Abstract
The gas‐phase thermal decomposition of 7,7‐dimethylbicyclo[3.2.0]hept‐2‐en‐6‐one (DBH) to yield cyclopentadiene and 1,1‐dimethylketene as primary products was studied in the temperature range of 470‐550 °K using a static reaction system. First‐order rate constants for the depletion of DBH based on the internal standard technique and gaschromatographic analyses were independent of the initial starting pressure (7‐68 torr) and of the conversion, ranging between 5% and 89%. (Throughout this paper, 1 torr = (101.325/760) kNm^−2^, and 1 cal = 4.184J). The reaction is essentially homogeneous, as the nature of the reaction vessel surface (Teflon or glass) had no effect on the observed rate constants which fit the Arrhenius relationship
where θ = 2.303 RT. These activation parameters, when compared with those for similar reactions involving the molecules bicyclo[3.2.0]hept‐2‐en‐6‐one, bicyclo[3.2.0]heptan6‐one, and cyclobutanone, demonstrate a very small effect of the alkyl substituents bonded to the carbon atom adjacent to the carbonyl carbon. Accepting the previously discussed concerted and pronounced polar nature of the mechanism for these retro‐ketene addition reactions, the present data suggest that considerable changes in charge densities between the ground and transition state are only occurring on the two opposite centers of the molecule, with the negative charge residing essentially on the oxygen atom and the positive charge on the opposing bridgehead carbon atom. It then appears that the charge separation in the transition state is more appropriately described as being pseudo‐zwitterionic rather than quadrupolar in nature.