I. BALAKRISHNAN AND M. P. REDDY
850
irradiation of nitric oxide alone has been carried out by other workers using fission fragments and y ray^,'^!^^ but in both cases the G(-NO) value was much lower than when trifluoroiodomethane was present. The occurrence of this chain reaction propogated by a regenerated trifluoromethyl radical casts doubt on the usefulness of nitric oxide as a radical scavenger in radiolysis work. In the present work the inference is that nitric oxide scavenges the initially formed excited or “hot” trifluoromethyl radicals followed by a chain reaction involving thermalized trifluoromethyl radicals. It has been suggested in the previous work that only excited trifluoromethyl radicals can abstract fluorine
from trifluoroiodomethane to give tetrafluoromethane, and this is now substantiated. The eventual fate of the thermal trifluoromethyl radicals is to form trifluoronitromethane, trifluoronitrosomethane, or some as yet unidentified product.
Acknowledgments. I. McAlpine wishes to thank the Science Research Council for a maintenance award, during the tenure of which this work was carried out. (15) P.Harteck and 9. Dondes, J. Chem. Phvs., 27,646 (1957). (16) M. T. Dmitriev and L. V. Saradzhev, Russ. J. Phys. Chem., 35, 354 (1961).
Mechanism of Reaction of Hydroxyl Radicals with Benzene in the y Radiolysis of the Aerated Aqueous Benzene System by I. Balakrishnan and M. P. Reddy Communication No. IS67 from the Natwnal Chemical Laboratory, Poona, India
(Received June 84, 1960)
The reaction of hydroxyl radicals with benzene in aerated aqueous solutions under y radiolysis produces phenol and 0-hydroxymucondialdehyde (PHMD). The respective G values are 1.7 and 1.2, in 0.8 N HzS04, and 1.78 and 0.7, in neutral solutions, the sum of the two yields accounting for all the OH radicals in each instance. The enhancing effect of ferrous ions on these yields has been correlated with the peroxide chain mechanism advanced originally to account for the influence of organic impurities on the G(Fe*+)of the Fricke dosimeter. At a benzene concentration of M and a ferrous ion concentration of M , G(Fea+)is -48; G(pheno1) and G(pHMD) are 8.8 and 6.6, respectively. This corresponds to a chain length of -2.
Introduction
case of Fenton’s reagent and later by Dewhurstl4)l5to explain the influence of organic impurities on the G
A program of study of the formation of phenolic compounds under radiolysis required reexamination of the aerated aqueous benzene system. Earlier (1) G.Stein and J. Weiss, J. Chem. Soc., 3245 (1949). has reported the formation of phenol in the y radiolysis (2) T.J. Sworski, J. Chem. Phvs., 20, 1817 (1952). of this system, Three other products, “mucondial(3) P.V. Phung and M. Burton, Radiation Rea., 7, 199 (1957), dehyde” with a G value ranging from 0,2to 1.7,7-10an (4) J. Goodman and J. Steigman, J. Phys, Chem., 62, 1020 (1968). unknown phenol-like compound, and an acid, have been (5) J. H. Baxendale and D. Smithies, J. Chem, $oca, 779 (1959). notably described. In a previous communication” (6) H. C. Christensen, Aktiebolaget Atomenergie Stoakholm, AE it was shown that these three products are one and the 142, 1964,p 34; Chem. Abstr,, 61, 12829 (1964). (7) M.Daniels, G. Scholes, and J. Weiss, J . Chem. Soc., 832 (1956). same, ie., ,B-hydroxymucondialdehyde (PHMD), (8) I.Loeff and G . Stein, i b i d . , 2623 (1963). Interference due to pHMD accounts for the wide varia(9) L.I. Kartasheva and A. K . Pikaev, Dokl. Akad. Nauk SSSR, 163, tion in the values of G(pheno1)reported earlier.1-6~9~10~12 1155 (1965). This problem has here been solved by effecting a separa(10) L. I. Kartasheva and A. K . Pikaev, Russ. J. Phys. Chem., 41, tion of the two compounds before analyzing them, and 1534 (1967). (11) T. K. K. Srinivasan, I. (Balakrishnan, and M . P. Reddy, J. reliable G(pheno1) and G(0HMD) values have been Phys. Chem., 73,2071 (1960). obtained. (12) I. Balakrishnan and M. P . Reddy, ibid., 72, 4609 (1968). Ferrous ions have been observed to enhance the yields (13) I. M.Kolthoff and A. I. Medalia, J. Amer. Chem. Soc., 71, 3784 of phenol and the diaLg This effect has been restudied (1949). and correlated with the peroxide chain mechanism (14) H.A. Dewhurst, J. Chem. Phys., 19,1329 (1951). advanced originally by Kolthoff and Medalia’3 in the (15) H.A.Dewhurst, Trans. Faraday Soc,, 48,905(1952). The Journal of Physical Chemistry
