Chapter 17 — Key Takeaways

What you should leave Chapter 17 with

  1. Alkynes have C≡C triple bonds: 1 σ + 2 π. sp hybridization gives linear geometry (~180°).

  2. Bond properties: C≡C bond length ~1.20 Å (shortest); bond strength ~200 kcal/mol (strongest of the C-C bonds).

  3. Terminal alkyne C-H pKa is ~25 — uniquely acidic among hydrocarbons. The sp orbital character (50% s-character) stabilizes the carbanion conjugate base.

  4. Hybridization-acidity relationship: - sp³ alkane C-H: pKa ~50. - sp² alkene C-H: pKa ~44. - sp alkyne C-H: pKa ~25. More s-character = more acidic.

  5. Bases that deprotonate terminal alkynes: NaNH₂ (standard), n-BuLi, NaH. Bases that don't: NaOH, NaOEt (too weak).

  6. Alkynide anion (acetylide) is a strong base AND a strong carbon nucleophile.

  7. Alkyne addition reactions are similar to alkene's but can go twice: - HX: Markovnikov → vinyl halide → gem-dihalide (with second HX). - X₂: trans-dihaloalkene (anti) → tetrahalide (with second X₂). - H₂O / HgSO₄ / H₂SO₄: Markovnikov → enol → methyl ketone (terminal alkyne; via tautomerism). - Hydroboration / H₂O₂: anti-Markovnikov → enol → aldehyde (terminal alkyne). - H₂ / Pd/C: full reduction → alkane. - H₂ / Lindlar Pd: → cis-alkene (poisoned Pd; syn). - Na in NH₃(l): → trans-alkene (radical anion mechanism; anti).

  8. Selective alkene formation from alkyne: - cis-alkene: H₂ + Lindlar Pd. - trans-alkene: Na in NH₃(l).

This is one of the most-used selectivity tools in natural product synthesis.

  1. Alkynide alkylation: RC≡C⁻ + R'X → RC≡C-R' (new C-C bond). SN2 mechanism. Works for primary alkyl halides (and methyl).

  2. Alkynide carbonyl addition: RC≡C⁻ + R'COR'' → propargyl alcohol (after workup). Same reactivity as Grignard, but C-end carbanion.

  3. Sonogashira coupling (Ch 37): aryl halide + terminal alkyne + Pd + Cu → aryl alkyne. Modern alternative to alkynide-SN2 for aryl halides.

  4. Why hydration gives methyl ketone vs hydroboration gives aldehyde:

    • Hydration is Markovnikov: OH at internal C → enol with OH on internal → methyl ketone.
    • Hydroboration is anti-Markovnikov: B at terminal C → vinyl borane with B on terminal → enol with OH on terminal → aldehyde.

  5. Industrial acetylene chemistry (Reppe, 1928+): vinyl chloride (PVC), vinyl acetate (PVA), acrylonitrile (acrylic fibers, ABS), 2-butyne-1,4-diol. Largely replaced by ethylene-based feedstocks today.

  6. Click chemistry (CuAAC): Sharpless 2022 Nobel. Cu-catalyzed azide + alkyne → 1,2,3-triazole. Used in bioconjugation.

  7. Cyclic alkynes: cyclooctyne is the smallest stable cyclic alkyne. Strain-promoted azide-alkyne cycloaddition (SPAAC) uses cyclooctyne without Cu.

  8. Spectroscopy:

    • IR: ≡C-H sharp at 3300 cm⁻¹; C≡C at 2100-2260 (weak).
    • ¹H NMR: terminal ≡C-H at δ 1.7-3.1 (less deshielded due to π anisotropy).
    • ¹³C NMR: C≡C at δ 70-90 (less downfield than C=C).

  9. Connection to alkene chemistry: alkynes + 1 equivalent of various reagents = Z-alkene (or specific functional groups). Alkynes + 2 equivalents = saturated/double-functionalized.

  10. Industrial scale: acetylene is still produced in millions of tons/year for welding and specialty chemistry. Modern alkyne chemistry is more often pharmaceutical-scale.

  11. Synthesis strategy: when planning a synthesis with a Z- or E-alkene, often the cleanest route is via an alkyne + selective reduction.

  12. Mastery of Chapter 17 sets up Chapter 18 (radicals) and Chapter 19 (Diels-Alder), and is essential for natural product synthesis (Chs 32-38).

Cross-references

  • Chapter 2 — sp hybridization.
  • Chapter 15-16 — Alkene chemistry (analogous patterns).
  • Chapter 18 — Radical reactions (alternative addition mechanism).
  • Chapter 19 — Diels-Alder (alkyne can be dienophile).
  • Chapter 25 — Carbonyl addition (alkynide as nucleophile, similar to Grignard).
  • Chapter 27 — α-carbon chemistry (alkynide is a special case).
  • Chapter 36 — Reduction methods (catalytic hydrogenation).
  • Chapter 37 — Sonogashira coupling.
  • Chapter 38 — Total synthesis (alkynes as alkene precursors).
  • Appendix C — Reaction summary.
  • Appendix F — Named reactions.

Study tip

For each alkyne reaction, ask: 1. Once or twice? Is one π bond enough or both? 2. Markovnikov or anti-Markovnikov? 3. Stereo of resulting alkene/dialkene? cis (Lindlar) or trans (Na/NH₃)? 4. Terminal alkyne special features? Acidity, alkynide formation, methyl ketone vs aldehyde.

If you can answer these for any alkyne reaction, you've internalized Chapter 17.