Bonding in Carbonyl Compounds – The Carbon-oxygen Double Bond

Where the carbon-oxygen double bond, C=O, occurs in organic compounds it is called a carbonyl group. The simplest compound containing this group is methanal.

We are going to look at the bonding in methanal, but it would equally apply to any other compound containing C=O. The interesting thing is the nature of the carbon-oxygen double bond – not what it's attached to.

Just as in ethene or benzene, the carbon atom is joined to three other atoms. The carbon's electrons rearrange themselves, and promotion and hybridisation give sp² hybrid orbitals.

Promotion gives:

Hybridisation of the 2s orbital and two of the 2p orbitals means that the carbon atom now looks like the diagram on the right.

Three sp² hybrid orbitals are formed and these arrange themselves as far apart in space as they can – at 120° to each other. The remaining p orbital is at right angles to them.

This is exactly the same as in ethene or in benzene.

Oxygen's electronic structure is 1s²2s²2p_x²2p_y¹2p_z¹.

The 1s electrons are too deep inside the atom to be concerned with the bonding and so we'll ignore them from now on. Hybridisation occurs in the oxygen as well. It is easier to see this using "electrons-in-boxes".

This time two of the sp² hybrid orbitals contain lone pairs of electrons.

The carbon atom and oxygen atom then bond in much the same way as the two carbons do in ethene. In the next diagram, we are assuming that the carbon will also bond to two hydrogens to make methanal – but it could equally well bond to anything else.

End-to-end overlap between the atomic orbitals that are pointing towards each other produce sigma bonds.

Notice that the p orbitals are overlapping sideways.

This sideways overlap produces a π bond. So just like C=C, C=O is made up of a sigma bond and a π bond.

Does that mean that the bonding is exactly the same as in ethene? No! The distribution of electrons in the π bond is heavily distorted towards the oxygen end of the bond, because oxygen is much more electronegative than carbon.

This distortion in the π bond causes major differences in the reactions of compounds containing carbon-oxygen double bonds like methanal compared with compounds containing carbon-carbon double bonds like ethene.