Photolysis of Mixtures of Cyclobutane and Acetone-d6. Reactions of the Cyclobutyl Radical

The activation energy for abstraction of hydrogen from cyclobutane by a methyl radical is 10.3±0.15 kcal/mole, with a pre-exponential factor relative to methyl radical abstracting D from acetone-d6 of 1.8. At temperatures of less than 200°, the cyclobutyl radicals disappear almost exclusively by combination with methyl radicals to form methyl cyclobutane. At higher temperatures the cyclobutyl radical (a) opens to form the ĊH2CH2CH=CH2 radical with an Eact of 18.1±0.8 kcal/mole, and (b) splits out a molecule of hydrogen. The latter reaction is considerably slower than the former. The ĊH2CH2CH=CH2 radical is unusually stable, and does not decompose appreciably even at 325°C. At 425°, the highest temperature which could be reliably studied in this system, the product analysis indicates that the radical decomposes slowly to ethylene and a vinyl radical. By contrast, n-butyl radicals and s-butyl radicals readily decompose at 250°C in a similar reaction media. The main reaction of the ĊH2CH2CH=CH2 radical is the abstraction of H (or D) from the parent mixture. It also combines with a methyl radical to form pentene-1. Evidence is presented to show that the ĊH2CH2CH=CH2 radical isomerizes at temperatures of 350° and above to the resonance stabilized CH3ĊHĊHĊH2 radical, with an energy of activation of 20.0±0.7 kcal/mole.