Aldol Condensation can be defined as an organic reaction in which enolate ion reacts with a carbonyl compound to form β-hydroxyketone or β-hydroxyaldehyde, followed by a dehydration to give a conjugated enone. Aldol Condensation plays a vital role in the organic synthesis, creating a path to form carbon-carbon bonds.
One of the common examples for base-catalyzed aldol condensation is stated below in which catalyst generally used is hydroxide ion.
Mechanism of Aldol Condensation
- In reverse order, The hydroxide ion deprotonates the aldehyde.
- Here Enolate ion 1 adds to the unreacted aldehyde.
- Alkoxide ion 2 is protonated by water.
- A small amount of aldol is converted into enolate ion (4) by hydroxide ion.
- Here Enolate Ion(4) loses a hydroxide ion.
Step 1 to step 3 illustrates aldol reaction.
Types of Condensation
It is important to differentiate aldol condensation from various reactions of carbonyl compounds.
- In a case of Perkin reaction, enolate generated by anhydride is aromatic.
- A Claisen condensation contains 2 ester compounds.
- A Henry reaction contains an aliphatic nitro compound and an aldehyde.
- Dieckmann condensation contains 2 ester groups present in the same molecule, which produces cyclic molecule.
- In Japp–Maitland condensation, water is removed by nucleophilic displacement.
Some frequently asked questions in aldol condensation reaction that may help you in exam preparation are as follows:
Question 1: What is aldol condensation?
Answer: When aldehydes and ketones having at least one α-hydrogen are treated with dilute alkali (which act as a catalyst) they form β-hydroxyaldehydes (aldol) or β-hydroxyketones (ketol) respectively. This reaction is known as aldol condensation.
Question 2: Explain the mechanism of aldol condensation.
Answer: Aldol condensation is an organic reaction in which an enolate ion reacts with carboxyl compound in order to form a β– hydroxyaldehyde or β– hydroxyketone.
- Hydroxide functions as a base and therefore moves the acidic a-hydrogen producing the reactive enolate ion. This reaction can be seen as an acid-base reaction.
- The aldehyde is attacked at the electrophilic carbonyl carbon by the nucleophilic enolate ion. This attack is a nucleophilic addition reaction and gives alkoxide intermediate.
- The alkoxide deprotonates water molecule, thereby producing hydroxide and the β–hydroxyaldehyde.
Which reference books can be followed to prepare for aldol condensation?
Answer: For studying aldol condensation one can follow the NCERT chemistry textbook part-2 for class 12th. The chapter named ‘Aldehydes, Ketones and Carboxylic acids’ in this book contains this process. For a better understanding of this concept, use the book Organic Chemistry by Morrison & Boyd or reference books such as Organic Chemistry written by Solomons & Fryhle. Solving previous year JEE papers will also be an added advantage on this topic.