Alkyne Structure and Nomenclature

Alkynes are unsaturated hydrocarbons with at least one carbon–carbon triple bond. They follow the general formula CₙH₂ₙ₋₂, are linear at the triple bond (sp hybridized, 180°), and are more reactive and notably more acidic at a terminal ≡C–H (pKₐ ~25–26) than the analogous alkenes or alkanes.

• How to name alkynes
• Geometry and formula
• At-a-glance properties
• Worked examples (naming + visualization)
• Summary

How to name alkynes

• Choose the longest chain containing the triple bond, number from the end that gives the triple bond the lowest locant, and use the suffix “–yne.”
• Indicate the triple-bond position with a number (e.g., pent-2-yne has the triple bond starting at carbon 2).
• Multiple triple bonds use “diyne,” “triyne,” etc., with locants for each.

Example:

Try our Organic Compound Namer to confirm structures and IUPAC names.

Geometry and formula

• Hybridization: sp on the triple-bond carbons → linear (180°) with one σ bond and two π bonds.
• Formula: CₙH₂ₙ₋₂ (same as a diene), reflecting two degrees of unsaturation.
• Electronegativity/acidic H: The 50% s-character of sp carbon stabilizes the conjugate base of terminal alkynes, making ≡C–H notably more acidic (pKₐ ~25–26) than alkene/alkane C–H.
• Terminal vs internal: Terminal alkynes (1-ynes) have a triple bond at C1 and an acidic ≡C–H that can form an acetylide with strong base; internal alkynes (2-ynes, 3-ynes, etc.) have no terminal ≡C–H.

At-a-glance properties

• Similar boiling/melting trends to alkenes/alkanes of the same size, but more reactive due to two π bonds.
• Linear geometry; nonpolar overall (terminal ≡C–H is only weakly polar).
• Undergo addition reactions (see the reactions overview) and can be deprotonated at a terminal ≡C–H to form acetylides.
• No cis/trans (E/Z) isomerism about a single C≡C: the substituents are colinear with the linear triple bond.
• Terminal ≡C–H acidity matters synthetically: deprotonate with very strong bases (NaNH₂, organolithiums); weaker bases like hydroxide/alkoxides are not sufficient.
• Physical properties (boiling point/solubility) are broadly similar to alkenes of the same carbon count; size and dispersion dominate trends.

Worked examples (naming + visualization)

Below are five sample alkynes (≥5 carbons). Atom numbers in each image are shown along the longest chain to make the locants for the triple bond clear. These mirror common alkyne nomenclature practice problems.

Molecule name	Structure (numbered)	Short explanation
pent-2-yne		Five carbons; triple between C2–C3 (internal alkyne, lowest locant).
hex-3-yne		Six carbons; symmetric internal triple bond at C3.
hept-2-yne		Seven carbons; triple between C2–C3 (internal alkyne; no terminal ≡C–H).
4-methylpent-2-yne		Pent parent; triple at C2; methyl substituent at C4 (numbers on the main chain).
5-bromopent-2-yne		Pent parent; triple at C2; bromo substituent at the terminal C5.

Summary

• Naming: Longest chain with the triple bond; lowest-numbered triple bond; suffix “–yne.”
• Geometry: sp, linear, 180°; formula CₙH₂ₙ₋₂.
• Reactivity: Two π bonds enable additions; terminal ≡C–H is significantly more acidic than alkene/alkane C–H (→ acetylide formation with strong base); no cis/trans assignment around a single C≡C.