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- Title
Kinetic and mechanistic studies of the reactions of 2-mercaptoethanol and thioglycolic acid with a Co(III)-bound superoxide complex.
- Authors
Mandal, Arabinda; Das, Ranendu Sekhar; Singh, Bula; Banerjee, Rupendranath; Mukhopadhay, Subrata
- Abstract
In acid media ([H+] = 0.01-0.06 M), 2-mercaptoethanol (HSCH2CH2OH, abbreviated as MERCAP) and thioglycolic acid (HSCH2COOH, abbreviated as TGA) reduce the superoxo complex [(en)(dien)CoIII(O2)CoIII(en)(dien)]5+ ( 1) to the corresponding peroxo complex [(en)(dien)CoIII(μ-O2)CoIII(en)(dien)]4+ ( 2). The observed rate ( ko), although proportional to both [MERCAP] and [TGA], is higher for TGA than for MERCAP, which is contrary to the expected trend on the basis of standard reduction potential (-0.14 versus -0.26 V, respectively). Moreover, ko values decrease with increasing media ionic strength ( I) and [H+] and thus, thiolate anions are supposed to be the reactive forms of the reductants. Under the experimental conditions, the concentrations of such reductants (-SCH2CH2OH and HOOCCH2S-, respectively) from MERCAP (p Ka = 9.7) and TGA (p Ka1, p Ka2 = 3.53, 10.10) are very small. However, for TGA, the tautomerization between HSCH2COO- and -SCH2COOH (p Ki = 7.0) plays a significant role in increasing the concentration of HOOCCH2S-. The rates of both of the reactions are limited by the rate of solvent diffusion and this fact is also supported by the relatively low activation energy ( Ea), for the reactions. The Ea values for the reactions with MERCAP and TGA are close enough (29.6 ± 1.3 and 27.0 ± 0.4 kJ M−1 respectively) to suggest a common reaction mechanism for both.
- Subjects
THIOGLYCOLIC acid; MERCAPTOETHANOL; COBALT compounds synthesis; SUPEROXIDES; REDUCTION potential; ACTIVATION energy
- Publication
Canadian Journal of Chemistry, 2015, Vol 93, Issue 11, p1276
- ISSN
0008-4042
- Publication type
Article
- DOI
10.1139/cjc-2015-0110