CBSE Class 12-science Answered

The aldol reaction may proceed via two fundamentally different mechanisms. Carbonyl compounds, such as aldehydes and ketones, can be converted to enols or enol ethers. These compounds, being nucleophilic at the alpha-carbon, can attack especially reactive protonated carbonyls such as protonated aldehydes. This is the "enol mechanism." Carbonyl compounds, being carbon acids, can also be deprotonated to form enolates, which are much more nucleophilic than enols or enol ethers and can attack electrophiles directly. The usual electrophile is an aldehyde, since ketones are much less reactive. This is the "enolate mechanism."

If the conditions are particularly harsh (e.g., NaOMe, MeOH, reflux), condensation may occur, but this can usually be avoided with mild reagents and low temperatures (e.g., LDA (a strong base), THF, ?78 °C).

Aldol condensation is of 2 types:-self aldol condensation and crossed aldol condensation .

In self-aldol condensation,the electrophile attacks the carbonyl carbon of another molecule of the same type of compound.

In cross aldol condensation, the electrophile attacks the carbonyl carbon of a different  molecule of carbonyl compound.

For example-

The Aldol Condensation of Aldehydes