Aldehydes and ketones have acidic hydrogens in the neighboring position of the carbonyl group, known as the alpha position. These hydrogens have a pKa between 18 and 21.


Enolates act as nucleophiles through carbon, attacking a large number of electrophiles (haloalkanes, epoxides, carbonyls, esters...). At this point we will focus on the reaction between enlolates and haloalkanes, which allows the addition of carbon chains to the a position of the chain.

Cyclohexanone is converted to 2-Methylcyclohexanone by treatment with LDA followed by methyl iodide.


Aldehydes and ketones exchange their hydrogens for deuteriums when treated with DO- /D2O or with D+ /D2O. In basic media the reaction proceeds through enolates and in acidic media the intermediates formed are enols.


Aldehydes and ketones react with halogens in acidic or basic media, producing the substitution of hydrogens for halogens.

Halogenation of propanone in an acid medium:


Aldehydes and ketones condense in basic media to form aldols. This reaction is called aldol condensation.


The aldol formed dehydrates in the basic medium by heating to form an a , b -unsaturated.


Ketones are less reactive than aldehydes and give a very low yield in aldol condensation. Thus, two propanone molecules condense to form the corresponding aldol with a 2% yield. High percentages of the product can be achieved by separating it from the reaction medium as it is being formed, or by heating it to dehydrate it. In both ways the aldol equilibria shift towards the final product.


Dicarbonyl compounds condense via the intramolecular aldol in basic media. In this reaction, cycles of five or six members are obtained.
Thus, 2,6-heptanedione condenses with methoxide in methanol to form 3-methylcyclohex-2-enone.


The reaction between two different carbonyls is called crossed or mixed aldol. This reaction only has synthetic utility in two cases:
1. Only one of the carbonyls can form enolates.
2. One of the carbonyls is much more reactive than the other.
In all other situations, the mixed aldol generates mixtures of four products. Let's see as an example the condensation of ethanal and propanal.

sintesis-ab-insaturados01.gif The a,b -unsaturated carbonyls are organic compounds that have a double bond between the a,b positions of an aldehyde or ketone.

Propenal or acrolein is an a,b -unsaturated carbonyl. Its two conjugated double bonds give it special reactivity.

The a,b -unsaturated are compounds that have two electrophilic positions: the carbonyl carbon and the b carbon.

Additions 1,2. Lithium organometallics attack the carbonyl carbon giving rise to 1,2 additions.

yoThe organometallics of lithium and magnesium attack the carbonyl carbon of the a,b-unsaturates

Enolates of aldehydes or ketones add to the a,b-unsaturated to form 1,5-dicarbonyls. This reaction is called the Michael addition.

Propanone reacts with a,b-unsaturated to form 1,5-dicarbonyl