Permanganate ion oxidations X. Kinetics and mechanisms of the initial stage in the oxidation of 4-hydroxy-2-pyrimidinethiols

✍ Scribed by Fillmore Freeman; Dorrece Lynn Bond; Steven Martin Chernow; Pamela Ann Davidson; Elaine Marie Karchefski

• Publisher: John Wiley and Sons
• Year: 1978
• Tongue: English
• Weight: 463 KB
• Volume: 10
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550100902

✦ Synopsis

Abstract

Permanganate ion rapidly oxidizes the thiol group in 2‐thiouracils (2,3‐dihydro‐2‐thioxo‐4(1H)‐pyrimidinones, 4‐hydroxy‐2‐pyrimidinethiols) to the corresponding uracil‐2‐sulfonates in phosphate‐buffered solutions. The oxidations, which are first order in oxidant and first order in reductants, are characterized by relatively small energies of activation (E~a~ ≅ 4.3–6.7kcal/mol) and relatively large negative entropies of activation (Δ__S__‡ = −24.9–−34.4 eu). The relative rates of oxidation of 2‐thiouracil (2), 6‐methyl‐ (4), 6‐n‐propyl‐ (6), 1,6‐dimethyl‐(8), and 3,6‐dimethyl‐2‐thiouracil (9) are 1.00:1.23:0.98:7.32:2.54. The rate of oxidation of 1,3,6‐trimethyl‐2‐thiouracil (12) is several orders of magnitude slower than that of the other thiouracils. Although the thiol group or thiolate anion is probably involved in the rate‐determining step, several other possible mechanisms are also considered.