The mechanism of thermal elimination. Part 17. Rate data for pyrolysis of vinyl acetate and 1,2-diacetoxyethane

Abstract

Between 721.7 and 636.4 K, vinyl acetate undergoes thermal decomposition according to the rate equation log (k/s^–1)= 10.43 – 182.4/2.303 RT(R= 8.312 J mol^–1 K^–1). Approximately 95% of reaction is decarbonylation to give acetone, with decomposition to ketene and acetaldehyde being the minor component. The latter reaction is an analogue of acetic anhydride pyrolysis which takes place at least 10⁶ times faster per β-hydrogen at 600 K. This very large rate difference parallels that between β-keto-acids and βγ-alkenoic acids and contrasts markedly with pyrolysis of alkyl acetates and alkyl vinyl ethers, which occurs at closely similar rates. The contrasting behaviour most probably reflects differences in the principal bond-breaking step of the reaction, which for vinyl acetate and acetic anhydride (and also the acids) is breaking of the β–X–H bond so that the nucleophilicity of the attacking group assumes major importance; for esters and vinyl ethers this is not the most important step so their reaction rates are similar. The relative reactivities to the acids support an alternative view that both vinyl acetate and acetic anhydride pyrolyse via their enol forms. The greater understanding of the factors affecting gas-phase elimination rates permits prediction of the relative rates of compounds not yet studied. Pyrolysis of 1,2-diacetoxyethane gave non-first-order plots, with rate acceleration due to formation of the more reactive vinyl acetate. The β-acetoxy-group (OCOMe) increased the rate of elimination (per β-hydrogen at 600 K)ca. 7-fold, which compares with factors of 388 and 144 for COMe and CO₂Me respectively, and a reduction of 3.6-fold by OMe.