The mechanism of thermal eliminations. Part 15. Abnormal rate spread in pyrolysis of alkyl methyl carbonates and S-alkyl O-methyl carbonates due to enhanced nucleophilicity of the carbonyl group

Abstract

Rate coefficients for pyrolytic elimination of ethyl, isopropyl, and t-butyl carbonates, and of di-t-butyl carbonate have been measured over a 50 K range for each compound. The relative rates at 600 K are 1 : 29.6 : 2 934 : 3 526 and the rate spread for the primary, secondary, and tertiary compounds is inconsistent with that obtained from elimination of a range of other esters including alkyl phenyl carbonates. The least reactive compounds are found to be more reactive than predicted, probably owing to a combination of the greater E_i character of their transition states and the high nucleophilicity of the carbonyl group in dialkyl carbonates. Rate data for pyrolysis of S-ethyl, S-isopropyl, and S-t-butyl O-methyl carbonates give the relative rates at 600 K as 1 : 22 : 1 074. The Bu^t : Prⁱ rate ratio (49 : 1) is therefore greater than the Prⁱ : Et ratio, as it is for all other related eliminations; this confirms that the literature results (which show the converse) are in error. The seemingly anomalous relative reactivities of thiolacetates and thiolcarbonates as compared with their oxygen-containing analogues is also shown to be consistent with the effect of variable polarity of the transition state in ester pyrolysis upon the importance of carbonyl group nucleophilicity, and this also accounts for the relative reactivities of thiol-, thion-, and dithio-acetates. Steric acceleration appears less important for carbonates than for acetates, since the rate for di-t-butyl carbonate (statistically corrected) is lower than for t-butyl methyl carbonate, whereas pivalates are more reactive than acetates.