Aldol condensation is occurred only in carbonyl compounds which have alpha hydrogen and alpha carbon atoms. Both aldehyde and ketone compounds are defined as carbonyl compounds. Aldol condesation can used to extend the carbon chain. Tha alpha hydrogen in carbonyl compounds has acidic charcteristics. Therefore they are removed easily with bases (alkalis) and give carbanion. Then this carbanion reacts with another carbonyl molecule to give the aldol product.
In this tutorial, we study about followings of aldol condensation.
Dilute strong alkalis such as NaOH, KOH, Ba(OH)2 are used. These alkalis start the reaction. You will see how it happens in mechanism part.
β-hydroxyketone or β-hydroxyaldehyde is given as the product. These compounds contains a hydroxyl group and a carbonyl group. If these priducts are heated alone or with I2, hydroxyl group is eliminated and form a double bond between carbon atoms (alkene group).
The carbon atoms where they are existing next to the carbonyl carbon. As example acetaldehyde has one alpha carbon. Propanone has two alpha carbons. But formaldehyde does not have alpha carbon.
All H atoms existing on alpha carbon (α carbon) are defined as α H. All carbon atoms which are adjacent to carbonyl carbon are defined as alpha carbon. Alpha hydrogen and alpha carbon are required for aldol condensation.
Aldehyde or ketone which has alpha hydrogen reacts 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.
If two or more aldehydes or ketones are mixed together with strong alkali, it gives a product mixture, not a single product.
Ethanal is an aldehyde. Ethanal reacts with dilute NaOH or KOH in aldol condensation. There are two carbon atoms in ethanal molecule. But in the product, there are four carbon. After aldol condensation , aldol product is heated with I2 or heated alone. The -OH group is eliminated (dehydrtion) and give double bond.
Propanone is a ketone. Propanone contains three carbon atoms. After aldol condensation of propanone , there are six carbon atoms in the carbon chain. First gives the aldol product with dilute NaOH or KOH. Then aldol product is heated alone or heated with I2. Alcohol group in the aldol product is eliminated (dehydration) by forming a double bond.
Aldol condensation of propanal also gives an aldol product.
Now, we discuss some examples about aldol condensation of mixture of aldehydes and ketones. When there are two or more aldehydes or ketones, it gives mixture of products.
Ethanal and propanone both are aldehydes. This ethanal and propanone mixture with NaOH(aq) forms four different products.
Four products can be given as a product mixture.
Dilue NaOH, KOH, Ba(OH)2 can be used as reagents for aldol condensation. Reaction is stared by OH- ions provided from strong alkali. At the end of reaction, OH- ions are regenerated again.
Therefore OH- ions are behaved as a catalyst.
Also OH- ions are behaved as nucleophiles. They can attack at positively charged parts such as carbonyl carbon atom.
Mechanism is very hard and complex to understand in mixture of aldehyde and ketone compounds. So try to understand the first mechanism when only one aldehyde or ketone is in the reaction.
Two carboanions are formed from propanal and acetaldehyde. So those two carboanions can react with both propanal and acetaldehyde to give four different products.
Formaldehyde does not have alpha carbon because it has only one carbon atom. therefore we cannot do aldol condensation of formaldehyde.
Benzaldehyde has alpha carbon. But that alpha carbon does not have alpha hydrogen. So benzaldehyde also cannot show aldol condensation.
In a acidic medium too, self condensation of acetaldehyde is occurred through a different mechanism. Acid behaves as the catalyst. But, as the product, aldol is not given always. As the reason we can think, in the presence of acidic medium, aldol is dehydrated and give crotanaldehyde as the product.
Two carbonyl compounds exist to show the aldol condensation. You know formaldehyde cannot show aldol condensation itself. Therefore, 4-pentanone is the compound which has the capability of producing the nucleophile to start the reaction. Reaction happens as below.
Aldehydes without alpha carbon and alpha hydrogen does not have the ability to do aldol condensation. As examples, formaldehyde and benzaldehyde can be given. Formaldehyde does not have alpha carbon and benzaldehyde does not have alpha hydrogen.
yes. In aldol condensation two molecules of carbonyl compound is participated. Alpha carbon of one molecule is oxidized and carbinol carbon of other molecule is reduced during the reaction.
Formaldehyde does not have alpha carbon. Therefore, there is no alpha hydrogen. Without alpha hydrogen aldol condensation is not occurred.
Yes. Carbonol carbon is positively charged and it is attacked by carbanion. And a new carbon group is attached to the aldehyde or ketone.
A ketone molecule become a carboanion due to attck of OH-. Carboanion attacks the carbonyl carbon atom of another ketone molecule. Water also contribute to the mechanism to form the alcohol group. As the product, a compound which has more carbon atoms than previous ketone is given.
To occur the aldol condensation, there should be both alpha hydrogen and alpha carbon atom in the aldehydde or ketone. In the benzaldehyde, there is an alpha carbon atom. But there is no alpha hydrogen atoms. So benzldehyde does not have chance to react with dilute NaOH to show aldol condensation reaction.
Methanal do nt have both alpha hydrogen and alpha carbon atom. So no aldol condensation for methanal.
When only one aldehyde compound is with dilute NaOH, one product is given. If there are two different aldehyde compounds, four different products are given.
Dilute strong alkalis such as NaOH or KOH or Ba(OH)2 are used as aldol reagents.
Mixture of products are given as the result when aldehyde and ketone are together and react with sodium hydroxide.
Due to acidity of those hydrogen atoms, these type of reactions are occurred. A base can react with these alpha hydrogen atoms.
Formaldehyde do not have an alpha carbon or alpha hydrogen atoms. Therefore, carbanion cannot be made by formaldehyde. But, formaldehyde can be attached by the carbanion made by methyl ketone compound.