2-Hydroxyacetophenone, reaction with benzaldehyde

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 p-chlorobenzaldehyde < 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. 

Reaction between 2 -hydroxyacetophenone and benzaldehyde (Claisen-Schmidt condensation) in the absence of a solvent at 423 K giving 2 -hydroxy chalcones and flavanones has been successfully performed with MgO as a solid base catalyst/581 A conversion of 40 % after 1 h with 67 % selectivity to chalcone was achieved. The influence of the solvent and the effects of a substituent on the aromatic ring were investigated by Amiridis et a//59,6"1 The reaction was carried out on MgO at 433 K. Dimethyl sulfoxide (DMSO) showed a strong promoting effect on the reaction, which was attributed to the ability of this dipolar aprotic solvent to weakly solvate anions and stabilize cations so that both become available for reaction. In this case, a conversion of 2-hydroxyacetophenone of 47 % with a selectivity to flavanone of 78 % was achieved after 30 min in a batch reactor. Further investigations1611 showed that DMSO significantly increases the rate of the subsequent isomerization of the 2 -hydroxychalcone intermediate to flavanone. 

Aldol and related condensation reactions such as Knoevenagel and Claisen-Schmidt condensations are also widely used in the fine chemicals and specialty chemicals, e.g. flavors and fragrances, industries. Activated hydrotalcites have been employed as solid bases in many of these syntheses. Pertinent examples include the aldol condensation of acetone and citral [107, 108], the first step in the synthesis of ionones, and the Claisen-Schmidt condensation of substituted 2-hydroxyacetophenones with substituted benzaldehydes [109], the synthetic... 

The condensation of substituted benzaldehydes with 2-hydroxyacetophenones to give a,P unsaturated ketones (hydroxychalcones) of industrial interest has been reported by Climent et al. [29] to reach a yield of 85 % at a temperature of 443 K on hydrotalcite. The rate went through a maximum for a Al/(Mg Al) ratio close to 0.3. The presence of electron-acceptor groups in the aromatic ring of benzaldehyde increased the reaction rate in proportion to the value of the Hammett constant, although bulky substituents such as NO2, Cl, or OCH3 resulted in geometrical effects as a result of diffusional problems or steric restrictions. 

Corma et al. have also tested MCM-410H in the aldol condensation between benzaldehyde and 2 -hydroxyacetophenone. They showed that this material catalyzed successive aldol condensation and intramolecular Michael reaction addition to give flavone with good selectivity. Chromenes can be also produced by condensation of salicylaldehyde derivatives and diethylglucotaconate under mild conditions (Figure 9). 

This reaction is based on the formation of a color in a mixture of an aqueous sodium nitroprusside solution with carbonyl compounds in an alkaline medium. A positive test is obtained with aliphatic and certain aromatic aldehydes and aromatic and aliphatic ketones. The literature data on this reaction are conflicting and no general rule can be expressed about which carbonyl compounds give the reaction and which do not. For example, the color is not produced with formaldehyde, glyoxal, benzaldehyde, o-hydroxy-benzaldehyde, chloral, vanillin, benzophenone, naphthyl phenyl ketone, trihydroxybenzophenone, benzil, acetophenones substituted on the aromatic nucleus with hydroxyl, etc. The reaction can therefore be used for the differentiation of certain types of aldehydes and ketones, as, for example, acetaldehyde from formaldehyde, and acetophenone from benzophenone or hydroxyacetophenone, etc. Further, various colors formed in alkaline media or after the subsequent acidification of the reaction mixture can also be used for differentiation. Sodium hydroxide can be replaced by ammonia, piperidine... 

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