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The very low‐pressure pyrolysis of phenyl ethyl ether, phenyl allyl ether, and benzyl methyl ether and the enthalpy of formation of the phenoxy radical
Author(s) -
Colussi A. J.,
Zabel F.,
Benson S. W.
Publication year - 1977
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550090202
Subject(s) - chemistry , radical , medicinal chemistry , ether , bond dissociation energy , benzene , enthalpy , alkoxy group , diphenyl ether , photochemistry , dissociation (chemistry) , organic chemistry , alkyl , physics , quantum mechanics
A value of the enthalpy of formation of the phenoxy radical in the gas phase, Δ H ° ,298K (ϕO·, g) = 11.4 ± 2.0 kcal/mol, has been obtained from the kinetic study of the unimolecular decompositions of phenyl ethyl ether, phenyl allyl ether, and benzyl methyl ether Trivial names for ethoxy benzene, 2‐propenoxy (allyloxy) benzene, and α‐methoxytoluene, respectively at very low pressures. Bond fission, producing phenoxy or benzyl radicals, respectively, is the only mode of decomposition in each case. The present value leads to a bond dissociation energy BDE(ϕO—H) = 86.5 ± 2 kcal/mol, 1 kcal = 4.18674 kJ (absolute) in good agreement with recent estimates made on the basis of competitive oxidation steps in the liquid phase. A comparison with bond dissociation energies of aliphatic alcohols, BDE(RO—H) = 104 kcal/mol, reveals that the stabilization energy of the phenoxy radical (17.5 kcal/mol) is considerably greater than the one observed for the isoelectronic benzyl radical (13.2 kcal/mol). Decomposition of phenoxy radicals into cyclopentadienyl radicals and CO has been observed at temperatures above 1000°K, and a mechanism for this reaction is proposed.