Alkynes: Preparation, Acidic Character and Reactions

Alkynes are hydrocarbons with at least one carbon–carbon triple bond (CnH2n-2); acetylene (HC≡CH) is the simplest. The triple bond is reactive toward addition, and the sp-hybridized C−H of a terminal alkyne is weakly acidic — a property no alkane or alkene has.

Acetylene's flame with oxygen reaches about 3000°C, hot enough to weld steel, and its acidic terminal hydrogen lets chemists join carbon chains together — the foundation of much of synthetic organic chemistry.

Key Concepts

• Preparation from vicinal dihalides: treating a 1,2-dibromoalkane with alcoholic KOH removes one HBr to give a vinyl bromide, and then strong base (NaNH2) removes the second HBr to give the alkyne.
• Preparation from calcium carbide: CaC2 + 2H2O → HC≡CH + Ca(OH)2. This was historically the main industrial route to acetylene.
• Acidic character: the terminal C−H bond uses an sp orbital (50% s-character), holding electrons close and stabilizing the anion. Acidity: HC≡CH (pKa ≈ 25) > alkenes (≈ 44) > alkanes (≈ 50).
• Acetylide formation: terminal alkynes react with Na, NaNH2, or ammoniacal AgNO3/Cu2Cl2 to give acetylides — the white silver and red copper precipitates are classic tests for terminal alkynes.
• Addition reactions: H2 with Lindlar's catalyst gives cis-alkenes; Na in liquid NH3 gives trans-alkenes; HX adds by Markovnikov's rule; water adds with Hg2+/H2SO4 to give aldehydes or ketones.

Formulas

• From calcium carbide: CaC2 + 2H2O → C2H2 + Ca(OH)2. Molar masses: CaC2 = 64.10 g/mol, C2H2 = 26.04 g/mol.
• Acetylide formation: HC≡CH + NaNH2 → HC≡CNa + NH3. Because NH3 (pKa ≈ 35) is a far weaker acid than acetylene (pKa ≈ 25), the equilibrium lies overwhelmingly to the right.
• Equilibrium constant from pKa: for an acid–base reaction, Keq = 10^(pKa of product acid − pKa of reactant acid) = 10^(35 − 25) = 10^10. The reaction is essentially complete.

Worked Example

Show numerically that sodium amide deprotonates acetylene essentially completely, using the pKa values 25 (acetylene) and 35 (ammonia).

1. Reaction: HC≡CH + NH2- → HC≡C- + NH3. The reactant acid is acetylene (pKa = 25); the product acid is ammonia (pKa = 35).
2. For proton transfer, Keq = 10^(pKa of product acid − pKa of reactant acid).
3. Keq = 10^(35 − 25) = 10^10 = 10,000,000,000.
4. Keq = 10^10 means the reaction goes to completion — every acetylene molecule becomes acetylide ion, which is then alkylated to build longer chains (HC≡C- + R−X → HC≡C−R).

Common Misconceptions

• All alkynes are acidic. Wrong. Only terminal alkynes (R−C≡C−H) have the acidic hydrogen; internal alkynes (R−C≡C−R) have no such hydrogen and give no acetylide tests.
• Acetylene is acidic like a carboxylic acid. Wrong in scale: pKa ≈ 25 makes it billions of times weaker than acetic acid (pKa ≈ 4.8) — only strong bases like NaNH2 deprotonate it.
• Lindlar's catalyst and Na/NH3 give the same alkene. Wrong. Lindlar's gives the cis-alkene, sodium in liquid ammonia gives the trans-alkene — opposite stereochemistry, a favourite exam distinction.

Real-World Applications

• Welding: the oxy-acetylene flame (~3000°C) cuts and welds steel, and was the original reason for large-scale acetylene production.
• PVC plastic: vinyl chloride monomer is made from acetylene in some regions, feeding the huge PVC industry.
• Organic synthesis: acetylide ions are carbon nucleophiles used to lengthen carbon chains step by step in drug and natural-product synthesis.

Alkynes combine a reactive triple bond with a uniquely acidic terminal hydrogen: they are made by dehydrohalogenation or from calcium carbide, extended via acetylides (Keq = 10^10), and converted by controlled additions into alkenes, aldehydes, and ketones.