Kinetics of the four-centre elimination of keten from bicyclo[3,2,0]hept-2-en-6-one in the gas phase

Abstract

The gas-phase pyrolysis of bicyclo[3,2,0]hept-2-en-6-one (BCH) to yield cyclopentadiene and keten as main products was studied in the range 471–534 K, using a static reaction system. The rate of depletion of BCH with time was followed gas chromatographically using cyclohexanone as an internal standard. The reaction is essentially homogeneous and consistent first-order rate constants were obtained for conversions ranging from 8 to 97% and a variation in total initial pressures from 15 to 485 Torr [= 2·0–64·66 kN m^–2](using propene as a diluent). The observed rate constants yield the following Arrhenius relationship (with standard errors): log k(s^–1)=(13·16 ± 0·11)–(37·53 ± 0·25)/θ where θ equals 2·303 RT(K) in kcal mol^–1.

The results are consistent with a concerted polar four-centre elimination mechanism and are in general agreement with the well documented observations for the back reaction, the 2 + 2-addition of ketens to conjugated dienes. The surprisingly low activation parameters observed in this study when compared with those for the similar cyclobutanone reaction (10^{14·56 – 52·0/θ}) are attributed partly to large stabilizing effects of the substituents onto the induced positive (ca. 8·5 kcal mol^–1) and negative (ca. 1·5 kcal mol^–1) charges on the bridgehead carbon atoms and partly to an additional stabilizing effect resulting from the interaction of the carbonyl group with the extra double bond in the proposed twisted transition state.