Alkynes - Nomenclature and isomerism, structure of acetylene

Alkynes:

• These are unsaturated hydrocarbons containing at least one triple bond between two carbons.
• General formula is C_n H_2n-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	Ethyne
3	C₃H₄	CH₃ - C ≡ CH	Methyl Acetylene	Propyne

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–	ethynyl
HC≡C—CH2–	2-propynyl
—C≡C—CH3	1-propynyl

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.
  
  
• Carbon–carbon bond strength is more than that of alkenes and alkanes and acetylene molecule is linear structure.

Isomerism:

• 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.