NOTES
1143
the localization energies of substrafes for nucleophilic, radical, and electrophilic attack, as well as the free valences and n-electron densities of each position, are shown. The occupied energy levels are so determined as t o give a good agreement with the structure of free radicals (vide supra). All the hydroxylated positions, i e . , position 5 (or 2) of furan, position 4 of furoic acid, position 5 of furfuryl, and position 5 of furfuryl alcohol, possess minimal localization energies for radical attack, and also maximal values of free valence (Table 11). Therefore, it can be concluded that reactions of both oxidation systems, HzOz and Fe(I1) HzOz, involve radical Ti(II1) species as a reactive intermediate. For consistency, four electrons were introduced from CHzOH to the ring of furfuryl alcohol. No other justification is offered a t this time. Nature of Reactive Species. The Ti(II1) HzOz system has reactive properties similar to the OH radical with u compounds, while Fenton’s reagent possesses an opposite character as an ~ x i d a n t . ~ - b JHowever, ~ concerning aromatic hydroxylation reactions, it is clearly shown that two oxidation systems behave similarly. The hydroxylated position of the intermediate free radical has been shown to be a most reactive site for radical attack of the original molecule. Similar results weie obtained with phenol derivatives.16 The chemical nature of the reactive species involved in these oxidation systems is still unknown, OH-like species are proposed for the Ti(II1) f HzOzsystem as attacking agents, because Livingston and Zeldes17have
+
+
+
shown that the intermediate free radicals obtained by photochemically generated OH radicals are similar to the intermediates produced by the Ti(II1) HzOz system. A number of kinetic studies are now being carried out using epr.lgdz2 Despite considerable efforts to clarify the nature of the reactive species involved in Fenton’s reagent since the 1930’s, no definite conclusion has yet been reached. Recently, a series of epr studies demonstrated that Fenton’s reagent possesses properties as an oxidant for saturated hydrocarbons3-6J1 different from those of Ti(II1) HzOz. Taking as an example the oxidation of alcohols, Fenton’s reagent extracts a hydrogen from the position farthest away from the alcoholic OH, while the Ti (111) HzOzsystem attacks the position nearest t o the alcoholic OH. Therefore, it was concluded that the two oxidation systems involve different reactive species in the abstraction of hydrogen from saturated compounds. However, the present study demonstrates that both oxidants have similar properties in yielding hydroxylated adducts from furans. Further studies on aromatic hydroxylation are now in progress.
+
+
+
(16) T.Shiga, in preparation. (17) R. Livingston and N. Zeldes, J. Chem. Phgs., 44, 1245 (1966). (18) L. H. Piette and G . Bulow, Preprints, 9,2-c, p c-9 American Chemical Society, 1967. (19) F. Sicilio, R . E. Florin, and L. A. Wall, J. Phys. Chem., 70, 47 (1967). (20) Y. 9. Chiang, J. Craddock, D . Mickewich, and J. Turkevich, i b i d . , 70, 3509 (1967). (21) H. Fischer, Ber. Bunsenges. Phys. Chem., 71, 685 (1967). (22) K . Takakura and B. R%nby,J. Phys. Chem., 72, 164 (1968).
NOTES
Effect of Density and Electron Scavengers
in Nitrous Oxide Radiolysis’*
NzO-scavenger experiments continue the investigation into the decomposition mechanism,
Experimental Section by John T. SearsIb Nuclear Engineering Department, Brookhaven National Laboratory, Upton. New York 11973 (Received August 28. 1 9 6 8 )
Primary processes in the irradiation-induced decomposition of nitrous oxide, which is a potential dosimetry s y ~ t e m , ~are J still incompletely understood. The present investigation presents results of the irradiation of solid, liquid, and high density (to 0.7 g/cc) gas which emphasize the relative constancy of G ( Nz). Additional
All gases were simply purified by freeze-thaw vacuum techniques. Low-pressure y-ray experiments were conducted in all-glass break-seal vessels (500 to 550 cc) . I n other experiments zzzRn CY rays (and daughter particles) were deposited internally in a 90-cc sealed vessel until the radon was exhausted. zlOPoa rays (1) (a) This work was performed under the auspices of the U. S. Atomic Energy Commission. (b) Esso Research and Engineering Co., Linden, N. J. 07036, (2) P. Harteck and 9. Dondes, NucZeonlcs, 14, 66 (1956). (3) R . W. Hummel and J. A. Hearne. Nature, 188, 734 (1960). Volume YS,Number 4 April 1869
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1144
