Silver (III) oxidation of the DL-mandelate ion

Abstract

The oxidation of DL-mandelate (MAN^–) by [Ag(OH)₄]^– in strongly alkaline media has been investigated by stopped-flow spectrophotometry. Product analysis shows that benzaldehyde is the sole organic product when MAN^– is in excess. The rate law is k_obs=(a+b[OH^–])[MAN^–], where a=(6.2 ± 0.2) dm³ mol^–1 s^–1 and b=(7.0 ± 0.3) dm⁶ mol^–2s^–1 at 25 °C and I= 1.3 mol dm^–3. Oxidation of benzaldehyde (to benzoic acid) by Ag^III is more than an order of magnitude slower than the mandelate reaction. Activation parameters for both hydroxide-dependent and -independent paths are ΔH_a^‡=(59.3 ± 2.1) kJ mol^–1, ΔS_b^‡=(–32.0 ± 0.5) J mol^–1 K^–1 and ΔH^‡_b=(48.0 ± 2.3) kJ mol^–1, ΔS_b^‡=(–68.5 ± 0.6) J mol^–1 K^–1 respectively. Two different mechanistic pathways are discussed. A base-catalysed processs, followed by C–C cleavage, seems to be the most probable mode of decarboxylation. The comparative non-reactivity of phenylglyoximate and α-methoxyphenylacetate ions underlines the importance of the α-OH group in the mandelate oxidation.