UC San Diego

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