Isomerization of chemically activated 1-buten-1-yl and 1-buten-4-yl radicals

Abstract

Publication costs assisted by the Institute for Chemical Research Chemically activated 1-buten-l-yl radicals were generated by the addition of ethyl radicals produced by the photolysis of diethyl ketone to acetylene at 75 and 123°. The unimolecular rate constant for the isomerization of the excited 1-buten-l-yl to l-buten-4-yl radicals through 1,4-hydrogen atom migration was measured. In addition, it was found that the l-buten-4-yl formed can isomerize to methylallyl radicals via 1,2-H atom shift. The average rate constants for isomerization were found to be 1.00 X 109 and 1.05 X 109 sec-1 for 1-buten-l-yl and 3.47 X 107 and 6.20 X 107 sec-1 for l-buten-4-yl radicals at 75 and 123°, respectively. The best agreement between the rate constants as calculated by the RRKM theory and the experimental results was found when the threshold energies, Eq, were chosen as 17.1 and 33.0 kcal/mol for 1,4and 1,2-H atom shifts, respectively. It is shown that these values satisfy the expression, Eq = \> + Est where Eab is the activation energy for a bimolecular H atom abstraction and Es is the ring strain energy.

The fragmentation and isomerization of hot (vibrationally excited) ground state unsaturated radicals

“…The transient formation of a cyclic 5-atom intermediate structure does not demand too much energy. The occurrence of process (27) is in agreement with earlier conclusions from work with somewhat less energized l-buten-l-yl radicals [21]. This study also showed that the rearrangement of this species to the ~-methylallyl structure is much slower than process (27).…”

“…In other words, the 1,4-sigmatropic transfer is faster than the 1,3-transfer. Conversely, the 1-buten-4-yl radical isomerizes to the ~-methallyl structure CH2=CHCH2~H2 t (1,2 shift) ÷ CH3CHCHCH 2 (28) -l E A = 138 kJ mol through an internal 1,2-hydrogen shift [21].…”

“…kO mol -l[21]. More recently, by using a somewhat different technique, Niedzielski et al showed that the three linear C3H 5 species can be linked together[lO].…”

Mechanistic Studies on the Combustion of Isotopically Labelled Cyclohexanes within a Single Cylinder Internal Combustion Engine

“…H' atom abstraction away from the allylic sites may occur to some degree, especially under the high temperature conditions experienced in the post-flame chemistry where the relative importance of activation barriers is lowered. Formation of cyclohex-3-enyl radicals and to a lesser degree, eyelohex-l-enyl radicals may provide a route for intramolecular hydrogen migration through 1,2 or 1,3-H shifts to form cyclohex-2-enyl (for an analogous reaction see Ibuki et a/., 1976). Despite the ring-strained transition states required for either of these isomerisations, the formation of the allylic eyelohex-2-enyl radical may well provide a sufficient driving force for these processes.…”

Thermal decomposition of 2, 3‐dimethyl butane: Experiments and mechanistic modelling

“…The precursor radicals of 2-methyl butadiene are the only radicals in the scheme which undergo intramolecular radical isomerization (five-membered cyclic transition state). Three-and four-membered cyclic intramolecular isomerizations are assumed to be much less likely [27]; indeed, they appear unnecessary for a satisfactory prediction of product dynamics. In agreement with theory, intramolecular isomerization reactions of higher alkane radicals (via 5-membered and 6-membered cyclic transition states) are important and must be included in the reaction model.…”