Two molecules of an aldehyde or ketone containing α- hydrogen atom, combine itself in the presence of dilute alkali to form β-hydroxyaldehyde or β-hydroxyketone. 

This reaction is called aldol condensation after the name of the product which contains both aldehyde (al) and a hydroxy (ol) group.

Similar condensation of two molecules of ketones ( α- hydrogen) is also considered as aldol condensation although the product in this case contains a keto and hydroxy group rather than an aldehyde group.

Which is the main product if the following reaction?

The aldol dimerization of propanone is difficult owing to steric effects. Phenyl conjugation with the double bond facilitates dehydration of the crossed aldol adduct, and the trans product will be more stable than the cis isomer.