Kinetics of condensation of benzaldehyde and its derivatives with acetone and methyl ethyl ketone catalysed by aluminium oxide

Kinetics of condensation of benzaldehyde with excess acetone and methyl ethyl ketone in the liquid phase and in the presence of aluminium oxide as the catalyst was investigated in the range of temperatures from 60 to 160°C. The pseudo-first order kinetics for the condensation of benzaldehyde with acetone and the positive value of the Hammett reaction constant (ρ = 1.43 ± 0.08 at 90°C) found for the condensation of substituted benzaldehydes with acetone, both lead to the conclusion that the formation of 4-hydroxy-4-phenylbutan-2-one is the rate-determining step. The ρ constant decreases with increasing temperature and the estimated isokinetic temperature corresponds to 449 ± 8 K. At temperatures ranging from 60 to 90°C, the retroaldolisation of the above β-hydroxy ketone is the minor and the dehydration of the β-hydroxy ketone the major reaction; the proportion of the dehydration reaction increases with increasing polarity of the reaction medium and decreases with increasing temperature. In the benzaldehyde condensation with methyl ethyl ketone at 90-160°C, the formation of 1-phenyl-1-penten-3-one, which is catalysed by basic sites of aluminium oxide, is the favoured reaction. The apparent activation energy for the reaction producing the latter ketone is c. 11.5 kJ mol-1 larger than that for the formation of 4-phenyl-3-methyl-3-buten-2-one resulting from the condensation catalysed by acidic sites of the catalyst.