Alkynes – Nomenclature and isomerism, Structure of acetylene


  • These are unsaturated hydrocarbons containing at least one triple bond between two carbons.
  • General formula is Cn H2n-2,               (n = 2, 3, 4, 5, …).
  • Acetylene is first member

Nomenclature and isomerism:

The IUPAC name is derived from the IUPAC name of alkanes by replacing ending ‘ane’ by ‘yne’ along with the position of triple bond e.g. Alkane–ane + yne = Alkyne.

Ex: CH3 – CH2C ≡ CH but – 1 – yne

Value of n

Formula Structure Common name IUPAC name
2 C₂H₂ HC ≡ CH Acetylene


3 C₃H₄ CH₃ – C ≡ CH Methyl Acetylene


Alkynyl groups:

Residual part left after the removal of one H atom from alkyne is known as alkynyl group. According to IUPAC nomenclature, these groups are named by replacing terminal (e) of alkyne by “yl” e.g.

HC ≡ C –

HC ≡ C – CH


– C ≡ C – CH3


Structure of acetylene:

  • Acetylene contains one ‘sigma’ bond and two ‘pi’ bonds.
  • Sigma bond is formed by the head on overlapping of the two sp hybridized orbitals of the two carbon atoms.
  • The remaining sp hybridized orbital of each carbon atom undergoes overlapping along the inter-nucleus axis with 1s orbital of the two hydrogen atoms forming two C – H bonds. H – C – C bond angle is 180⁰C.
  • The unhybridised orbitals are perpendicular to the plane of C – C sigma bond these are on sidewise overlapping gives two ‘pi’ bonds.
    Structure of Acetylene
  • Carbon–carbon bond strength is more than that of alkenes and alkanes and acetylene molecule is linear structure.


  • Isomerism in alkenes starts from four carbons.
    CH₃ – CH₂ – C ≡ CH – 1 – butyne
    CH₃ – C ≡ C – CH₃ – 2 – Butyne
  • These are positional isomers due to different positions of triple bond.