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Introduction to the Aldol Reaction

A helpful carbon-carbon bond-forming reaction termed as the Aldol Reaction or the Aldol Condensation is yet other instance of electrophilic substitution at the alpha carbon in enols or enolate anions. Three instances of the base-catalyzed aldol reaction are displayed in the diagram, and corresponding acid-catalyzed reactions also take place. The basic transformation in this reaction is a dimerization of an aldehyde (or ketone) to a beta-hydroxy aldehyde (or ketone) by alpha C-H addition of one reactant molecule to the carbonyl group of a second reactant molecule. A display depicting the nucleophilic enolic donor molecule and the electrophilic acceptor molecule jointly with the newly formed carbon-carbon bond will be shown.

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In the existence of acid or base catalysts the aldol reaction is reversible, and the beta-hydroxy carbonyl results might revert to the initial aldehyde or ketone reactants. In the nonexistence of such type of catalysts these aldol products are completely stable and isolable compounds. Due to this reversibility, the yield of aldol results is associated to their relative thermodynamic stability. In the example of aldehyde reactants (like in reactions no.1 & 2 above), the aldol reaction is modestly exothermic and the yields are good. Though, aldol reactions of ketones are less favorable (for example no.3 above), and the equilibrium result concentration is small. A good way of conquering this disadvantage has been found. A relatively insoluble base, Ba(OH)2, is employed to catalyze the aldol reaction of acetone, and the result is removed from contact with this base by filtration and recirculation of the acetone.

A. Dehydration of Aldol Products

The results of aldol reactions frequently go through a subsequent elimination of water made up of an alpha-hydrogen and the beta-hydroxyl group. The result of this beta-elimination reaction is an α,β-unsaturated aldehyde or ketone, as displayed in the diagram. Acid-catalyzed conditions are more generally employed to influence this elimination (instance no.1, 2 & 5), but base-catalyzed elimination also takes place, particularly on heating (instance no.3, 4 & 5). The additional stability offered by the conjugated carbonyl system of the product makes some ketone aldol reactions thermodynamically favorable (no.4 & 5) and mixtures of stereoisomers (E & Z) are get from reaction no.4. Reaction no.5 is an interesting instance of an intramolecular aldol reaction; such type of reactions forms a new ring. 
Reactions where a larger molecule is created from smaller components, with the elimination of a extremely small by-product like water are termed as Condensations. Though, the following instances are properly considered as aldol condensations. Dehydration step of an aldol condensation is also reversible in the existence of acid and base catalysts. Subsequently, on heating with aqueous solutions of strong acids or bases, several α, β-unsaturated carbonyl compounds fragment into smaller aldehyde or ketones, a process termed as the retro-aldol reaction.

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B. Mixed Aldol Condensations

The previous instances of aldol reactions and condensations employed a common reactant as both the enolic donor and the electrophilic acceptor. The product in such type of cases is all the time a dimer of the reactant carbonyl compound. Aldol condensations among distinct carbonyl reactants are called crossed or mixed reactions, and under specific conditions such type of crossed aldol condensations can be effective. Some instances are displayed below, and in most cases beta-elimination of water takes palce under the circumstances used. The exception, reaction no.3, is carried out under mild circumstances with an excess of the reactive aldehyde formaldehyde serving in the role of electrophilic acceptor. The first reaction illustrates that ketones having two sets of alpha-hydrogens may react at both sites if enough acceptor co-reactant is supplied. The interesting dissimilarity in regioselectivity displayed in the second reaction (the reactants are in the central shaded region) demonstrates some subtle variations between acid and base-catalyzed aldol reactions. The base-catalyzed reaction proceeds through an enolate anion donor species and kinetically favored proton removal is from the less substituted alpha-carbon. The acid-catalyzed aldol continues through the enol tautomer and the more stable of the two enol tautomers is that with the more substituted double bond. At last, reaction no.4 has two reactive alpha-carbons and a reversible aldol reaction might take place at both. Only one of the two aldol results can go through a beta-elimination of water, so the eventual isolated result comes from that reaction sequence. The Claisen-Schmidt reaction are aldol condensation of ketones with aryl aldehydes to form the α,β-unsaturated derivatives.

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