Atoms, bonding and types of reactions

Ethyne

In a triple bond six electrons are shared. Let’s consider ethyne, C₂H₂, as an example.

Each Carbon in ethyne has four bonds so, as before with ethane and ethene, one of Carbon’s 2s electrons is promoted to a 2p orbital.

As we saw in the last section, the double bond in ethene is composed of one σ bond and one π-bond. Similarly, the triple bond in ethyne is composed of one σ-bond and two π-bonds.

This means that each Carbon in ethyne has two σ-bonds (one to bond to the Hydrogen and one to bond to the other Carbon). To form the two σ-bonds, two hybrid sp orbitals are formed from one s orbital and one p-orbital. This leaves two p-orbitals remaining.

The two sp orbitals rearrange themselves as far from each other as possible (180° from each other). The two-remaining p-orbitals are at right angles (90°) both to the sp orbitals and to each other.

Sigma bonds form by the merging or end-to-end overlap of atomic orbitals, with resulting bonds containing a bonding pair of electrons. Two σ-bonds are formed – one between Hydrogen’s 1s orbital and Carbon’s sp hybrid orbital and the other between each of the Carbon’s sp orbitals.

As seen before with ethene, the remaining p-orbitals cannot undergo end-to-end overlap, however they are in close enough proximity to overlap sideways. This creates 2 π-bonds.

For a π-bond to exist, the p-orbitals of each Carbon must be aligned. Therefore, there is no rotation around the Carbon-Carbon triple bond as this would break the π-bond.