Because of the lack of leaving group on these carbonyls (the leaving group would have to be an alkyl group leaving as a carbanion – not generally very stable), they react differently to the general reactions discussed on the previous page. The C=O bond is still polar and attractive to nucleophiles however, so they can attack here. However, it is important now to consider whether the nucleophile is a ‘reversible’ one or not. Consider the following scheme;

So, to summarize the above scheme; some nucleophiles will be stable enough that it is more energetically favourable for them to remain separate from the carbonyl, and for the stable C=O group to remain (examples of these nucleophiles are OH, Cl, MeO etc. – i.e. relatively stable anions). Other nucleophiles are not stable enough to be ejected from the intermediate, and so the C=O group is lost and the O is usually protonated (examples of these nucleophiles – the ‘irreversible’ nucleophiles – are MeLi (Me), MeMgBr (Me again) – mainly carbanion nucleophiles).

There is another important reaction type that ketones and aldehydes undergo, and it stems from keto-enol tautomerism. This essential concept is shown below;

So the α proton of a ketone or aldehyde is partially acidic, and can be removed by strong bases to give a reasonably stabilised anion which can then act as a nucleophile. The anion could of course act as a nucleophile from either the oxygen or the carbon, but reacting with the latter leaves the C=O group intact, which is usually very favourable. An example reaction is the halogenation of ketones;

Where there is a choice of α protons, the most acidic one will be taken;