savvy-chemist: Carbonyl Compounds (1) Structure of the Carbonyl Group.

Friday 1 April 2016

Carbonyl Compounds (1) Structure of the Carbonyl Group.

I’m going to begin a series of posts on the chemistry of the organic functional group the Carbonyl group.

Let’s ask first what is a carbonyl group?

We can simply say it is a C=O group but that would be too simplistic really.

There are some very interesting and fairly good atomic orbital representations of the carbonyl group on line and we’ll look at a few soon enough.

First let’s reference the carbon –carbon double bond in ethene.

You’ll find my page on the ethene double bond here.

That page shows you how the molecular orbital theory attempts fairly successfully to account for the formation of the ethene double bond.

The two excited state carbon atoms form a sigma (σ) and a pi (π) bond.

The sigma bond is formed along the axis joining the two carbon nuclei and the pi bond forms in two halves above and below the plane of the molecule.

The effect of the pi bond is to restrict rotation about the sigma bond.

The effect of this restricted rotation is that it is possible for the formation of geometric isomers of some types of alkene.

Now in a similar way the carbonyl bond forms from an excited carbon and oxygen atom.

In their ground state, oxygen and carbon have the following electron configurations:

In the formation of the carbonyl double bond, certain changes must take place on the excitation of the atomic orbitals of both atoms for the atoms to form the corresponding molecular orbitals that then fit the chemistry of carbonyl compounds such as aldehydes.

To fit the carbonyl reality, the molecular orbital model must produce an oxygen atom with two lone pairs of electrons since this feature is known to account for the solubility in water of some carbonyl compounds like aldehydes through hydrogen bonding with water.

Furthermore, the molecular orbital model must account for the polarity of the carbonyl group since its polarity results in nucleophilic addition reactions between aldehydes and nucleophiles like HCN

And thirdly, the molecular orbital model has to account for the planarity of the carbonyl bond since this feature of aldehydes gives rise to racemic mixtures of products from some nucleophilic addition reactions.

The resultant model suggests that oxygen forms an excited state in which three sp² orbitals form leaving a p^z orbital.

The model also suggests that the carbon atom forms three sp² atomic orbitals by the excitation of an electron into the vacant 2p^z orbital (see the large arrow in the diagram above.)

The resultant excited atomic states for oxygen and carbon look like this:

The p^z orbitals side–on overlap to form the π bond.

Two sp² orbitals end–on overlap to form the σ bond.

The situation has been variously pictured on different sites on the internet as follows:

First, this pictorial representation shows the situation before the formation of the molecular orbitals:

The second example shows the simplest carbonyl compound methanal or formaldehyde HCHO and shows how each bond forms quite nicely, again showing the situation before the formation of the molecular orbitals.

The next image comes from a youtube video cut, I think, and shows the formation of the carbonyl bond but you’ll note I hope a couple of mistakes in it.

First, the p^z orbital is not shown at a different energy level to the sp² atomic orbitals.

Second, there’s a nice attempt to show the planarity of the structure but the problem is that the bond angles around the carbon atom have been shown to be all 120^o when as we shall see shortly they are not all equal. VSEPR rules apply.

The next image below helps in the way it attempts to show how the more electronegative oxygen atom draws the carbonyl bonding electrons towards itself. Oxygen here is electron pulling.

The next two images are helpful but for the fact that they too show the bond angles around the carbonyl bond to be 120^o when they are not because of the extra electron density of the carbonyl bond itself repelling the single bonds attached to the carbon atom. (see example below).