Alkynes

From WikidChem

Adrianne Smits and Lily Yan

An alkyne is molecule that contains a triple bond between two carbons.

Carbon has 4 valence electrons. One is used in bonding to an R group (e.g. Hydrogen), and the other three are used in the triple bond, which is one sigma bond and two pi bonds.

The sigma bond between the two carbons are two p orbitals oriented toward each other, with good overlap and good energy splitting. The HOMOs and LUMOs are not unusual in the sigma bond.

However, in both pi bonds, the p orbitals involved have poor overlap, which leads to poor energy splitting. Since a p orbital is higher in energy than an s orbital, pi bonds start out higher than usual C-C bonds. And because energy splitting is poor, the HOMOs are unusually high and the LUMOs are unusually low. The HOMOs are higher than the LUMOs are low due to the original higher energy of p orbitals.

Since there are two pi bonds, there are two unusually high HOMOs. This means an alkyne is very reactive and unstable.