In the 'aldol addition' reaction an enol or enolate of the aldehyde or ketone reacts by a second aldehyde or ketone making a new carbon-carbon bond. This makes the aldol reaction a significant reaction for organic synthesis. Originally, the aldol reaction employed ethanal and thus the product contained both an aldehyde and an alcohol functional group; therefore it became termed as the aldol reaction.
Mechanism of Aldol Condensation:
Methylene groups which are α to the carbonyl function are acidic. They can thus be abstracted via a base, generally NaOH, Na2CO3, t-amine, or n-BuLi. This produces a nucleophilic species that attacks the carbonyl group of the other carbonyl compound. The carbonyl compounds which might be involved comprise the given: (a) Aldehyde + aldehyde (b) aldehyde + ketone and (c) ketone + ketone. In the above cases the aldehyde might be similar or different or the ketones might be similar or different. Whenever they are different it is termed as mixed aldol condensation. The product of aldol condensation is a β-hydroxycarbonyl therefore the name aldol. This is a significant reaction in organic chemistry as on dehydration of the aldol it provides an α,β-unsaturated carbonyl compound.
Fig: Aldol Condensation-mechanism
Aldol-type Condensation reactions:
The above is the common aldol condensation reaction. There are other aldol-like reactions that are very significant in the organic synthesis. They comprise reactions like Reformatsky reaction and Perkin reaction.
The Reformatsky reaction is an organic reaction that condenses aldehydes (or ketones), by α-halo esters, by employing metallic zinc to form β-hydroxy-esters. This on dehydration provides an α,β-unsaturated ester that can be hydrolyzed to the corresponding carboxylic acid. If applied to the aromatic aldehyde it leads to the formation of cinnamic acid derivatives.
Fig: Reformatsky reaction
=> Mechanism of the Reformatsky Reaction:
Organozinc compounds are made up from α-halogenesters in the similar way as Grignard reagents. This reaction is possible because of the stability of esters against the organozincs. Because of very low basicity of zinc enolates, there is hardly any competition from the proton transfer and the scope of carbonyl addition partners is fairly broad. In presence of ketones or aldehydes, the organozinc compounds react as the nucleophilic partner in an addition to provide β-hydroxy esters.
The ester-stabilized organozinc reagent compared to organolithiums and organomagnesium halides (that is, Grignard reagents), the organozinc halide reagents employed in the Reformatsky reaction are comparatively stable, and most of them are available commercially.
The Perkin reaction is an organic reaction build by William Henry Perkin which can be employed to form cinnamic acids that is, α-β-unsaturated aromatic acid via the aldol condensation of aromatic aldehydes and acid anhydrides in the presence of an alkali salt of the acid. The Perkin condensation is an extremely helpful organic reaction that provides a manner to add two carbon atoms to the side chain of an aromatic ring. The method of the Perkin reaction is complex. It comprises the attack of enolate anion of the acetic anhydride on the aromatic aldehyde. A sequence of rearrangement follows, by the overall effect of making a carbon-carbon double bond (that is, loss of water) and hydrolyzing the anhydride function. Generally the trans-isomer makes preferentially as it is more stable.
Fig: Perkin Reaction
Note: This reaction is applicable only to the aromatic aldehydes.
Synthetic Application: Aldol Cyclization
The Intramolecular aldol reactions of diketones are often helpful for making five- and six- membered rings. Aldol cyclizations of rings bigger than six and smaller than five are less common as larger and smaller rings are less favored via their energy and entropy. The given reactions exhibit how a 1,4-diketone can condense and dehydrate to provide a cyclopentenone and how a 1,5-diketone provides a cyclohexenone.
Fig: Aldol Cyclization
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