Silver (III) oxidation of the DL-mandelate ion

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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 kobs=(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 AgIII is more than an order of magnitude slower than the mandelate reaction. Activation parameters for both hydroxide-dependent and -independent paths are ΔHa‡=(59.3 ± 2.1) kJ mol-1, ΔSb‡=(-32.0 ± 0.5) J mol-1 K-1 and ΔH‡b=(48.0 ± 2.3) kJ mol-1, ΔSb‡=(-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. © 1990, The Royal Society of Chemistry. All rights reserved.

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Journal of the Chemical Society, Perkin Transactions 2