mechanism via carbocation. Same first step as $S_N1$. First-order kinetics.

8. $E1$ products: usually Zaitsev (the more-stable alkene forms preferentially from the cation). Rearrangements possible if cation can rearrange.

9. $E1 + S_N1$: share the carbocation intermediate. From the cation, water/methanol attack gives SN1; β-H loss gives E1. Both happen in solvolysis; the ratio depends on T (high T favors E1).

10. Acid-catalyzed dehydration of alcohols is E1: protonation of OH, loss of water to form cation, loss of H to give alkene.

11. Hofmann elimination ($R_4N^+OH^-$ + heat → alkene + amine): gives Hofmann (less-substituted) alkene because the bulky $R_4N^+$ leaving group prefers the less-hindered β-H.

12. Cope elimination ($R_3N^+O^-$): syn-periplanar 5-membered cyclic TS, different stereochemistry from anti-periplanar E2.

The habit to leave with: every time you see a substrate with a leaving group AND a β-H, ask yourself: can elimination compete with substitution? The substrate substitution class, base bulk, and temperature determine the answer. By Chapter 13, you'll be running this analysis automatically.

Connections forward:

• Chapter 13: the SN/E decision framework — given a substrate, base, solvent, T, predict which of $S_N2$, $S_N1$, $E2$, $E1$ dominates.
• Chapter 15: alkenes and electrophilic addition — the reverse of E1/E2. Once you have an alkene from elimination, you can do many additions.
• Chapter 16: alkene addition reactions — directly use the alkenes you make by elimination.
• Chapter 27: α-carbon chemistry — α to a carbonyl, the α-H is acidic enough to be removed even with weak bases. This is conceptually parallel to E2 elimination but on a carbonyl rather than a halide.