MECHANISM OF OH RADICAL REACTION WITH BENZENE
85 1
(Fe3+) of the Fricke dosimetric system. This in turn helps to evolve a mechanism for the formation of the two products, taking the clue from the pulse radiolysis study of this system produced by Dorfman.16
Experimental Section Materials. A B D H sample of benzene, washed with pyrosulfuric acid, alkali, and waker, dried over sodium sulfate, and purified by five crystallizations, was used. p-Nitroaniline, laboratory grade, was purified by recrystallization from alcohol. o-Phenanthroline, pure, colorless, BDH sample, was used as such. Ferrous ammonium sulfate, a pure grade, was recrystallized from the triply distilled water used for all solutions in this study. Solvent ether, B.P., was further purified by washing with 1N NaOH, water, 0.8 N H2S04,and distilled water, before it was used for extraction. This was found to be essential for proper coupling of aqueous solutions containing dissolved ether. Procedures. A kilocurie e°Co y source was used on the basis of the 15.6 value for the Fricke dosimeter. The solutions studied were 0.8 N in sulfuric acid, for the most part. Some were neutral. Analyses were made using a Perkin-Elmer 350 spectrophotometer, as well as a Beckmann 3783 ratio recording spectrophotometer. 1. Phenol. p-Nitrobenzenediazonium chloride reagent (PNDC) forms deeply colored indicator dyes with phenols which are useful for unequivocal qualitative identifi~ation’~ of these compounds in many cases. The dyes have high extinction coefficients and are recommended for quantitative analysis of phenols. The procedure for the analysis of phenol given in Snell’s treatise1*was used, but the addition of sodium acetate was not essential. The irradiated solution was extracted four times with ether and the extract was washed five times with 0.1 N NaOH. The alkali layer was made barely acidic to litmus before coupling. The absorption maximum at 475 mp was measured against a reagent blank. The absorption of the latter a t 475 mp is negligible and there is no interference due to it. The ether extraction method was applied to a standard phenol solution of comparable strength and gave quantitative results. 2 , PHMD. PNDC also couples with PHMD to give a yellow dye. To estimate the product, the irradiated solution after ether extraction at a pH of 7 (pure sodium bicarbonate), to remove phenol, was made acidic to litmus and treated with sodium acetate (1 ml of a 25% solution) and then with PNDC (twice the theoretical amount). After 1 hr the yellow test solution was run against a practically colorless blank containing only sodium acetate, acid and PNDC, and the broad absorption maximum at 370 mk (cf. curve D in the inset in Figure 1) was measured. Coupling in sodium carbonate solution gives curve E of Figure 1, but this is
w V
2
4
m E
0
vr
m Q
/
m/U I
I
I
I
I
sx
IO-~M
CONCENTR A T 1 0 N
Figure 1. Standardization of the P N D C method for phenol: a, spectrum of irradiated aqueous solution of benzene treated with small excess of PNDC; b, the same, treated with large excess of PNDC; e, spectrum of PNDC dye of pure phenol; d, spectrum of BHMD solution treated with P N D C a t pH -5 (acetate buffer); e, spectrum of DHMD solution treated with P N D C a t p H -10 (sodium carbonate).
