The alkyne functional group consists of two sp hybridized C atoms bonded to each other via one σ and two π bonds. The 2 π bonds are produced by the side-to-side overlap of the two pairs of p-orbitals not utilized in the hybrids. The substituents are attached to the C≡C via σ bonds.The 2 C of the C≡C and the 2 atoms attached directly to the C≡C are linear. Since alkynes are linear, they cannot exist as cis- / trans- isomers.
As with alkenes, the more highly substituted internal alkynes are more stable. The "extra" π bond is weaker than that of an alkene π bond.
The π bonds are a region of high electron density (red) so alkynes are typically nucleophiles. Terminal alkynes, R-C≡C-H, are quite acidic (indicated by blue) for hydrocarbons, pKa = 26. Deprotonation of a terminal acetylene gives an acetylide ion.The acetylide ion is a good nucleophile and can be alkylated to give higher alkynes