Radiation Chemistry-II

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A Pulsed-Radiolysis Study o f the Gas-Phase R e a c t i o n o f O x y g e n A t o m s w i t h Benzene a n d R e l a t e d C o m p o u n d s : Rate Constants

Downloaded by AUBURN UNIV on February 20, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0082.ch008

a n d T r a n s i e n t Spectra

INDER MANI and MYRAN C. SAUER, JR. Chemistry Division, Argonne National Laboratory, Argonne, Ill.

Pulsed-radiolysis of gas-phase systems containingCO2or N2O results in the production of oxygen atoms, which are in the triplet ground state. In the presence of small amounts of benzene, for example, an optical absorption is found to be formed after the pulse, with a maximum at 275 mµand a shoulder in the region of 300 mµ. Similar absorption spectra are found for other compounds. Rate constants for the processO(3P)+ X-->products are, where the compound X is given in parentheses and the rate constant is in units of 10 litermole-1sec.-1, 0.36 (benzene), 1.4 (toluene), 3.2 (ethylbenzene), 6.7 (o-xylene), 7.7 (m-xylene), 4.5 (p-xylene), 3.1 (chlorobenzene), 0.27 (fluorobenzene), and 1.0 (pyridine). The estimated limits of error are about ±25%. 8

'"phe production of oxygen atoms by the pulsed-radiolysis of gas-phase systems containing C0 or N 0 has been described (6) and the reaction with molecular oxygen has been studied. The conclusion was reached that the oxygen atoms are most likely in the ground state ( P) when they undergo reaction with the oxygen. In the present work, we have studied the reactions of oxygen atoms produced in such systems with various aromatic molecules and have obtained information on the absolute rate constants of these reactions and on the optical absorption spectra of the resulting transients. 2

2

3

142 In Radiation Chemistry; Hart, Edwin J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

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A N D SAUER, J R .

Oxygen Atoms with Benzene

143

Experimental The experimental details of the preparation of samples, the pulseirradiation procedure, and the recording and analysis of data have all been described (6, 7, 9). Product analysis was done as previously de scribed (9), using temperature programmed gas chromatography (25°C. to 230°C.) and a 2 % Versamid-900 on silanized Chromosorb (white, acid washed, 60-80 mesh) column. Results and Discussion


Transient Spectra. The spectra are shown i n Figures 1, 2, and 3 and were obtained point by point, as has been previously described ( I , 8),

ETHYLBENZENE

260

280

300

320

λ (m/x.)

Figure 1. Transient absorption spectra resulting from reaction of oxygen atoms with benzene, toluene, and ethylbenzene

In Radiation Chemistry; Hart, Edwin J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

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RADIATION CHEMISTRY

taking readings every 5 m/x. The absorption spectra shown correspond to a time approximately 20 /xsec. after the electron pulse. As w i l l be discussed in the section on transient decay, the absorption decayed little over a time of a few milliseconds at wavelengths around the maximum, but the shoulder at longer wavelengths decayed to a plateau (about 50% of the maximum O . D . ) i n a time of less than a millisecond. Although absorption coefficients could not be determined accurately, they are estimated to be in the range of 1 0 - 1 0 M " c m . ' based on what is known about the concentration of oxygen atoms produced per pulse. The spectrum for each compound shown in Figures 1-3 is normalized to 1.0 at the maximum. In the case of benzene, one possible species absorbing in the region of 275 m/x is phenol. B y measuring the absorption of phenol vapor in one arm. of air at 24° C. in a Cary spectrophotometer, we found the absorption coefficient to be 4--5 X 1 0 M " cm." at 275.7 m/x; however, the absorption peak at 275.7 was very sharp, the absorption reaching zero by about 280 m/x. W e do not know how much the 50 fold increase in pressure used in the pulsed-radiolysis experiment would affect the width of the absorption, but it is very unlikely that phenol is the only absorbing species. In a recent study (10) on the synthesis of benzene oxide ( A ) in isooctane solution, this compound was found to have a maximum absorption at 271 m/x, and to be in equilibrium with oxepin ( B ) , with a maximum at 305 m/x. 3


II

3

1

4

1

1

1

The equilibrium is about 2:1 in favor of Β at room temp, in isooctane solution. As w i l l be discussed in the section on transient decay, these species are possible products of the reaction of oxygen atoms with ben zene. In addition, the biradical ( C ) produced


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A N D SAUER, JR.


when triplet ground state oxygen atoms react with benzene may have a long enough lifetime with respect to conversion to A, B, or phenol to be responsible for part of the absorption. (It should be noted, however, that work on the gas-phase reaction of triplet oxygen atoms to olefins (3) indicates that the lifetime of the biradical must be much shorter than the approximately 100 /*sec. lifetime which would be required here.)

