Ring opening of chemically activated cyclopentyl and methylcyclobutyl radicals

Abstract

The endothermic ring opening of vibrationally excited cyclopentyl radicals generated in the gas phase by adding H to cyclopentene has been studied. The results obtained were combined with literature results to test activated complex models for the ring-opening reaction, and the data were fit by a model which gives a thermal A factor of 1014-0±°•4 sec-1, and a critical energy of 34.6 ± 0.6 kcal/mol. The exothermic ring opening of vibrationally excited methyIcyclobut-1 -yl generated by adding H to methylenecyclobutane in the gas phase was also studied. Assuming a model for the ring-opening activated complex which corresponds to a thermal A factor of 10141±0•4 sec-1, the critical energy is calculated to be 31.7 ± 0.7 kcal/mol. These results suggest that the critical energies for ring openings of cycloalkyl radicals may be relatively insensitive to ring strain.

Mutual isomerization of cyclopentyl and 1-Penten-5-y1 radicals

“…The excited cyclopentyl radical (Rl) isomerizes into the 1-penten-5-yl structure (R2) in reaction (1). The product of this reaction undergoes either reverse rearrangement in reaction (2) or dissociates yielding ethylene and an allyl radical…”

“…The RRKM calculations of the threshold energies for reactions (1) and (2) reported in the literature were based on experimental data obtained at either one; [1,3] or two [2] excitation energies of the radical. Obsolete thermochemical input data, lower than those presently accepted U21, were used for bond dissociation energies of the C -H bonds.…”

“…The range of excita- tion energies is broad enough to obtain accurate threshold energies and Arrhenius parameters for reactions (1) and (2). The use of two different initial radicals (cyclopentyl and 1-penten-5-yl) is an additional advantage.…”

“…(1) In the case of recalculation of the data reported by Carter and Tardy [2] and by Stein and Rabinovitch [3], excitation energy of the cyclopentyl radical is so low that the distribution of energy becomes important. This distribution was calculated according to the well known methods [13-151. Vibrational frequencies of the activated complex for the reverse reaction were taken from Carter and Tardy.…”

“…Ring openings in the cyclopentyl radical as well as reverse intramolecular ring closure in the 1-penten-5-yl radical have been studied by chemical activation methods [l-31. (1) 0' (RI) -CH, = CH-CH,-CH,--CH, (R2) (2) 624 GIERCZAK, GAWLOWSKI, AND NIEDZIELSKI tynes, and a molecular sieves 5A column to remove other straight chained impurities. 1 ,CPentadiene (Fluka AG) was purified gas chromatographically using a Porasil B column.…”