t ' 5 7 8 A. to.002 A
25"7S •515" A-
2-6% s
to make a series of bridgehead dimers with hybridization at the central bond ranging from 24 to 42 % s character. The San Diego group has yet to complete the series, but three compounds measured so far allow a tentative conclusion. One compound, 1-biapocamphane, has 2 8 % s character in the central bond, which measures 1.544 ± 0.001 A. long. Another, 1-biadamantane, has a central bond 1.578 ± 0.002 A. long with 2 5 % s. These two are directly comparable because each has six hydrogen-hydrogen repulsions across the central bond and the distances are the same in both dimers, according to the x-ray data. Conjugation says that these two central bonds should be identical. The hybridization theory predicts that the bond in 1-biapocamphane should be 0.02 A. shorter because of its greater s character. Actually, it is 0.034 A. shorter than in 1biadamantane, thus supporting the hybridization theory. In a third molecule, 1-binorbornane, the San Diego chemists find a central bond length of 1.515 ± 0.005 A. with 28% s. The equation from the Dewar plot predicts a length of 1.518 A. Even here, they note, the bond is probably stretched somewhat by two hydrogen-hydrogen repulsions. Dr. Traylor, Dr. Kraut, and Dr. Alden suggest that these observations show that the concepts of conjugation and hyperconjugation are not required to explain bond length shortening in such compounds as propylene and butadiene. Nonbonded repulsions are not excluded, though, and it seems likely that repulsions and hybridizations are important. The work to come on bridgehead dimers with higher s character in the central bonds may shed more light on their respective roles.
Detailed information regarding the ease of formation and chemical reactivity of peroxy and phenoxy radicals in hydrocarbon systems has been obtained by Dr. Lee R. Mahoney and his coworkers at Ford Motor Co.'s scientific laboratory in Dearborn, Mich. This achievement was the result of a systematic study of hydrocarbon oxidation retarded by nonhindered phenols [/. Am. Chem. Soc, 89, 1895 (1967)]. In the study, the rates of oxygen absorption were measured in a solution of 9,10-dihydroanthracene (DHA) and 2,2',3,3'-tetraphenylbutane (TPB) containing varying amounts of 3-hydroxyphenol, 4-methoxyphenol, 4phenylphenol, 2-hydroxynaphthalene, 3-hydroxypyrene, and hydroperoxide. In a solution with DHA (the standard hydrocarbon) and TPB (the initiator), phenoxy radicals are generated as intermediates during free-radicalinitiated liquid-phase oxidation of the hydrocarbon. Dr. A. F. Bickel and Dr. E. C. Kooyman (Koninklijke/ Shell Laboratories, Amsterdam) in their studies of trialkylphenols and aromatic amines introduced the use of DHA as the standard hydrocarbon and TPB as an initiator for studies of strongly retarded oxidations.
Dr. Mahoney and his coworkers established several goals at the beginning of their study. These include: • Analysis of the oxidation data by means of a generalized rate expression, avoiding the use of limiting rate expressions. • Use of the numerical results from the analysis to yield information about relative rates of formation, of hydrogen abstraction, and of modes of destruction of phenoxy radicals. • Analysis of the data of other workers, who utilized different hydrocarbons, by means of the same generalized rate expression to derive estimates of the reactivity ratios for peroxy and phenoxy radicals in hydrogen abstraction reactions. The reactions of intermediate phenoxy radicals with hydroperoxides and with hydrocarbons have been proposed to explain the nonideal kinetic behavior of hydrocarbon oxidations retarded by nonhindered phenols. The hydroperoxide effect was first noted by Dr. J. R. Thomas, at Chevron's Richmond Laboratory. He found that the retarded oxidation rate is not constant with time because of the hydroperoxide produced by the reaction. This behavior is more pronounced with the larger kinetic chain lengths and tends
9 8
ff
I
ec.
+ 0 . 0 0 1 A.
First reliable ratios of rate constants obtained for nonhindered phenols
t/>
7
TH
V—
a> •«-»
mol esl
1.544 A-
6 5 4
t-
O X
Si-
3 2 1U
4-Methoxyphenol in chlorobenzene containing 0.099M 9,K)-dihydroanthracene and 0.0031 M 2,2'3,3-tetraphenylbutane at 60c
J_ .010
_L .020
.030
ft
.040 .100
( Q moles liter-1 0CH3 The dependence of the rate of oxygen absorption on 4-methoxyphenol concentration cannot be described by integral orders throughout a wide range of concentrations. At low concentrations of the phenol, the predominant chain-carrying radical is the peroxy radical; at high concentrations, the phenoxy radical is the main species MAY 1, 1967 C&EN
35
to give an apparent kinetic order, with respect to hydrocarbon concentration, greater than the true value. At low concentration of hydroperoxide, reactions of the intermediate phenoxy radicals with hydrocarbon must be considered in the analysis. Dr. Mahoney includes all of these reactions in the scheme: ki
R-R
-*
2Rko
R - + 0 2 -» R 0 2 k3
R 0 2 - + RH- -» R 0 2 H + Rk4
2R0 2 * -> 0 2 + inert products k5
R 0 2 - + AH
