Alkynes

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Alkynes — Key Facts for SLMC Medical (Sri Lanka)

• Alkynes are unsaturated hydrocarbons with a C≡C triple bond
• General formula: CₙH₂ₙ₋₂ (for one triple bond)
• Each carbon in the triple bond is sp-hybridized (180° bond angle, linear geometry)
• The triple bond consists of one σ-bond + two π-bonds
• ⚡ Exam tip: Addition reactions and acid-base chemistry of terminal alkynes are high-yield for SLMC

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Alkynes — SLMC Medical (Sri Lanka) Study Guide

What Are Alkynes?

Alkynes contain at least one carbon-carbon triple bond (C≡C). The simplest alkyne is ethyne (acetylene, C₂H₂): H–C≡C–H. Their general formula is CₙH₂ₙ₋₂ for one triple bond.

Nomenclature

Alkynes use the “-yne” suffix. Number the chain to give the triple bond the lowest possible number:

• Ethyne (C₂H₂): HC≡CH
• Propyne (C₃H₄): CH₃–C≡CH (methylacetylene)
• But-1-yne / But-2-yne

For compounds with both double and triple bonds, use “-en-yne” with the lowest possible numbers for both.

Electronic Structure

Each carbon in a C≡C triple bond is sp-hybridized:

• Two sp orbitals form σ-bonds at 180° angles (linear geometry)
• Two unhybridized p orbitals on each carbon form the two π-bonds (at 90° to each other)
• Total bond energy of C≡C is ~839 kJ/mol (strongest carbon-carbon bond)

The σ-framework: C(sp)–C(sp) σ-bond plus C(sp)–H σ-bonds. The π-system: two perpendicular π-bonds above and below/beside the internuclear axis.

Bond lengths: C≡C (~120 pm) < C=C (~134 pm) < C–C (~154 pm) Bond angles: C≡C–H = 180° (linear)

Physical Properties

Property	Observation
Boiling point	Increases with molecular size
Solubility	Insoluble in water; soluble in organic solvents
Density	Less than water
State	Ethyne: gas; higher alkynes: liquids/solids

Acetylene (ethyne) is a colorless gas with a garlic-like odor (due to impurities). It is supplied dissolved in acetone in compressed gas cylinders (with porus filling material).

Chemical Properties

1. Combustion

• 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O (complete combustion)
• Acetylene burns with a very hot, luminous flame (~3000°C in O₂)
• Used in oxy-acetylene welding — important industrial/medical equipment knowledge

2. Addition Reactions

Alkynes undergo addition reactions stepwise — first to alkene, then to alkane:

Hydrogenation

• Alkyne + H₂ → Alkene → Alkane (with Pt/Pd/Ni)
• With Lindlar’s catalyst (Pd/PbO/CaCO₃, poisoned): syn addition → gives cis-alkene only
• With Na/NH₃ (dissolving metal reduction): anti addition → gives trans-alkene only

Halogenation

• Alkyne + 2X₂ → Tetrahalide (vicinal dihalide first, then geminal tetrahalide)
• C₂H₂ + Br₂ → C₂H₂Br₂ (bromoethene) → C₂H₂Br₄ (tetrabromoethane)

Addition of Hydrogen Halides (HX)

• Follows Markovnikov’s rule
• HC≡CH + HCl → CH₂=CHCl (vinyl chloride — important industrial monomer!)
• HC≡CH + H₂O → CH₃CHO (acetaldehyde) — only after Hg²⁺/H₂SO₄ catalysis

Hydration

• Alkyne + H₂O → Aldehyde or Ketone (Hg²⁺ catalyst, H₂SO₄)
• Markovnikov addition: water adds with H to the terminal carbon
• Tautomerization gives the carbonyl compound

3. Acid-Base Reactions (Terminal Alkynes)

Terminal alkynes (R–C≡CH) are weakly acidic:

• pKa ~ 25 — much more acidic than alkanes (pKa ~ 50) or alkenes (pKa ~ 44)
• They react with strong bases (Na, NaNH₂, AgNO₃, CuCl) to form acetylides
• R–C≡CH + Na → R–C≡C⁻ Na⁺ + ½H₂ (hydrogen gas evolved)
• R–C≡CH + AgNO₃ → R–C≡CAg↓ (white precipitate) + HNO₃

This is a test for terminal alkynes — silver nitrate test gives a white precipitate.

Heavy metal acetylides (Ag, Cu) are explosive when dry — an important safety fact.

4. Ozonolysis

C≡C bond cleavage → two carboxylic acids (or CO₂ if terminal)

• HC≡CH + O₃ → 2CO₂ + H₂O (fully oxidized)

5. Polymerization

• 3HC≡CH → C₆H₆ (benzene) (Trimerization with Ni(CN)₂ catalyst at 60°C)
• This is an industrially important route to benzene!

Preparation of Alkynes

1. Dehydrohalogenation of vicinal dihalides

R–CHX–CHX–R + 2KOH/alc → R–C≡C–R + 2KBr + 2H₂O (2 steps of elimination, heat)

2. From calcium carbide

CaC₂ + 2H₂O → Ca(OH)₂ + HC≡CH (acetylene from calcium carbide) This is how acetylene was historically produced industrially — important for SLMC!

CaC₂ is produced from limestone and coke: CaO + 3C → CaC₂ + CO (electric arc furnace)

Clinical and Medical Relevance

• Acetylene is used in oxy-acetylene welding for surgical instrument repair
• Vinyl chloride (from acetylene + HCl) polymerizes to PVC — essential in IV tubing, blood bags, endotracheal tubes
• Trichloroethylene (TCE) is an anesthetic alkyne derivative (now banned — hepatotoxicity)
• Propofol (anesthetic) synthesis involves alkyne chemistry
• Heavy metal acetylides are explosive — relevant to industrial safety and hazard management

Common SLMC Exam Traps

• Ethyne + H₂O (hydration) gives acetaldehyde, NOT ethanol — students often confuse this
• Terminal alkynes are acidic enough to react with AgNO₃ — this distinguishes them from non-terminal alkynes
• Lindlar’s catalyst gives cis product; Na/NH₃ gives trans product — remember which is which
• When adding 2 equivalents of HX to an alkyne, the second addition always gives a geminal (same carbon) dihalide — Markovnikov’s rule applies at each step

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