Chapter 15 — Key Takeaways

What you should leave Chapter 15 with

  1. An alkene has a C=C double bond consisting of one σ + one π. The π bond's two electrons sit above and below the plane of the C=C — they are the nucleophile in alkene reactions.

  2. Alkene structure: - sp² hybridized carbons. - Trigonal planar geometry; ~120° bond angles. - C=C bond length ~1.34 Å; bond strength ~146 kcal/mol (π contributes ~66 kcal/mol). - Restricted rotation (~60 kcal/mol energy cost).

  3. E/Z nomenclature: based on CIP priority. - E: higher-priority substituents on opposite sides. - Z: higher-priority substituents on the same side. - Often (but not always) corresponds to trans/cis.

  4. Alkene stability increases with substitution (more alkyl groups → more stable): $$\text{tetrasubstituted} > \text{trisubstituted} > \text{disubstituted (trans)} > \text{disubstituted (cis)} > \text{monosubstituted} > \text{unsubstituted}$$ Reason: hyperconjugation (alkyl C-H σ bonds donate into π* and stabilize the C=C).

  5. Electrophilic addition is the central reaction of alkenes. Mechanism: - Step 1 (slow): π bond attacks electrophile (e.g., H of HCl); carbocation forms. - Step 2 (fast): nucleophile (e.g., Cl⁻) attacks the carbocation.

  6. Markovnikov's rule (1869): when HX adds to an unsymmetrical alkene, H goes to the C with more H's; X goes to the C with fewer H's (the more-substituted one).

  7. Why Markovnikov works: the more-substituted carbocation is more stable (3° > 2° > 1°). By Hammond postulate, the lower-energy intermediate goes through the lower-energy TS.

  8. Carbocation stability ranking: $$3° > 2° > 1° > methyl$$ Due to hyperconjugation and inductive effects.

  9. Anti-Markovnikov addition occurs in two scenarios: - HBr + peroxides: radical chain mechanism (Ch 18). - Hydroboration-oxidation: B-H adds with B at the less-substituted C (Ch 16).

  10. Br₂ addition to alkenes goes through a bromonium ion (3-membered cyclic intermediate). Br⁻ attacks the bromonium ion from the opposite face → anti addition (stereospecific).

  11. For HX addition, stereochemistry of the product is mixed because the carbocation is planar and halide can attack either face. So HX gives a mixture of cis and trans products.

  12. Acid-catalyzed hydration: alkene + H₂O + H⁺ → alcohol (Markovnikov). Used industrially for ethanol, sec-butanol, etc.

  13. Carbocation rearrangements (1,2-hydride shift, 1,2-methyl shift) can occur. Watch for: 2° cation adjacent to a quaternary C (potential 1,2-methyl shift to give 3° cation).

  14. Spectroscopy of alkenes:

    • IR: C=C stretch 1620–1680 cm⁻¹; vinyl C-H stretch 3000–3100 cm⁻¹.
    • ¹H NMR: vinyl H at δ 4.5–6.5; trans coupling 12-18 Hz, cis coupling 6-12 Hz.
    • ¹³C NMR: vinyl C at δ 100–145.

  15. Industrial alkenes are central:

    • Ethylene: 200+ million tons/year (polymerized to polyethylene).
    • Propylene: 100+ million tons/year (polypropylene).
    • Butadiene: 10+ million tons/year (rubber).
    • Styrene: 25+ million tons/year (polystyrene).

  16. Natural rubber is cis-1,4-polyisoprene. Vulcanization (Goodyear, 1839) cross-links with sulfur — alkene addition chemistry on industrial scale.

  17. Alkene chemistry symmetry with Part III: Part III had alkyl halide + nucleophile (electrophilic C, nucleophile attacks). Part IV has alkene + electrophile (nucleophilic π, electrophile attacks). Same chemistry; flipped roles.

  18. Cis vs trans alkene properties: trans is generally more stable; cis has more steric strain. Both have characteristic IR and NMR features.

  19. Connection to elimination: alkene formation by elimination (Ch 12) is the reverse of alkene addition. Same mechanism in reverse; same product preference (more-stable alkene = more-stable cation).

  20. Mastery of Chapter 15 sets up Chapter 16 (broader survey of alkene additions) and the rest of Part IV.

Cross-references

  • Chapter 2 — Bonding and hybridization (foundation).
  • Chapter 5 — Conformational analysis (alkenes have specific geometry).
  • Chapter 7 — Stereochemistry (E/Z, cis/trans).
  • Chapter 10-11 — SN2 and SN1 (electrophile-nucleophile partner inversion).
  • Chapter 12 — Elimination (reverse of addition).
  • Chapter 16 — Full alkene addition reaction survey.
  • Chapter 17 — Alkynes (different π system).
  • Chapter 18 — Radical reactions (anti-Markovnikov HBr).
  • Chapter 19 — Diels-Alder cycloaddition (alkene as dienophile).
  • Chapter 37 — Olefin metathesis and Ziegler-Natta polymerization.
  • Appendix C — Reaction summary.
  • Appendix F — Named reactions.

Study tip

For each alkene reaction, predict: 1. Regiochemistry: where do H and X go (Markovnikov)? 2. Stereochemistry: anti (Br₂ via bromonium) or mixed (HX via cation)? 3. Rearrangements: is a better cation accessible?

If you can answer these for any HX, X₂, or H₂O addition, you've internalized Chapter 15. Then Chapter 16 (more reaction types) follows logically.