I

I 260

ι

I

ι

280 λ

Figure 2.

I 300

ι

I

.

I

320

(m/x)

Transient absorption spectra resulting from reaction of oxygen atoms with o-, m-, and p-xylene

In the case of the other compounds studied, the spectra are similar, as are the decays of the longer wavelength parts of the spectra. However, we can only surmise that analogous transient species are involved.



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RADIATION CHEMISTRY

260

280

300

II

320

λ (m/x)

Figure 3. Transient absorption spectra resulting from reaction of oxygen atoms with fluorobenzene and pyridine Transient Formation. That oxygen atoms in the P ground state are produced when a system of argon and C 0 is pulse irradiated has been previously demonstrated ( 6 ) . Furthermore, the oxygen atoms were shown to be formed nearly simultaneously with the microsecond puke. When a small amount of benzene (1 to 8 cm.) is added to the system, optical absorption is found in the region of 275 m/x, the spectrum being shown in Figure 1. This absorption was observed to form with a half time of about 4 /xsec. or greater, depending on the benzene pressure. A typical formation curve is shown in Figure 4. It was found that the first order rate constant derived from such transient curves was independent of benzene concentration, as expected, with the following qualifications. As the benzene concentration was lowered, it was found that high values 3

2


8.

Oxygen Atoms

M A N I AND SAUER, J R .

With

147

ΒβΠΖβΠβ

were obtained for the rate constant unless progressively lower intensity pulses were used. ( O f course, the optical density was thereby lowered, and more amplification had to be used, resulting i n greater noise levels. However, analysis of the curves was possible, several curves being ana lyzed at each intensity and the results averaged.) This behavior can be explained if the formation of the transient by Reaction 1 O + CeHe-^CeHeO

(1)

is interfered with by any reaction involving two transient species such as Ο + C H O -> products

(2)

0 + 0->0

(3)


e

e

2

C H O + C H O -> products e

e

e

(4)

e

Lower pulse intensity makes smaller the probability of Reactions 2, 3, and 4 occurring during the formation of C H O by Reaction 1. 6

e

Figure 4. Transient formation at 285 m μ in system containing 56 aim. Ar, 2 atm. CO , and 0.09 atm. benzene. The pulse was 80 ma. and 0.4 pjsec.; the optical pathlength was 26 cm. g

The rate constant obtained for Reaction 1 was 3.6 =b 0.7 X 1 0 M 7

_ 1

sec." independent of benzene pressure from 2 to 8 cm. (However, at 1

1 cm. benzene, low enough pulse intensity could not be used to overcome the importance of Reactions 2, 3, and 4 relative to 1, and the apparent rate constant obtained was higher.) W i t h i n 20%, the same rate constant for Reaction 1 was obtained i n systems containing 30 to 90 atm. A r plus about 2 atm. C 0 , 50 ajQ^^^^^g^q^^^^o^j^^^^ ^SScJ^^^^' 2

^

Library

1155 16th SL, K.W. In Radiation Chemistry; Hart, Edwin J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

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(no A r ) . Also, the use of C D had no effect on ki. (It should be emphasized that the rate constants reported here are actually a sum of the rate constants for all reaction paths by which the oxygen atom reacts with a given compound. The intermediate C H 0 formed in Reaction 1, for example, may represent more than one species.) e

e

6

6

As has been discussed previously (6, 7), the oxygen atom which reacts with benzene is likely to be F , and therefore the CeHeO species produced in Reaction 1 must be initially the biradical species C . A l though we do not know the lifetime of this species, it may be rapidly converted to species A and/or B, and thereby not contribute directly to the observed absorption. These species w i l l be further considered in the section on transient decay.


