carbon-carbon triple bond

A carbon-carbon triple bond is the C#C link that defines alkynes — two carbons bonded three times over, sharing six electrons. Drawn as three parallel lines, it is the strongest and shortest carbon-carbon connection found in everyday organic molecules. Picture the two carbons pulled tightly together by a triple cable, with everything else attached to them stretched out into a straight line.

The three lines break down into one sigma bond and two pi bonds. Each carbon is sp hybridized, using two sp orbitals: one points to the other carbon (the sigma bond) and one points to whatever else it is attached to, and because there are only two such directions they sit 180 degrees apart, making the bond perfectly linear. The two leftover p orbitals on each carbon stand perpendicular to the axis and to each other, overlapping sideways to form two pi bonds arranged like a cylinder of electron density wrapping the bond axis.

Like the double bond, this electron-rich link undergoes addition; but a triple bond holds two pi bonds, so it can often add a reagent twice (for example, adding two molecules of HX, or two of hydrogen to become first an alkene and then an alkane). The triple bond's extra rigidity and linearity also show up in real molecules, and in terminal alkynes the sp carbon's high s-character is what makes the attached C-H acidic enough to be removed and used as a nucleophile.