alkyne

An alkyne is a hydrocarbon containing at least one carbon-carbon triple bond, written C#C (the # standing in for three lines). If an alkene is two carbons holding hands twice, an alkyne is two carbons holding hands three times — the tightest, shortest, and most electron-dense carbon-carbon link in ordinary organic chemistry. The simplest and most famous example is acetylene, H-C#C-H, the gas burned in welding torches because its combustion is extraordinarily hot.

Each triple-bond carbon is sp hybridized, meaning it is linear: the carbon and its two attached groups line up in a straight line at 180 degrees. The triple bond is one sigma bond plus two pi bonds (two perpendicular sideways overlaps), which is why the bond is so short and strong overall. Alkynes come in two flavours that behave quite differently: internal alkynes have carbon groups on both ends, while terminal alkynes carry a hydrogen on a triple-bond carbon (R-C#C-H), and that particular C-H is unusually acidic — a defining piece of alkyne chemistry.

Alkynes matter both as fuels and as flexible building blocks. The triple bond, like the double bond, is electron-rich and undergoes addition reactions, often able to add twice. More distinctively, removing the acidic terminal hydrogen gives an acetylide anion, a carbon nucleophile that can attack other molecules to forge brand-new carbon-carbon bonds — one of the relatively few ways to lengthen a carbon skeleton at will. The general formula for one triple bond is CnH2n-2 (two degrees of unsaturation).