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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

✍ Scribed by A. J. Colussi; F. Zabel; S. W. Benson

Book ID
102928606
Publisher
John Wiley and Sons
Year
1977
Tongue
English
Weight
734 KB
Volume
9
Category
Article
ISSN
0538-8066

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✦ Synopsis

Abstract

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 etherTrivial 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.

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