not recommended because of interference due to the strong color developed by the blank. The extinction coefficient for the maximum a t 370 mp was obtained using milligram quantities of the dye isolated from a large volume of solution irradiated intermittently to a high dose. At each interruption, benzene and air were dissolved to compensate for depletion. The ether solution of the dye was washed twice with small amounts of water and evaporated. The solid dye was dried at 80’. A weighed amount was dissolved in a, little alcohol and diluted to a suitable volume with water and 1 ml of acetate-acid buffer (pH -5) (m0 7850). In sodium carbonate solution €890 M p is ,
9950. (18) L. M. Dorfman, I. A. Taub, and R. E. Bahler, J . Chem. Phys., 36, 3051 (1962).
(17) S. Palkin and H. Wales, J . Amer. Chem. Soc., 46, 1488 (1924). (18) F. D. Snell and C. T. Snell, “Colorimetric Methods of Analysis,” Vol. 111, D. Van Nostrand Co., Inc., Princeton, N. J., 1953, pp 117, 127. Volume 74, Number 4 February 19,1970
I. BALAKRISHNAN AND nti. P. REDDY
852
71
/4
a w
t -J \ v)
w A 0
I
IO
3 0
0
a a
~
DOSE
~~
2
1
x
1021
(~VILITER)
Figure 2. Yields of phenol in the 7 radiolysis of aerated aqueous benzene: 0, 0.8 iV H#&; a), neutral; 0, in presence of 10-8M ferrous ion; oxygenated.
-0-,
Irradiated solutions above pH 4 show an absorption maximum a t 346 mpj4which is entirely due to PHMD and has an E of 4080 in solutions neutralized with pure bicarbonate. The value is about 2% lower at pH 5 and 5% higher at pH 10. This value was obtained by matching the 345-mp peak with that of the dye a t 370 mp. The order of magnitude of this B value proves that the OH is situated ,8 to an aldehyde group, as pointed out earlier." Figure 5 describes the p H dependence of the complete spectrum of PHMD and contrasts it with the behavior of a few related ketoenols.19-21 The 345-rn~ peak is particularly useful in determining the effect of ferrous ions upon the yield of PHMD. For this purpose, the iron was precipitated a t about pH 9 using sodium hydroxide and centrifuged and the absorption compared at the same pH with that of irradiated solutions containing no iron. I n neutral solutions the formation of 2,4-dinitrophenylhydrazone from DHMD was found to be linear up to a very large dose. Assuming this to be a trihydrazone, a G value of 0.7 was obtained (see inset in Figure 3) in agreement with the PNDC method. The implied proof that the derivative is a trihydrazone is crucial to the identification of the compound as Phydroxymucondialdehyde. l1 The Journal of Physical Chemistry
DOSE (eV/ LITER)
5x 1 0 '
Figure 3. Yields of PHMD in the y radiolysis of aerated aqueous benzene: 0, 0.8 N H$3O, in absence of ferrous ion; 8, the same in presence of M ferrous ion; a,neutral; 0, neutral (by weighing the dinitrophenylhydrazone); 0, the p-nitrophenyl hydrazone.
