Question

ALDOL FORMATION: The Aldol product is removed from the water by CRYSTALLIZATION + H20 H3CO OCH3

Answer 1

All aldol reactions proceed due to the formation of an enolate ion that exists due to the abstraction of the acidic alpha proton in the carbonyl compound with such a proton, stabilizing the enol form of the carbonyl compound. This enolate oxygen attacks the electrophilic carbonyl carbon on the other carbonyl molecule, forming an aldol product which will be a hydroxy carbonyl, which due to the strongly acidic/basic conditions used proceeds to dehydration to yield the chalcone product.

From this it should be noted that the reaction proceeds due to the formation and stabilization of enolate ions. All carbonyl compounds with an alpha hydrogen are constantly in equilibrium with their enol forms. This rapid interconversion, termed tautomerism is called in this case as keto-enol tautomerism. This equilibrium, unless a species stabilizes enols, shifts more towards the keto-form rather than the enol-form. Thus, aldol reactions involve consumption of the less stable species in an equilbrium. This leads to the equilibrium tipping to the right to form more enol to try and re-establish the equilbrium as per Le Chatlier's principle. However, as the minor product formed is rapidly consumed in the reaction, more and more of the ketone starts to decompose and form more of the enol, which being the suitable substrate for the reaction is continously consumed, thus propagating the reaction.

If however, any strong acid/base was not used, the rapid interconversion leads to very small quantity of the product, that will be so low that the conversion is negligible. The use of the catalyst here helps tip the equilibrium in favour of product formation by stabilizing the less stable species. Further, crystallization removes the product from water, which is also a product of the aldol condensation. This will initiate greater tendency for the equilibrium to shift towards the right as the aldol product is being constantly removed while the other product - water is left in the medium.

Thus, two principles - stablizing the minor product in an equilibrium and removal of a product from the equilibrium, sustains and completes this reaction owing to the tendency of the system to try and re-establish equilibrium under the new given conditions.