Explain the following reactions :
Aldol condensation

Explanation

Two molecules of an aldehyde or a ketone containing `alpha`- hydrogens undergo condensation in the presence of a base to yield an aldol (`alpha-` hydroxyaldehyde). Aldol on heating loses a molecule of water to give unsaturated aldehyde or ketone. Aldol condensation also proceeds under acidic conditions.


Aldehydes such as formaldehyde, 2, 2-dimethyl propanal, benzaldehyde, etc. which do not contain `alpha`-hydrogen/s do not under go aldol condensation.
Cross aldol condensation
Aldol condensation between two different aldehydes or ketones is called cross aldol condensation.
However, it is a useful synthetic reaction only of one of the aldehydes does not contain `alpha`-hydrogen atoms, i.e., formaldehyde, benzaldehyde, etc. For example,

Intramolecular aldol condensation
If a compound contains two aldehydic/ketonic or one aldehydic and one ketonic group at 1, 6 or 1, 7 positions w.r.t. to each other, intramolecular aldol condensation occurs to yield alicylic `alpha,beta-` unsaturated aldehyde/ketone. For example,