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Atoms, bonding and types of reactions

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Molecular orbitals – Triple bonds, Ethyne


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

Ethyne 1

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.

Ethyne 2

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.

Ethyne 3

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.

Ethyne 3 (1)

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.

Ethyne 5

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.