Aldol reaction is an important organic reaction of aldehydes and ketones. This reaction is shown by aldehydes and Ketones having alpha-hydrogen atom in the compound. Such a-hydrogen atom containing aldehydes and ketones will undergo self condensation in presence dilute alkalies like dil.NaOH (dilute Sodium hydroxide), dil.Ba(OH)2(dilute Barium hydroxide) forming alpha, β-hydroxyaldehyde (an aldol) or β-hydroxyketone (a Ketol) .The Aldol or the Ketol thus formed in the reaction upon heating eliminates a molecule of water (i.e. it undergoes dehydration) forming alpha, β-unsaturated Aldehydes or Ketones respectively. This reaction now is called Aldol condensation reaction.
Example: 1 of Aldol condensation reaction involving an aldehyde.
When acetaldehyde is treated with dil.NaOH it undergoes self condensation as it contains alpha-hydrogen atom in its compound forming β-hydroxyaldehyde (an aldol) namely 3-Hydroxy butanal. This compound upon further heating will eliminate a molecule of water forming aldol condensation product namely Crotonaldehyde Or But-2-en-al.
Example: 2 of Aldol condensation reaction involving a ketone.
When acetone-the ketone is treated with dil.Ba(OH)2 it undergoes self condensation as it contains a-hydrogen atom in its compound forming β-hydroxyketone (a ketol) namely 4-Hydroxy-4-methyl Pentan-2-one. This compound upon further heating will eliminate a molecule of water forming aldol condensation product namely 4-Methyl pent-3-en-2-one.
Note: Even though ketones form ketols the general name as aldol condensation is used to the reactions involving ketones due to the reaction similarity with aldehydes.
In this Aldol condensation reaction an enol or an enolate ion reacts with a carbonyl compound of either aldehyde or ketone to form a β-hydroxyaldehyde (an aldol) or β-hydroxyketone (a Ketol), followed by a dehydration to give a conjugated enone.
What are Enols and Enolates?
Come let us try to understand what are these Enols and Enolates & how are they formed and what is their role in Aldol condensation reaction.
Enols, or more formally, alkenols, are a type of reactive structure or intermediate formed in the reaction that is represented as an alkene with a hydroxyl group attached to one end of the alkene double bond.
In simple terms, enols are compounds that have alcohol groups, (-OH), directly substituted onto alkenes, C=C, hence “alkene-ols” or enols.
Enols can be viewed as alkenes with a strong electron donating substituent. Since alkenes themselves typically react as nucleophiles, the -OH group makes them even more reactive than a simple alkene.
In other words, enols are isomers of aldehydes or ketones in which one alpha hydrogen has been removed and placed on the oxygen atom of the carbonyl group. The molecule has a c=c and an -OH group, so it is called an ene/ol, i.e., an enol. Enols can be formed only from carbonyl compounds which have a-hydrogen. They can be formed by acid or base catalysis, and once formed are highly reactive toward electrophiles, like chlorine, bromine etc,.
Note: Even though the carbonyl group is reactive towards nucleophiles at the carbonyl carbon, it is found that the carbonyl carbon is typically not reactive toward electrophiles, (but the oxygen end of the Carbonyl group is reactive). In contrast, the isomeric enol formed during the reaction is reactive towards electrophiles even at the carbonyl carbon end.
The process of enol formation is called “enolization”. It requires either acid or base to catalyze the reaction.
Enolates are the conjugate bases or anions of enols (like alkoxides are the anions of alcohols) and can be prepared using a base.
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