Carbonyls

The other unsaturated system that will commonly undergo nucleophilic addition.

However, a carbonyl system is most likely to undergo nucleophilic substitution with a different mechanism to S_N1, S_N2 etc. and this is dealt with in a separate section. The reason for the attractiveness of carbonyl systems to nucleophiles is that the Carbon-Oxygen double bond is quite polar, as shown by the resonance structures which leave a δ⁺ on the carbon.

Compared to nucleophilic addition to alkenes, the anion produced in carbonyls is much more stable because it lies on the electronegative oxygen rather than carbon. The diagram below shows the two possible outcomes of a nucleophile attacking a carbonyl;

Again, the pathway that involves substitution will be dealt with in much detail in the Carbonyls section. The stability of the carbonyl system means that many of these reactions will be reversible (unless the nucleophile is very strong - or in other words cannot act as a leaving group) and thus held in some sort of equilibrium. An example is the reaction of carbonyls with alcohols;

The approach of the nucleophile is very specific - it attacks the π bond at the correct angle to form a new sp³ hybridised centre (104.5^o);

Further detail on carbonyl mechanisms is covered in the carbonyls section.