Carbon-oxygen double bond is the active group of carbonyl compounds. There are two kinds of carbonyl compounds as aldehydes and ketones. Reactions of carbonyl group depends on nature and polarity of double bond.
R - alkyl or aryl groups
R1, R2 - alkyl or aryl groups
Aldol condensation is occurred only in carbonyl compounds which have a α hydrogen. These α H is acidic, therefore they are removed easily with bases and give carbanion.
All H atoms existing on α carbon are defined as α H. All carbon atoms which are adjacent to carbonyl carbon are defined as α carbon.
Aldehyde or ketone which have αH react with any strong bases such as NaOH, KOH and Ba(OH)2 and give aldol as the product. This reaction doubles the number of carbon atoms of initial aldehyde or ketone. To dehydrate the aldol compound, it is heated alone or with I2.
Reaction is stared by OH- ions provided from NaOH or KOH. At the end, OH- is regenerated.
Therefore OH- ions are behaved as a catalyst.
Related Topics of Carbonyl Compounds | Aldehydes and Ketones