traveled through a stainless steel window into N 2 0 in a to produce decomposition. Present interest concerned 605-cc vessel, which had a stopcock to allow periodic electron reactions as deduced from scavenger-addition sampling. High density y-ray experiments were conexperiments. The present results, low and high denducted in 6-mm diam 3-cc sealed vessels; in gas-phase sity, indicate that G(N2) decreased by only mG(e) = experiments, the vessels were situated inside stainless 3.05 because of electron scavenging. steel pressure vessels (7 cc) , which were filled with N 2 0 Thermal electrons might react to form nitrogen from to nearly the same density to equalize pressure, and the following over-all reaction" irradiated in a controlled-temperature water bath. ethN20 + Nz 0(1) Energy deposition by 210Poa particles was calculated by Slater4 from his semiconductor calibration of the It was then suggested that 0- further reactedlo source at various angles of emission, integrated over 2 r 0N20 +Nz 0 2 (2) radians. Energy deposition from radon decay was computer calculated by approximation of vessel 0N20 +NO NO(3) geometry, a particle and recoil ranges in N20j5v6and Reactions 1 and 2 were suggested14 as the mechanism assumed location on the vessel surface of daughter leading to N2 formation in electron scavenger-hydronuclei. I n situ Fricke dosimetry with appropriate carbon radiolysis. As preferential s~avenging'~J~ of electron density corrections was used for 6oCo y irthe electron by SF6 and CCL occurs, these additives radiation. should prohibit nitrogen formation via reactions 1 and Noncondensible gases (at - 196') were collected over 2, which have been estimated16 from scavenger-alkane a Toepler pump and analyzed by a gas chromatograph studies as (molecular sieve column) for Nz and Oz. In 2 1 0 P ~ G(N2) = [l kz/(k2 ka)]G(e) = 1.55G(e) = 4.7 a-ray experiments, the irradiated gases were instead fed directly into a calibrated GPC. The products, in NzO. Further, SF6 should not interfere with positive NO and NO2, were not measured in this study. ion reactions as its I.P. w 19 eV.17 Now carbon dioxide decreased the nitrogen yield in scavengerResults and Discussion alkane work to G(e), which suggested'* that reaction 2 Table I summarizes the present results. All the was prohibited because of preferential formation of pure N 2 0 results, in all phases below 40°,including the CO,-. In the present experiments, COz only slightly dense gas data at doses > 3 X lo6 rads, demonstrate decreased the nitrogen yield, while CC14, SFe, and NO2 the remarkable relative constancy of G(N2)(m10 to decreased G(Nz) by m3.2 f 0.5. React>ion 1 seems 11.5). The 210Poa-ray series gave G(N2) = 10.0 f to occur then, but the probability that reaction 2 is 0.4, in agreement with the value and analysis by Jones important is not supported by the present evidence. and Sworski [G(Nz) = 10.0 f 0.21.' Increase in dose The suggestion by Burtt and Henis,lg upon the observabove 4.4 X lo6 rads (0.2% N2) causes a decrease in ance of Oz- from N 2 0 gas bombarded by 0- in a mass nitrogen production rate until a new linear yield dose spectrometer, is often cited as evidence for reaction 2. region (1 to 5% N2) is obtained with G'(N2) = 6.7;s However, Paulson20observed 02-also, but a t a much the zzzRn experiment was in reasonable agreement with (4) L. Slater, Intern. J . A p p l . Radiation Isotopes, in press. these results. Irradiations of solid or liquid NzO (5) R . Evans, "The Atomic Nucleus," McGraw-Hill Book Go., produced only slight increases in the gas yields (Table New York, N. Y., 1955. (6) D. L. Baulch and J. F. Duncan, Aust. J . Chem., 10, 112 (1957). I). It is unclear why the present liquid phase results are slightly lower than those of Robinson and Freernanj9 (7) F. T. Jones and T . J. Sworski, J . Phys. Chem., 70, 1546 (1966). (8)J. Sutherland, private communication. Note that a t these doses G(Nz) = 12.9 f.0.2, 02/N2 = 0.2. Oxygen yields also considerable product NOZ, etc., has formed, and i t is tempting to appear fairly stable; however, little mechanistic evicompare the NO%-addition experiments where G(N2) decreased to 6.8 and NO and 0%were readily formed by photolysis/radiolysis dence should be deduced from this because of possible of NOz. reaction during analysislo or scavenger attack (see (9) M. G. Robinson and G. R . Freeman, J . Phys. Chem., 72, 1394 (1968). results of additive-CC14 experiments). (10)B. P. B u t t and J. F. Kircher, Radiation Res., 9, 1 (1958). Applied electric field e ~ p e r i m e n t s ~ Jand ~ - ~ethylene ~ (11) H. Essex and N. T. Williams, J . Chem. Phys., 16, 1153 (1948). scavenger experimentsIZ indicate that ionic recombina(12) R. Gordon and P. Ausloos, J . Res. Nat. Bur. Stand.. 69A, tion produces only small amounts of nitrogen (G
