Aldol reactions and condensations

The reaction of a carbonyl (aldehyde or ketone) with a base produces an enolate ion (a nucleophile). This nucleophile attacks any electrophile. What happens when you add a base to a carbonyl with no electrophile present  

It turns out that a reaction still occurs because the carbonyl group itself is an electrophile. As the enolate forms, it can attack the carbonyl group of another aldehyde or ketone molecule. This is an aldol reaction or aldol condensation, also called an aldol addition. 

Ketones are less reactive towards the nucleophile. In Organic Chemistry I, you saw that alkyl groups are electron donating. In ketones, the presence of the two alkyl groups attached to the carbonyl do a better job at compensating for the 8+ on the carbon atom than do one alkyl group and a hydrogen atom in an aldehyde. For this reason, aldehydes are more reactive than ketones. 

Forming a stronger base from a weaker base is very highly unlikely. 

An aldol reaction/condensation occurs when the enolate ion from an aldehyde or ketone attacks a molecule of the parent compound. If, however, two different carbonyl compounds are present, a crossed aldol reaction/ condensation occurs. 

---

The aldol formed by the aldol reaction, especially if heated, can react further. The heating causes dehydration (loss of H2O), and the overall reaction involving an aldol reaction followed by dehydration is the aldol condensation. The product of an aldol condensation, favored by the presence of extended conjugation, is an a,(3-unsaturated aldehyde (an enal) or ketone. The mechanism for dehydration (Figure 11-13) begins where the mechanism of the aldol reaction (Figure 11-12) ends. This process works better if extended conjugation results. The aldol reaction and condensation are reversible. 

---