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Permanganate ion oxidations X. Kinetics and mechanisms of the initial stage in the oxidation of 4‐hydroxy‐2‐pyrimidinethiols
Author(s) -
Freeman Fillmore,
Bond Dorrece Lynn,
Chernow Steven Martin,
Davidson Pamela Ann,
Karchefski Elaine Marie
Publication year - 1978
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550100902
Subject(s) - chemistry , permanganate , thiouracil , kinetics , medicinal chemistry , thiol , ion , inorganic chemistry , phosphate , reaction rate constant , uracil , photochemistry , organic chemistry , medicine , dna , biochemistry , physics , thyroid , quantum mechanics
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.