What is Crossed Aldol Condensation?
First, we need to understand about aldol condensation reaction. When there is a dilute base containing α-hydrogen, a reaction occurs in the aldehydes present in the solution and the result of the reaction is the β-hydroxy aldehydes and these aldehydes are called aldols and the reaction is called aldol condensation. Crossed Aldol Condensation is nothing but an aldol reaction that takes place between carbonyl compounds that are of two different types.
Aldol Condensation Reaction
Aldol condensation is a chemical process in which the enolate ion interacts with a carbonyl molecule to produce a -β hydroxy ketone or -β hydroxy aldehyde, which is then dehydrated to produce a conjugated enone. In the study of organic synthesis, aldol condensation is an important contributor paving a way for the creation of carbon-carbon bonds.
Base-catalysed aldol condensation is a common example of an aldol condensation where hydroxide ion is commonly used as the catalyst.
CH3CHO > CHCHO=CH3CH
Aldol Condensation Mechanism
An organic process in which an enolate ion interacts with a carboxyl molecule to create a –β hydroxy aldehyde or –β hydroxy ketone is known as aldol condensation.
- Hydroxide acts as a base, causing the acidic a-hydrogen to migrate, resulting in the reactive enolate ion. An acid-base reaction may be seen in this process.
- The nucleophilic enolate ion attacks the aldehyde where the electrophilic carbonyl carbon is located. This phenomenon is an addition reaction that is nucleophilic and results in the production of alkoxide intermediates.
- The water molecules get deprotonated by alkoxide and the by-product of this reaction are–β hydroxy aldehyde and hydroxide.
Crossed Aldol Condensation
Crossed aldol condensation is a condensation reaction taking place between two different molecules of a ketone or aldehyde in alcohol or water, which are protic solvents. When carbonyl compounds of two different types constitute a condensation reaction, it is known as crossed aldol condensation. Both aldehydes can generate carbanions and function as carbanion acceptors when they have alpha hydrogens. As a result, a combination of four compounds emerges with minimal synthetic value.
If alpha hydrogen is not present in one of the aldehydes, then a carbanion acceptor is the only thing the alpha hydrogen is capable of and if such cases happen, only two products can be formed. An aromatic aldehyde with no alpha position is a frequent substrate for the crossing aldol reaction. In addition, the rapid dehydration that occurs in the initial condensation process produces α, β–unsaturated ketone and does not allow the retro aldol to take place.
Types of Condensation Reactions
It is crucial to be able to tell the difference between the various carbonyl compound reaction from aldol condensation.
- Water is eliminated from Japp–Maitland condensation by nucleophilic displacement.
- An aliphatic nitro compound and an aldehyde are used in a Henry reaction.
- There are two ester compounds in a Claisen condensation.
- Dieckmann condensation occurs when two ester groups in the same molecule combine to form a cyclic molecule.
- In the Perkin reaction, the enolate produced by the anhydride is aromatic.
Applications of Aldol Condensation
- Preparation of large molecules from small primitive materials can be relatively easy using the method of the aldol condensation reaction.
- Aldol condensation is useful in the synthesis of repellents against insects like 2-ethyl-3-hydroxyhexanal.
- In the field of biosynthesis, condensation reaction is the most effective way to obtain large molecules from simple molecules comprising of 2 or 3 carbons.
- Carbon-carbon bonds are formed as a result of an aldol condensation. In photosynthesis and gluconeogenesis, this reaction process produces C-C bonds which are important for metabolism biochemistry. This is the 5th step in the process of Glycolysis.
- The process is frequently used to create solvents like isophorone and diacetone alcohol. It catalyses the creation of fragrances. It’s also utilised in the production of medicines like a, b unsaturated ketones, and aromatic ketones called chalcones. It’s also often utilised in the production of plasticizers.
Aldol condensation is a well-known chemical process. It’s been well-understood for decades and is still frequently utilised today. In reality, with the passage of time, the majority of responses have fallen out of favour. This is not the case with this response. Its significance has remained constant, if not increased, over time.
Another type of condensation reaction is the Claisen condensation. This reaction results in the by-product of C-C between two esters or between either a carbonyl compound and an ester. This reaction occurs in the presence of a strong base and the result of the reaction is a beta-diketone or a keto-ester. Rainer Ludwig Claisen, a German scientist, is credited with discovering the reaction. This response is depicted in the diagram below. The Claisen condensation reaction is initiated by the removal of an alpha proton by the action of a strong base, resulting in the creation of an enolate ion.
If you want to get in-depth information about complex topics like this, refer: Aldehydes,ketones And Carboxylic Acids from Class 12 Chemistry– Find All the Questions & Answers