The mechanisms of thermal eliminations. Part 12. Relative rates of pyrolysis of ethyl, isopropyl, t-butyl, and n-butyl vinyl ethers: rate spread as an index of elimination mechanisms

Abstract

Rate coefficients have been measured between 594.5 and 681.0 K for pyrolysis of ethyl, isopropy, t-butyl, and n-butyl vinyl ethers. The relative rates of elimination at 600 K are : 1.0, 10.0, 178, and 1.52, the Arrhenius data [E/kJ mol^–1, log (A/s^–1)] being: ethyl, 185.92, 11.826; isopropyl, 182.51, 12.527; t-butyl, 164.05, 12.169; n-butyl, 177.64, 11.284. These results show that the literature data for t-butyl vinyl ether elimination are considerably in error, confirming the prediction based on statistical mechanics. The relative primary, secondary, and tertiary elimination rates are consistent with those obtained from a range of thermal eliminations, whereas the relative rates based on the literature data are not. Analysis of the relative primary, secondary, and tertiary elimination rates from this and other reactions permits prediction of rates for secondary and tertiary compounds as yet unmeasured, and also suggests that some literature data may be in error through the incursion of side reactions. A β-ethyl group increases the rate of elimination 2,3-fold per β-hydrogen at 600 K (cf. 1.62 for acetates) and may likewise be due to steric acceleration, though the possibility of electron acceptance by alkyl groups in the gas-phase cannot be excluded. The product distribution in pyrolysis of 1-methylpropylvinyl ether is shown to be accounted for by the nature of the transition state. Vinyl ethers do not pyrolyse 50° below the corresponding esters as recently claimed. Tertiary vinyl ethers are for example less reactive than tertiary esters.