3

Similar transient formation curves were obtained when other organic molecules, listed in Table I, were used instead of benzene. The rate constants obtained are given in Table I along with that obtained for benzene. The values for benzene and toluene are in good agreement with the relative rate constants given by Cvetanovic (3). Although the estimated error limits are rather large (approximately 2 5 % ) , the general trends in reactivity are clear. In the series benzene, toluene, xylenes, the rate constant for the reaction of oxygen atoms with the molecule increases by a factor of approximately 20. It is clear that increasing the number of methyl groups has the effect of increasing the rate constant, although the differences between the three xylenes may not be meaningful since the rate constants are the same within the estimated error. The same order of reactivity (and approximately the same absolute rate constants) have been observed for the reaction of hydrogen atoms with these substances (4, 9). The values obtained (9) for the reaction of hydrogen atoms with benzene and toluene are 0.37 and 1.0 X 10 M~ sec." , both of which are within the experimental error of the corresponding oxygen atom results in Table I. In the case of xylenes, preliminary results (4) on the reactions of hydrogen atoms indicate that the rate constants are about half as great as the corresponding values of Table I. The rate constant for the reaction of H - with ethylbenzene is likewise about half that of oxygen atoms with ethylbenzene. Chlorobenzene shows about the same rate constant for both hydrogen and oxygen atoms, as does pyridine. Fluorobenzene shows the greatest difference, the hydrogen atom reaction being about 2.5 times faster than the oxygen atom reaction. 8

1

1

Cvetanovic (3) has concluded that the reaction of triplet oxygen with olefins is electrophilic, and that the reaction of hydrogen atoms is free radical in nature, the oxygen atom reaction rate constants showing larger variations with structural changes in the olefins. In view of the


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AND SAUER, J R .

similarity of the rate constants for hydrogen atom and oxygen atom reac tions with the compounds in Table I, the question arises as to whether the same classifications are reasonable for the reactions with aromatic compounds. Table I.

Gas Phase Rate Constants for Reactions of Oxygen Atoms with Organic Compounds Pressures (atm.) of Sample Constituents


Ar Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene Chlorobenzene Fluorobenzene Pyridine Benzene α

97 51 54 54 58 58 55 28, 54 a

co

t

2.5 2 2 2 2 2 2 2

Organic 0.032 0.011 0.007 0.009 0.011 0.012 0.09 0.024

10'° X Rate Constant liter mole' sec.' 1

1.4 3.2 6.7 7.7 4.5 3.1 0.27 1.0 0.36

1

± 0.3 ± 0.8 ± 1.6 ± 2.0 ± 1.4 ± 0.8 ± 0.06 ±0.3 ± 0.07

Various conditions, see text.

The classification of the hydrogen atom reactions as "free radical" seems reasonable, as the presence of any of the substituents tested on the benzene ring has an activating effect, as would seem to be expected (11); furthermore, the rather small changes in rate constants caused by the different constituents supports the "free radical," i.e., homolytic classifica tion (11). However, several puzzling aspects of the oxygen atom rate con stants i n Table I should be pointed out with respect to the question of the electrophilic nature of the oxygen atom reactions. The increase in reac tivity in the series benzene, toluene, xylenes is expected in view of the ex pected hyperconjugative effect of the added methyl groups which would make more electrons "available" to the ring, but on this basis, ethylbenzene should react more slowly than toluene because of the smaller hypercon jugative effect of ethyl compared with methyl. Thus, the data require an additional electron donating effect, which is stronger for ethyl than methyl. This is in agreement with work on the formation of ττ-complexes in solu tions of HC1 in these aromatic compounds (2), where the conclusion was reached that the alkyl groups promote the formation of the ττ-complex with HC1 primarily through their inductive effect, which is expected to be stronger for ethyl than for methyl. Therefore, in the oxygen atom reac tions, the alkyl groups must contribute to the electron density of the aro matic nucleus primarily through the inductive effect. The rate constant for chlorobenzene is also higher than one would expect in view of the ex-



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pected inductive effect of the chlorine atom, which would be expected to decrease the electron density in the ring. However, recent work (5) which considers the effect of substituents on the optical absorption spec trum of benzene leads to the conclusion that a chlorine atom substituent contributes electrons to the ring by the mesomeric effect, which quali tatively supports our data on chlorobenzene. A t the same time, the latter work finds a very small mesomeric effect for the fluorine atom, which correlates with the low value for Ο · -f- fluorobenzene in Table I. Therefore, we conclude that the attack of oxygen atoms on the com pounds in Table I may be of an electrophilic nature, but that the rather small differences in comparison with the analogous hydrogen atom reac tions leaves some doubt as to the meaning and correctness of this conclusion. Decay of Transient Absorption. The optical absorption decayed significantly over a few milliseconds only at the longer wavelength side of the maximum absorption—i.e., in the region of 300 m/x. (The behavior of the absorption on appreciably longer time scales was obscured by the combination of low optical density and fluctuations in lamp intensity.)

