Uncatalyzed and General Acid Catalyzed Decomposition of Alkyl Xanthates and Monothiocarbonates in Aqueous Solutions

Article abstract of DOI:10.1021/ja00349a045

The decomposition of potassium alkyl xanthates and alkyl monothiocarbonates follows the rate law k_obbsd = k_H3O++> + k_HA + k_H2O in aqueos buffer solutions.The Broested coefficient β_lg for the k_H2O terms is -1.1 for the monothiocarbonates and -1.3 for the xanthates.These results complement the value of -1.1 observed for the alkyl monocarbonates and have been interpreted as late transition states in the decomposition direction.The reactions are also subject to general acid catalysis with α values of 0.9 +/- 0.2 and 0.8 +/- 0.1 for ethyl and methoxyethyl xanthates; α values for the monothiocarbonates are 0.58 +/- 0.05 and 0.57 +/- 0.01 for the methyl and methoxyethyl compounds, respectively.Solvent kinetic isotope effects (k_D3O+/k_H3O+) for ethyl and methoxyethyl xanthates are 2.53 and 2.12, respectevely; for ethyl, methyl, and methoxyethyl monothiocarbonate they are 1.95, 1.94, and 1.91, respectevely.The high Broensted value for ethyl xanthate and the resultant uncertainly in its numerical value coupled with the high inverse solvent isotope effect place this reaction on the borderline between a specific acid catalyzed mechanism and a concerted mechanism.The latter is favored because the value for k_H3O+ is larger than that predicted for the protonation of the alcohol oxygen of the xanthates and monothiocarbonates; further, the formation of such protonated species is estimated to be sufficiently unfavorable as to require a breakdown step faster than a molecular vibration to account for the observed rates.Acetic acid does not catalyze the k_H3O+ term for ethyl xanthate at very low pH; the small amount of catalysis at higher pH must therefore be true general acid catalysis and not a solvent effect.Broensted β_lg values for the acid-catalyzed decompositions of the xanthates and monothiocarbonates are small.These results have been interpreted in terms of concerted general acid catalysis where little change in charge occurs on the leaving group oxygen in going from the starting material to the transition state.