SUBSTITUENT EFFECT IN THE OXIDATION OF CINNAMIC-ACIDS BY QUINOLINIUM DICHROMATE

Oxidation of cinnamic acid and substituted cinnamic acids by quinolinium dichromate is first order each in [oxidant] and [cinnamic acid]. The reaction is acid catalysed, and Hammett's acidity function H-0 is used as a measure of the acidity of the medium. A medium of low dielectric constant favours the oxidation process. Among the substituents used for the oxidation process, the order of reactivity is found to be p-OCH3 >> p-CH3 > H > p-Cl > m-Cl > m-NO2 > p-NO2. Correlation of log k(2) with sigma(+) gives a linear plot with a negative slope and the reaction constant rho(+) is found to be - 0.53. The mechanism proposed involves a slow attack of (QH)H2CrO4 on cinnamic acid forming a cyclic complex which ultimately decomposes to give benzaldehyde.