The kinetics of chalcone formation via aldol condensation was studied using UV spectrophotometry. Chalcone was formed from substituted benzaldehyde (ArCHO) and acetophenone (Ar'COCH₃) with catalyst sodium hydroxide (NaOH) dissolved in acetonitrile and H₂O or deuterium oxide (D₂O). The rate of appearance of each chalcone was measured at its [lambda]max using a UV spectrophotometer, and time curves of Absorbance vs. Time were analyzed. The rate expression for the formation of chalcone is third order: Rate = k₃ [ArCHO][Ar'COCH3][ ⁻OH]. With excess [Ar'COCH₃], the reaction becomes pseudo first order in [ArCHO], where the observed rate constant (kobs) is simply proportional to the concentrations of [Ar'COCH₃] and [⁻OH]. Kinetic runs with different initial conditions of [ArCHO], [Ar'COCH₃], and [⁻OH] were prepared in H₂O and in D₂O to measure the rates and the kinetic isotope effect kD /kH. The formation of chalcone or substituted chalcone was observed to be faster in D₂O than in H₂O, which indicates that Deprotonation 2 is not the rate-limiting step. Using a process of elimination, we can conclude that the rate- limiting step for chalcone mechanism via aldol condensation is the Elimination step in the dehydration process. The intermediate [Beta]-hydroxy ketone in each chalcone formation was synthesized. The [Beta}-hydroxy ketone intermediates were reacted with NaOH to revert to starting materials ArCHO and Ar'COCH₃, and to dehydrate to product chalcone. The ratio of concentration of starting material to product was determined and the rate constants from each [Beta]-hydroxy ketone intermediate to its starting materials and product were evaluated to help complete the understanding of the mechanism of chalcone formation

Although aldol condensation is one of the most important organic reactions, capable of forming new C-C bonds, its mechanism has never been fully established. We now conclude that the rate-limiting step in the base-catalyzed aldol condensation of benzaldehydes with acetophenones, to produce chalcones, is the final loss of hydroxide and formation of the C═C bond. This conclusion is based on a study of the partitioning ratios of the intermediate ketols and on the solvent kinetic isotope effects, whereby the condensations are faster in D2O than in H2O, regardless of substitution.