Carbonyl Compounds : Introduction

00:01 Right. Okay. So, now, I would like to talk to you about a very key functional group and that is the carbonyl functional group. Here, we can see on the board two examples of carbonyl functional groups, the aldehyde and the ketone. Note the difference between the two.

00:19 In the case of the ketone, we have a single substitution on the carbonyl carbon. This is the carbon double bond oxygen with the remaining valency on that carbon being taken up by a carbon-hydrogen sigma bond. In the case of the ketone, however, we have substitution on both sides, either via a mixture or combination of aromatic and aliphatic substituents.

00:44 Going back to what we said about aromatics, of course, benzene rings, naphthalene etc.

00:51 or indeed alkyl rings such as methyl, propyl or butyl and this fundamentally informs how they react and also some of their properties. So, let’s have a look at carboxyl compounds just briefly. There will be a lecture, the next lecture, that covers other reactivity of these carboxyl compounds, but I want to introduce right now the concept of them because they feature our friend, the carbonyl functional group, where we have R as an aliphatic or aromatic joined to a carbon double bound to an oxygen, and it is this that actually informs the reactivity both of carbonyl and carboxyl reagents due to the properties of the carbon double bound to the oxygen. I’ve shown here Z correlating to a group or groups which can be attached to that carbon such as the OH group, the OR or alkoxy group, the NR or amine group that’s giving us an amide and a hydride which can be formed from another equivalent of acetic or alkanoic acids and indeed, halide group. I’m not going to go into too much detail in this lecture, suffice to say that their reactivity is based to a large extent on the properties of that carbonyl group.

02:15 So, let’s have a look at some of the nomenclature. The ending, for example, of a ketone, the suffix is always “–one”. A ketone group is numbered to give the carbonyl group the lowest number. Remember what we said before about priority. We were talking, in this particular case, about what the atomic mass was or the heteroatom attached to a carbon.

02:40 So, let’s, for example, look at our friends, the aldehydes, shown at the top. Propanal is... and butanal take the “–al” suffix because they are aldehydes. So, whenever you see a carbonyl group in a terminal position to which is attached only hydrogen, you should be able to recognise it as being an aldehyde. Now, because the carbon-oxygen bond there shows that the oxygen actually has a mass of 16, the carbon has a mass of 12, this gives the priority to the carbon of 1 and of course, therefore, every subsequent backward count would move around from that position all the way to the longest chain.

03:23 So, in the case of the carbonyl, we would count 1, 2, 3 all the way around to where the methyl group is in the 3 position. If we have a look at the ketone, we can see that in the case of propanone or acetone, there is no need to number it, since you cannot have a ketone in a terminal position because it then becomes an aldehyde. So, there is no need to say propane-2-one because there is only one possible position for that carbonyl to be in and still remain a ketone. If, on the other hand, we have these long chain asymmetric ketones such as the 3-methyl-2-hexanone, we use the same rules that we’ve applied in the nomenclature of haloalkanes and of alcohols earlier on in this module. That is to say where we have an oxygen, a heteroatom attached to the carbon, where it is not the terminal position, we find the nearest terminal position to call one. We do use that in the case of OH when it is not terminal and we do that in the case of halogens when they are not terminal. So, this, therefore, gives us a counting structure which goes from the terminal, there is the carbonyl 1, 2, 3 all the way through the hexane longest chain and the substituent in the 3 position, which is the methyl group, carries that prefix 3-methyl-2-hexanone where the 2 correlates to exactly where the carbonyl fits in the longest chain of that alkane structure.