Chapter 4 Summary & Reaction Map (Alkynes)

Alkynes parallel alkenes but add twice and offer unique C–C bond-forming paths via acetylides.

Big Reactions

• Acetylide formation: RC≡CH + NaNH₂ → RC≡C⁻; SN2 on 1° halides to extend chains.
• Double elimination: Vicinal/geminal dihalides + strong base → alkynes.
• Hydrogenation: H₂/Pt → alkane; Lindlar → cis-alkene; Na/NH₃(l) → trans-alkene.
• Hydrohalogenation: 1 equiv HX → vinyl halide (Markovnikov); excess → geminal dihalide. Peroxide HBr can give anti-Markovnikov (less common).
• Halogenation: 1 equiv X₂ → trans dihaloalkene; excess → tetrahalide.
• Hydration: Hg²⁺/H₂O/H⁺ → ketone (Markovnikov, terminal → methyl ketone); Sia₂BH then H₂O₂/NaOH → aldehyde from terminal (anti-Markovnikov), ketones from internal.
• Oxidative cleavage: Hot KMnO₄ or O₃/oxidative workup → carboxylic acids (terminal carbon → CO₂).

Reaction Map Notes

• Build up: Alkyl dihalide → (2× base) → alkyne; terminal alkyne → acetylide → SN2 → internal alkyne.
• Interconvert: Alkyne ⇄ alkene via partial reductions (cis/trans); alkene → alkyne via Br₂ then double elimination.
• Functionalize: Add HX/X₂/H₂O to get vinyl/gem dihalides, alkenes, carbonyls; cleave to acids under strong oxidants.

Quick Checks

• Terminal alkyne pKₐ ~25 → needs strong base.
• Choose Lindlar vs Na/NH₃ for cis vs trans alkenes.
• Terminal hydration: Hg²⁺ → ketone; hydroboration → aldehyde.
• Excess HX/X₂ drives to geminal/tetra products.