9. Polyhydroxybenzenes. Sensitive color tests for quinol and quinone,22 catechol, and all the higher polyhydroxybenzenes18 showed the absence of these compounds in the irradiated solution. Resorcinol gives a deep purple color with PNDC which was never observed with irradiated solutions. 4. Ferrous I o n . Ferrous ion oxidation in the Fricke system saturated with benzene was followed using the o-phenanthroline methodz3standardized excluding acetate or citrate. With a large excess of reagent (-30 times the ferrous content), even in 0.8 N HzS04 the extinction coefficient is only slightly lower than the reported value, provided a reaction time of 1 hr is allowed. Acetate seems to interfere with color formation in the case of irradiated solutions and the results are erratic. (19) C. J. Cavallito, C. M. Martini, and F. C. Nachod, J . Amer. Chem. Soc., 73,2546 (1951). (20) G. Schwarzenbach, K. Lutz, and E. Felder, Helv. Chim. Acta, 27,
579 (1944). (21) Y. Kojima, N. Itada, and 0. Hayaishi, J . Biol. Chem., 236, 2227 (1961). (22) R.J. Lacoste, J. R. Covington, and G. J. Frisone, Anal. Chem., 32, 990 (1960). (23) M. Lefort and J. Pucheault, J . Chim. Phys., 50, 580 (1953).
MECHANISM OF OH RADICAL REACTION WITH BENZENE
853 lower curve represents the actual ferrous ion disappearance (o-phenanthroline method) while the upper gives the absorption at 305 mp comprising evidently the pHMD contribution and the ferric ion contribution. The few points obtained with oxygenated solutions clearly indicate restored linearity. Oxygen consumption in this system is very large.
I
Discussion The Mechanism of Reaction of Hydroxyl Radicals with Benzene. In continuation of the various observations on pHMD reported previously” a reliable method of quantitative determination has been developed here based upon its reaction with PNDCZ4which, according to present views25 on such compounds coupling with diazonium salts, can be written 0
2
N
-
e
:
+
CHO -CH /
)CO
c“ 2
5 DOSE
x
1020ev
( e v / L I T E R )
Figure 4. The disappearance of ferrous ion in the y M radiolysis of aerated aqueous benzene containing ferrous ion: 0, disappeayance of ferrous ion; (D, appearance of 305-mp peak; and -0-, oxygenated.
-9-
)\CH
OHd
Ck
\\,CH
Results Figure 1 shows the standardization of the PNDC method for phenol ( e 4 n r n p 25,000). Curve C of the inset is the spectrum of the phenol dye. Curve B is the absorption of the irradiated solution upon treatment with an excess of PNDC. I n the present work it was this two-peak spectrum which first revealed the formation of the two products. When the excess of reagent is reduced progressively, the 475-mp peak rises and the spectrum resembles that of the phenol dye. The G (phenol) value of 1.54 reported earlieP is the highest obtainable by this procedure, which was abandoned when it was later realized that ether extraction removes the phenol exclusively. Figure 2 describes phenol formation in the presence and absence of ferrous ion in 0.8 N HzS04 solution. The yield in neutral benzene solution is practically indistinguishable from that in 0.8 N H2S04,the values quoted being the least-squares figures for the experimental points in each case. Figure 3 describes the formation of PHMD. Comparison with Figure 2 shows that the ferrous ion increases the yield of the two products by the same factor. At large doses, in acid solutions, the G value of the two compounds drops. Figure 4 describes the effect of benzene on G(Fea+) of the Fricke dosimetric system at a ferrous ion concentration of M , saturated with benzene. The
OHd
With the use of this method it now becomes evident that the assignment G(pheno1) = G(OH), originally made by Phung and B ~ r t o nshould ,~ read G(pheno1) G(pHMD) = G(0H). The values in Table I, for 0.8N HzSOdand neutral water, bear this out very clearly. The table also contains values taken from earlier work8Joafter applying a correction to the molar extinction of the hydrazone. It may be mentioned here that, in the present work, acid solutions gave precipitates whose weights varied in an erratic manner, in complete contrast t o neutral solutions. The reason was not investigated. The ratio of phenol to pHMD remains the same in the presence of ferrous ions as in their absence, in spite of the profound change in the mechanism and the onset of a chain reaction. Clearly, therefore, the two products are from the same primary source (ie., the OH radical). I n addition, the mechanisms of their formation must have other features in common so that ferrous ions could influence them both identically. The mechanism given in Scheme I, involving the formation of the HOz radical together with phenol or DHMD, meets these requirements.
