Hydrolysis of tert-butyl formate:: Kinetics, products, and implications for the environmental impact of methyl tert-butyl ether

Assessing the environmental fate of methyl tert-butyl ether (MTBE) has become a subject of renewed interest because the large quantities of this compound that are being used as an oxygenated additive in gasoline. Various studies on the fate of MTBE have shown that it can be degraded to tert-butyl formate (TBF), particularly in the atmosphere. Although it is generally recognized that TBF is subject to hydrolysis, the kinetics and products of this reaction under environmentally relevant conditions have not been described previously. In this study, we determined the kinetics of TBF hydrolysis as a function of pH and temperature. Over the pH range of 5 to 7, the neutral hydrolysis pathway predominates, with k(N) = (1.0 +/- 0.2) x 10(-6)/s. Outside this range, strong pH effects were observed because of acidic and basic hydrolyses, from which we determined that k(A) = (2.7 +/- 0.5) x 10(-3)/(M.s) and k(B) = 1.7 +/- 0.3/(M.s). Buffered and unbuffered systems gave the same hydrolysis rates for a given pH, indicating that buffer catalysis was not significant under the conditions tested. The activation energies corresponding to k(N), k(A), and k(B) were determined to be 78 +/- 5, 59 +/- 4, and 88 +/- 11 kJ/mol, respectively. In all experiments, tert-butyl alcohol was found at concentrations corresponding to stoichiometric formation from TBF. Based on our kinetics data, the expected half-life for hydrolysis of TBF at pH = 2 and 4 degrees C (as per some standard preservation protocols for water sampling) is 6 h. At neutral pH and 22 degrees C, the estimated half-life is 5 d, and at pH = 11 and 22 degrees C, the value is only 8 min.