Question

Describe the following:
(i) Cross aldol condensation
(ii) Decarboxylation

Answer 1

Hint The answer here is dependent on the basic concept of organic chemistry which deals with the reaction mechanism where aldol condensation gives the $\beta -$ hydroxy aldehyde or $\beta -$ hydroxy ketone and decarboxylation is the removal of carboxyl group.

Complete answer:
Let us now see in detail about these two reactions given and write their mechanism of reaction.
(i) Aldol condensation:
Aldol condensation as the name itself says is the condensation of an enol or enolate ion with the carbonyl compound to form a $\beta -$ hydroxy aldehyde or $\beta -$ hydroxy ketone which is followed by dehydration to give a conjugated enone.
The step by step mechanism by citing an example as general reaction can be explained as shown below,

The carboxylic addition to the enol group reacts in a protonated form where the addition product is obtained which on dehydration yields the condensation product.
(ii) Decarboxylation is the chemical reaction which removes a carboxyl group with the release of carbon dioxide gas. Here, there is removal of one carbon atom which comes out as $C{{O}_{2}}$
The general reaction for this process is given as shown below,

Therefore, the product forms is the alkane and carbon dioxide in the above case

Note: Note that the aldol condensation takes place only if there is $\alpha -$ hydrogen atom because the aldol condensation requires the formation of the enolate ion or enol and the absence of$\alpha -$hydrogen do not give these intermediates.