+
(24) A. P. Altshuller and I. R. Cohen, Anal. Chim. Acta, 24, 61 (1961). (25) S. Hunig and 0. Boea, Liebigs Ann. Chem., 579,28 (1953) I
Volume 74?Number 4 February 19, 1970
I. BALAKRISHNAN AND M. P. REDDY
854 Table I: The Yields of Phenol and p-Hydroxymucondialdehyde in the Dose rate,
eV ml-1 min-1
5
x
1015
---------pH
7
13.8 X 10I6 7 7.644 X 1015(200 5.4
Neutral----
Phenol
BHMD
1.77 f
0.68 f 0.04 0 . 87c 0 . 70d
0.05 2.00 1.87
-
Sum
2.45
G(OH)
2.4a
7
y
Radiolysis of Aerated Aqueous Solutions of Benzene
__--
,
----
Phenol
pHMD
Sum
0.4b
1.71 A
1.20 i 0.07 1.23~ 1 .2gd
2.91
0.1
2.87 2.57
Acid-
pH
0.4b 2.2
2.20 1.87
3.43 3.16
Ref
G(0H)
2,ga
Present study 10
8
kvp X-rays) a Standard values from A. 0. Allen, "The Radiation Chemistry of Water and Aqueous Solutions," D. Van Nostrand Co., Inc., Princeton, N. J., 1961,Chapter 4. 0.8 N H&3Od. After correction using €550 mp of 44,000obtained in this study for pHMD in place of 36,000 reported in ref 9 for mucondialdehyde dinitrophenylhydrazone. After correction using €ago mp of 9400 obtained here for the p-nitrophenylhydrazone of pHMD, in place of the value of 8240 for that of mucondialdehyde reported in ref 8. We may add that when mass spectrometry of the two hydrazones was attempted, they decomposed at 250-280' and did not yield the peak of the parent ion. The PNDC dye did likewise.
I
I
x Figure 5 . Spectra of ketoenols conjugated with C=O (see ref 19-21). pHMD: a, pH 1; b, pH 7-8; c, pH 12-6; benzoyl acetaldehyde: d, 95% alcohol; e, pH 2; glutaconic aldehyde : f, sodium salt ; a-hydroxymuconicsemialdehyde : g, p H 7.5; h, pH 6.6,5.5,and 1. The c for the peak in the 360 mp region for p hydroxy a,p-unsaturated aldehydes (lower set of curves) is one order of magnitude lower than for 6 hydroxy a,p : y,&unsaturated aldehydes (upper curves).
The Effect of Ferrous Ions. Proskurnin26 and Kartashevag measured the "catalytic" effect of ferrous ions upon the aerated aqueous benzene system and found an enhancement of G(pheno1) and G("mucondia1dehyde"). On the other hand, Vermeil2' and other^*^^^^ studied systems made up of benzene and other hydrocarbons, aromatic and aliphatic, dissolved in acid ferrous sulfate solution and found in all cases a large increase in G(Fe3+). The two effects are evidently related, and quantitative correlation should be possible. The Journal of Physical Chemietry
A situation similar to the above arises in the case of the Fenton system, in which hydroxyl radicals are produced in a reaction between ferrous ions and hydrogen pero~ide.~O-~* When benzene is added to this system, the yield of ferric ion is greatly enhanced,33an effect obtainable by the addition of many other organic compounds. To account for this, Kolthoff and Nedalia13 suggested a mechanism in which the hydroxyl radicals react with the organic compound in preference to the ferrous ion, to produce radicals which pick up oxygen and become peroxy radicals which either oxidize two ferrous ions and carry a chain reaction or yield three ferric ions before becoming a product. Dewh ~ r s t ' ~first ~ ' studied ~ the Fricke dosimetric system in presence of a series of alcohols and used the above chain mechanism to account for the high yields of ferric ions. He did not measure the products arising from the organic impurity for correlation with the yield of ferric ion. Scheme I incorporates the chain mechanism of Kolthoff and Medalia, modifying it to include an important reaction visualized by Dorfman,Ie in which the organic