Aldol condensation of aldehydes and ketones, mechanism

Aldol condensation is occurred only in carbonyl compounds which have a alpha hydrogen. Carbonyl compounds are aldehydes and ketones. These alpha hydrogen is acidic, 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 studt about followings of aldol condensation

  1. Reagents used and products of the reaction
  2. Examples of aldol reaction
  3. Aldol condensation mechanism
  4. Questions

Reagents using in aldol condensation

Dilute strong alkalis such as NaOH, KOH, Ba(OH)2 are used.


Products of aldol condensation

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.


Alpha hydrogen

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.

What is alpha hydrogen?

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.

alfa carbon alfa hydrogen

Aldol condensation of same type aldehyde or ketone

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.


Aldol condensation of mixture different types of aldehydes or ketones

If two or more aldehydes or ketones are mixed together with strong alkali, it gives a product mixture, not a single product.

Examples of Aldol condensation of mixture different types of aldehydes or ketones

  • Mixture of ethanal and propanal with NaOH(aq) gives mixture of four products.
  • Propanone and propanl also react with KOH(aq) and give four products in the mixture.

Ethanal aldol condensation

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.

ethanal aldol condensation

Propanone aldol condensation

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.

propanone aldol condensation

Aldol condensation of propanal

Aldol condensation of propanal also gives an aldol product.

Aldol condensation of propanal

Aldol condensation of mixture of aldehydes and ketones

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.


Aldol condensation of ethanal and propanone

Ethanal and propanone both are aldehydes. This ethanal and propanone mixture with NaOH(aq) forms four different products.

Aldol condensation of ethanal and propanone

Steps

  • Two carboanions can be given in the initial step (from ethanal and propanone).
  • One carboanion can attack ethanal or propanone to give two products.
  • Also other carboanion can attack with ethanal or propanone to give two products.

Four products can be given as a product mixture.



Aldol condensation mechanism of aldehyde and ketone


Aldol condensation with dilute strong alkali

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. So try to understand the first mechanism when only one aldehyde or ketone is in the reaction.


Aldol condensation mechanism steps

  1. Hydroxyl ions attack alpha hydrogen atom in the carbonyl compound. Then bond between alpha hydrogen and alpha carbon is broken and electrons of that bond are concentrated to alpha carbon. That forms a carbanion. Now that alpha hydrogen atom connects with hydroxyl ion and forms a water molecule.
  2. Next, carbanion attacks the carbonyl carbon of another molecule of aldehyde or ketone. A new carbon carbon bond is formed. Due to form of new carbon carbon bond, C=O bond is turned to C-O with negative charge on oxygen atom.
  3. Finally, O- attacks a water molecule and get a hydrogen atom to form alcohol group. It forms OH- ion.

Acetaldehyde aldol condensation mechanism

acetaldehyde aldol condensation mechanism

All of the aldol products formed between propanal and acetaldehyde in NaOH

  • Propanal - CH3CHCHO - Aldehyde
  • Acetaldehyde - CH3CHO - aldehyde

Two carboanions are formed from propanal and acetaldehyde. So those two carboanions can react with both propanal and acetaldehyde to give four different products.

propanone aldol condensation mechanism

acetone - propanone aldol condensation mechanism

What aldehydes are unable to do aldol condensation?

Formaldehyde does not have alpha carbon because it has only one carbon atom. therefore we cannot do aldol condensation of formaldehyde.
formaldehyde: HCHO


Benzaldehyde has alpha carbon. But that alpha carbon does not have alpha hydrogen. So benzaldehyde also cannot show aldol condensation.



Summary of aldol condenstion

Aldol condensation of aldehydes and ketones

Is aldol condensation is an oxidation - reduction ( redox) reaction?

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.


Why formaldehyde does not give aldol condensation ?

Formaldehyde does not have alpha carbon. Therefore, there is no alpha hydrogen. Without alpha hydrogen aldol condensation is not occurred.



Is aldol condensation a nucleophilic addition reaction?

Yes. Carbonol carbon is positively charged and it is attacked by carbanion. And a new carbon group is attached to the aldehyde or ketone.



What will be given when ketone is attacked by NaOH and H2O

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.



Why benzaldehyde do not react with NaOH to produce aldol product.

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.



What about methanal

Methanal do nt have both alpha hydrogen and alpha carbon atom. So no aldol condensation for methanal.



Aldehyde + NaOH products

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.



What are the aldol reagents?

Dilute strong alkalis such as NaOH or KOH or Ba(OH)2 are used as aldol reagents.



For aldol condensation, why need H on alpha carbon

Due to acidity of those hydrogen atoms, these type of reactions are occurred. A base can react with these alpha hydrogen atoms.



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