Alkynes: The Triple-Bond Hydrocarbons

🔍 What are Alkynes?

Alkynes are unsaturated hydrocarbons with at least one carbon-carbon triple bond (C≡C). Their general formula is CnH2n-2. The simplest alkyne is ethyne (C2H2), commonly called acetylene 🔥, used in oxyacetylene welding torches!

📛 Naming & Isomerism

  • IUPAC naming: Replace “-ane” with “-yne” (e.g., butyne).
  • Position of triple bond is indicated by the lowest-numbered carbon in the bond.
  • Isomer types:
    • 👯 Position isomers: Triple bond in different locations (e.g., but-1-yne vs. but-2-yne).
    • 🌿 Chain isomers: Different carbon skeletons (e.g., pent-1-yne vs. 3-methylbut-1-yne).

Example for C5H8 (pentyne isomers):

StructureIUPAC Name
HC≡C-CH2-CH2-CH3Pent-1-yne
H3C-C≡C-CH2-CH3Pent-2-yne
H3C-CH-C≡CH
│
CH3		3-Methylbut-1-yne

🔬 Triple Bond Structure (Ethyne)

  • Each carbon has sp hybridization.
  • Bond angles: 180° (linear shape).
  • Bond components:
    • 1 C-C σ bond (sp-sp overlap)
    • 2 C-H σ bonds (sp-s overlap)
    • 2 C-C π bonds (p-p lateral overlap)
  • Bond strength: C≡C (823 kJ/mol) > C=C (681 kJ/mol) > C-C (348 kJ/mol).
  • Bond length: C≡C (120 pm) < C=C (133 pm) < C-C (154 pm).

⚗️ Preparation Methods

  1. From Calcium Carbide:
    \[ \ce{CaC2 + 2H2O -> Ca(OH)2 + C2H2} \] (Calcium carbide is made via: \(\ce{CaCO3 ->[\Delta] CaO + CO2}\) and \(\ce{CaO + 3C -> CaC2 + CO}\))
  2. From Vicinal Dihalides:
    \[ \ce{CH2Br-CH2Br ->[alc. KOH][-HBr] CH2=CHBr ->[NaNH2][-NaBr] HC≡CH} \]

⚛️ Properties of Alkynes

Physical:
Gases (C2-C4), liquids (C5-C12), solids (>C12). Lighter than water, soluble in organic solvents (ether, benzene).

Chemical Reactions:

  • Acidic Nature (NEET Favorite!):
    Only terminal alkynes (≡C-H) show acidity!
    \[ \ce{HC≡CH + Na -> HC≡C^- Na+ + 1/2 H2} \] Why? sp-hybridized carbons have high electronegativity (50% s-character) → H+ easily released.
    Acidity order: \(\ce{HC≡CH > CH3-C≡CH > CH3-C≡C-CH3}\)
  • Addition Reactions:
    1. With H2 (Catalyst: Pt/Pd/Ni):
      \[ \ce{HC≡CH ->[H2] H2C=CH2 ->[H2] CH3-CH3} \]
    2. With Halogens (X2) (Test for unsaturation 🧪):
      \[ \ce{CH3-C≡CH + Br2 -> CH3CBr=CHBr ->[Br2] CH3CBr2CHBr2} \]
    3. With HX (Markovnikov’s rule):
      \[ \ce{HC≡CH + 2HBr -> CH3-CHBr2} \]
    4. With H2O (Hg2+/H+ catalyst):
      \[ \ce{HC≡CH + H2O -> CH3CHO} \quad \text{(Ethanal)} \] \[ \ce{CH3-C≡CH + H2O -> CH3COCH3} \quad \text{(Propanone)} \]
  • Polymerization:
    • Linear: \(\ce{HC≡CH -> polymerize -> [-CH=CH-]_n}\) (Polyacetylene – used in batteries!)
    • Cyclic: \(\ce{3HC≡CH ->[873 K][Red hot Fe] C6H6}\) (Benzene)

🎯 NEET Super Shorts (Key Concepts)

  1. Acidity of Terminal Alkynes: Explained by sp-hybridization → higher % s-character → stronger C-H bond polarization.
  2. Markovnikov Addition in Unsymmetrical Alkynes: HX adds such that H attaches to carbon with more H atoms.
  3. Distinction Test: Only terminal alkynes react with Na/NaNH2 → H2 gas evolution (alkanes/alkenes don’t!).
  4. Bond Parameters: C≡C bond length (120 pm) and enthalpy (823 kJ/mol) are critical comparisons.
  5. Benzene from Alkyne: Cyclic polymerization of ethyne over red-hot iron tube.