Figure 5. Transient decay at 305 m μ. in system containing 56 atm. Ar, 2 atm. CO , and 0.09 atm. benzene. The pulse was 80 ma. and 0.4 ^sec; the optical pathlength was 26 cm. %

A typical oscilloscope trace is shown in Figure 5. (The decay rate d i d not change when the argon pressure was varied from 58 to 95 atm., or when the benzene pressure was changed from 2 to 8 cm.). The absorp tion d i d not decay to zero, but seemed to reach a plateau of about half of the maximum optical density. Because of "noise" and the difficulty of establishing the plateau, first and second order tests on such decay


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curves were ambiguous. However, by varying the intensity, and thereby the concentration of transient, it was found that the decay became faster with increasing intensity in a manner which indicates that the main path of decay is by bimolecular interaction between transient species except at the lowest intensities where a first order decay of the transient perhaps begins to contribute to the decay. (The first order rate constant is < 2 Χ 10 sec." ). The fact that use of deuterated benzene had no appreciable effect on the decay substantiates the argument that the decay is mainly bimolecular, since a first order process such as 3

1


H

would be expected to be slower with deuterated benzene. The bimolecular decay of the transient which absorbs at 300 m/x is probably not caused by the interaction of two bi-radical species—e.g.,

products,

because this would leave unexplained the adsorption at lower wave lengths which does not decay. However, fortuitous combinations of ab sorption spectra can be imagined which would allow the decay to be ascribed to the bi-radical species ( C ) being converted to A, B, or phenol. (This phenomenon of partial decay of the longer wavelength portion of the spectrum was also observed for the transients formed from the other organic compounds studied, and the intensity behavior was in general similar.) It is possible that the decay may be related in some way to a conversion involving one or more of the isomeric species benzene oxide ( A ) , oxepin ( B ) , and phenol. The species A and Β were found (10) to be stable in isooctane solution, and to exist in equilibrium with each other and to have optical absorptions at 271 and 305 m/x respectively. Phenol absorbs in the vapor phase at 275.5 m/x, as has been mentioned. Hence, it is tempting to ascribe the decay observed at 305 m/x to a



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disappearance of oxepin. However, it is not clear why this would be a second order decay, and if the oxepin converted to benzene oxide or phenol, the absorption in the 275 region should increase, which was not observed. It appears that there is not enough information to permit a conclusive statement as to which species are responsible for absorptions in the wavelength regions mentioned. It should be mentioned that an end-product analysis was carried out by gas chromatography. The only product detected corresponded to the retention time of phenol. However, if benzene oxide and oxepin were present, they may have isomerized to phenol during the process of collection and passage through the chromatographic column, or the retention times may have been indistinguishable from that of phenol. (Authentic samples of oxepin and benzene oxide were not available to test these points). Acknowledgment The authors wish to thank B. E. Cliftt, B. J. Naderer, and D. Donkersloot for operating the Linac. Literature Cited (1) Arai, S., Sauer, M.C.,Jr.,J.Chem. Phys. 44, 2297 (1966). (2) Brown, H.C.,Brady, J. D.,J.Am. Chem. Soc. 74, 3570 (1952). (3) Cvetanovic, R. J., "Advances in Photochemistry," Vol. 1, p. 142, W. A. Noyes, Jr., et al., eds., Interscience Publishers, New York and London, 1963. (4) Mani, I., Sauer, M.C.,Jr. (unpublished work). (5) Murrell, J. N., Lecture Series 1963, No. 2, The Royal Institute of Chem istry, London, p. 6. (6) Sauer, M.C.,Jr.,J.Phys. Chem. 71, 3311 (1967). (7) Sauer, M.C.,Jr., Dorfman, L. M.,J.Am. Chem. Soc. 87, 3801 (1965). (8) Sauer, M. C., Jr., Arai, S., Dorfman, L. M., J. Chem. Phys. 42, 708 (1965). (9) Sauer, M.C.,Jr., Ward, B.,J.Phys. Chem. 71, 3971 (1967). (10) Vogel, E., Günther,H.,Angew. Chem. Intern. Ed. 6, 385 (1967). (11) Williams, G.H.,"Homolytic Aromatic Substitutionp. 19, 25, Pergamon Press, New York, 1960. RECEIVED January 8, 1968. Based on work performed under the auspices of the U. S. Atomic Energy Commission.


Radiation Chemistry-II

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