ABSTRACT
Sirena C. Hargrove-Leak, Janine Lichtenberger, and Michael D. Amiridis University of South Carolina, Department of Chemical Engineering, Swearingen
Engineering Center, Columbia, SC 29208
amiridis@engr.sc.edu Abstract The Claisen-Schmidt condensation of 2΄-hydroxyacetophenone and different chlorinated benzaldehydes over MgO has been investigated through kinetic and FTIR spectroscopic studies. The results indicate that the position of the chlorine atom on the aromatic ring of the benzaldehyde substantially affects the rate of this reaction. In particular, the rate increases in the following order: pchlorobenzaldehyde < m-chlorobenzaldehyde < o-chlorobenzaldehyde. The difference between the meta and para-substituted benzaldehyde can be attributed to electronic effects due to the difference in the Hammett constants for these two positions. Steric effects were found to be responsible for the higher rate observed with the o-chlorobenzaldehyde. Introduction The synthesis of fine chemicals and pharmaceuticals has traditionally been achieved via homogeneous catalytic methods. Heterogeneous catalysis is an attractive alternative to these processes because of its inherent waste minimization and the potential to consolidate steps in a process. The synthesis of flavanones (i.e., condensation of 2΄-hydroxyacetophenone and benzaldehydes to form a 2΄-hydroxychalkone followed by an isomerization leading to the corresponding flavanone) represents an example of an important, homogeneously-catalyzed reaction that can be successfully heterogenized over solid catalysts [1]. This reaction has been previously investigated in our group in an effort to understand the reactant-solvent-catalyst interactions occurring when it is carried out over MgO in various solvents [2 - 4]. Substitution effects have been observed for both the homogeneously and heterogeneously base-catalyzed Claisen-Schmidt condensation of ketones and aldehydes with functional groups substituted in the para-position [5 - 12]. In this
Investigation of Chlorine Substitution 386
study, we extend our investigation of observed substitution effects in the reaction between 2΄-hydroxyacetophenone and benzaldehydes to include metaand ortho-substitution. It is anticipated that both steric and electronic effects may play a role in these cases. In order to explore these effects we have combined kinetic and infrared studies and focused on the reaction of 2΄- hydroxyacetophenone with substituted chlorobenzaldehydes. Results and Discussion Normalized 2΄-hydroxyacetophenone concentration versus time data obtained during the reaction of equimolar amounts of 2΄-hydroxyacetophenone with different chlorobenzaldehydes are shown in Figure 1. These data indicate substantial differences in the reactivity of the different chlorobenzaldehydes, depending on the position of the Cl atom. In particular, the initial reaction rate (equal to the slope of the concentration versus time curve) nearly doubles in going from the para-to the ortho-chlorobenzaldehyde (i.e., parachlorobenzaldehyde: 160 kmol/g-cat/s, meta-chlorobenzaldehyde: 191 kmol/gcat/s and ortho-chlorobenzaldehyde 268 kmol/g-cat/s). A similar high initial rate was also observed with 2,3-dichloro-benzaldehyde (298 kmol/g-cat/s). The difference in reactivity between meta-and para-chlorobenzaldehyde is consistent with anticipated electronic effects and can be explained by means of the Hammett constants for Cl substitution in these two positions (i.e., 0.37 for 3chlorobenzaldehyde versus 0.23 for 4-chlorobenzaldehyde). Steric effects have been previously reported to affect the reactivity of ortho-substituted molecules, due to the close proximity in this case of the additional functional group to the reaction center. Consequently, the orientation of the functional group in the ortho position may sterically affect the ability of the active center to participate in the reaction [13 - 14].
