Nomenclature, Structure of Triple Bond — Core Principles

Core Principles

Alkynes are unsaturated hydrocarbons characterized by at least one carbon-carbon triple bond. Their general formula for acyclic structures with one triple bond is $\text{C}_n\text{H}_{2n-2}$. The triple bond consists of one sigma ($\sigma$) bond and two pi ($\pi$) bonds.

The carbon atoms involved in the triple bond are sp hybridized, resulting in a linear geometry around these carbons with a bond angle of $180^\circ$. This sp hybridization arises from the mixing of one $2s$ and one $2p$ atomic orbital, forming two sp hybrid orbitals and leaving two unhybridized $2p$ orbitals for pi bond formation.

IUPAC nomenclature for alkynes involves identifying the longest carbon chain containing the triple bond, numbering it to give the triple bond the lowest possible locant, and replacing the '-ane' suffix of the corresponding alkane with '-yne'.

Substituents are named and positioned alphabetically. Ethyne (acetylene) is the simplest alkyne, widely used in welding and as a chemical feedstock. Terminal alkynes, with a C-H bond directly attached to the triple bond, exhibit acidic properties due to the high s-character of the sp-hybridized carbon, making them more reactive with strong bases.

Important Differences

vs Alkanes and Alkenes

Aspect	This Topic	Alkanes and Alkenes
General Formula (acyclic, one multiple bond)	Alkanes: $\text{C}_n\text{H}_{2n+2}$	Alkenes: $\text{C}_n\text{H}_{2n}$
Type of C-C Bond	Single bond	Double bond
Hybridization of C atoms in multiple bond	sp$^3$	sp$^2$
Bond Angle around C atoms in multiple bond	$109.5^\circ$ (tetrahedral)	$120^\circ$ (trigonal planar)
Number of Sigma ($\sigma$) and Pi ($\pi$) bonds in C-C multiple bond	1 $\sigma$, 0 $\pi$	1 $\sigma$, 1 $\pi$
Reactivity towards Electrophiles	Least reactive (saturated)	More reactive (one $\pi$ bond)
Acidity of C-H bonds	Non-acidic (pKa $\approx 50$)	Very weakly acidic (pKa $\approx 44$)

Alkynes stand apart from alkanes and alkenes primarily due to the presence of a carbon-carbon triple bond. This structural feature dictates their unique sp hybridization, leading to a linear geometry and a $180^\circ$ bond angle around the triple-bonded carbons, contrasting with the tetrahedral ($109.5^\circ$) of alkanes and trigonal planar ($120^\circ$) of alkenes. The triple bond comprises one sigma and two pi bonds, making alkynes highly unsaturated and generally more reactive towards electrophilic addition than alkenes, albeit often in a stepwise manner. Furthermore, terminal alkynes exhibit distinct acidity due to the high s-character of their sp-hybridized C-H bonds, a property not found in alkanes or alkenes.