Substituent effect in the oxidation of cinnamic acids by quinolinium dichromate

Abstract

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 H₀ 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 <i>p</i>-OCH₃ » <i>p</i>-CH₃> H > <i>p</i>-Cl > <i>m-</i> Cl > <i>m-</i>NO₂> <i>p-</i>NO₂<i>• </i> CorreIation of log <i>k₂ </i> with σ⁺ gives a linear plot with a negative slope and the reaction constant ρ⁺ 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.