peroxy radicals decompose to yield HOz instead of attacking ferrous ions. The result is that the identical influence of ferrous ions upon the yields of the two products finds a natural explanation. The number of ferrous ions required to convert the primary species into OH radicals and reduce the latter in the chain-terminating step is 16.6. The remaining ferric yield is formed in the propagation of the chain (26) E. V. Barelko, L. I. Kartasheva, P. D. Novikov, and M. A. Proskurnin, Proc. All-Union Conf. Radiat. Chem., l s t , 1967, 2, 84 (1957). (27) C. Vermeil, J. Chim. Phys., 52, 587 (1955). (28) H. Le Bail and J. Sutton, ibid., 53,430 (1956). (29) K. C. Kurien, P. V. Phung, and M. Burton, Radiat. Res., 11, 283 (1959). (30) F. Haber and J. Weiss, Proc. Roy. SOC.,A147, 332 (1934). (31) W. G. Barb, J. H. Baxendale, P. George, and K. R. Hargrave, Trans. Faraday Soc., 47,462 (1951). (32) W.Taylor and J. Weiss, J. Chem. Phys., 21,1419 (1953). (33) J. H. Baxendale and J. L. Magee, Discussions Faraday Soc., 14, 160 (1953).
MECHANISM OF OH RADICAL REACTION WITH BENZENE
855
Scheme I
O
H
+
O
-
HO
eOH-
HZ02
CHO CHO
CHO
and should be twice the total yield of organic product, Le., G(pheno1) C(@HMD). The two values for M ferrous sulsaturated solutions of benzene in fate are 32.2 and 15.4, respectively. Allowing for errors, the agreement with the requirements of Scheme I is satisfactory. Per 100 eV, the number of cycles of the chain is 15.4 while the number of hydroxyl radicals participating in the initiation is 7.4. ( i e . , 2.9 3.7 0.8, being the sum of the yields of OH, HO?, and HzOz, since the last two are converted to OH by the ferrous ion, as shown in Scheme I). Thus, each OH radical on an average gives rise to 2.1 cycles of the chain before it is reduced by a ferrous ion. The virtual absence of an effect on the G(Fea+) upon addition of phenol to the Fricke dosimetric system has a simple explanation. It has been shown34va5 that the attack of OH on phenol produces the phenoxy radical. Understandably, the ferrous ion simply reduces this radical back to phenol and there is no chain in this case. The Homolytic Hydroxylation of Benzene. In connection with the hydroxylation of naphthalene,lZit has been pointed out that oxygen plays an important part. The HOz elimination visualized by DorfmanlB has
+
+
+
t CH-tCHAIN
CHC
further support from the present work. The inclusion of this step in the mechanism of formation of pHMD is justifiable on the ground that there is a substantial gain in resonance energy in the formation of pHMD as in the case of phenol. Such a reaction has been recently suggested in the case of the a-hydroxyisopropylperoxy radical30which is an intermediate in the formation of acetone in the radiolysis of 2-propanol solutions. There is a striking pH effect upon the course of the hydroxylation reactions (Table I), The only plausible explanation for it is a steric effect caused by the protonation of the OH adduct in the more acidic solutions and solvation of the *OHz+. The ortho positions are thus rendered less accessible to 02 addition and the phenol yield is reduced relative to that of PHR4D.
Acknowledgment. We wish to express thanks to K. G. Deshpande and V. K. Bhalerao for numerous spectra taken toward this work.
(34) J. H. Merz and W. A. Waters, J. Chem. Soc., 2427 (1949). (35) AM. Ebert and E. J. Land, Trans. Faraday SOC.,63, 1181 (1967). (36) G. Hughes and H. A. Makada, ibid., 64, 3276 (1968).
Volume 74, Number 4 February 19, 1970
