2098
J. Phys. Chem. 1981, 85, 2098-2103
y-Radiolysis of Aqueous Benzo[ b ]thiophene Solutions' Paul Y. Feng,l2 Chemistry Department, Marquette Unlvershy, Milwaukee, Wisconsin 53233
Kundan Patel,' Louis Kapian, and Max S. Matheson Chemlstry Dlvlsion, Argonne National Laboratory, Argonne, Illinois 60439 (Received: June 24, 7980; In Flnal Form: January 26, 1981)
y-Radiolysis of aqueous benzo[blthiophene solutions saturated with NzO has been studied. In the absence of other reagents, the only products detected are 2- and 3-hydroxybenzothiopheneand their corresponding lactonic and ketonic tautomers and a small amount of dimeric species. In the presence of potassium hexacyanoferrate or sodium hydroxide, however, products attributable to hydroxylation at all six available carbon sites are detected and dimer formation is reduced. Analysis of the experimental data suggests that both benzothiophene and its hydroxylation products are efficient scavengers for the radiation-produced OH radicah and that the initial yield of hydroxylation products is equal to that of hydroxyl radicals. Consistent with the electrophilic characteristics of the OH radical, k(OH+CEH6S):k(OH+CEH~soH) = 0.5 f 0.1. All six available carbon sites are capable of forming OH adducts, but the radicals formed by addition to the 4,5,6, and 7 positions tend to revert to the original compound or its ion in a neutral or acidic medium. In the case of the adducts formed by addition to the thiophene ring, on the other hand, the 3 position is seen to be favored, possibly as the result of the preservation of the aromatic benzene resonance structure.
Introduction Although thiophene does exhibit typically aromatic characteristics, the introduction of a heteroatom leads to distinctive differences between the chemical behavior of thiophene and benzene both in chemical processes initiated by conventional means and in those induced by high-energy r a d i a t i ~ n . ~ -As ~ the simplest molecule containing both the aromatic benzene and the heterocyclic thiophene structure, therefore, benzo[b]thiophene constitutes a logical compound for studying the reactions of free radicals with more complex heterocyclic substances. Reactions of this type are of significance from the view point of coal chemistry inasmuch as the sulfur in coal is often found in benzothiophene-type structures. The present paper reports on our product identification, mechanistic, and kinetic studies of y-radiolysis of aqueous NzO-saturated solutions of benzothiophene and is a complementary effort to an ongoing pulse radiolytic investigation of the transient species formed in the same system. The principal advantage afforded by such a system is that quantitative conversion of the radiolytically produced electrons yields a radical mix of 90% OH and 10% of the less reactive H.'O A simplification of the reaction scheme may therefore be expected, facilitating thereby the interpretation of the experimental results. Experimental Section Materials. Fluka AG pure grade benzothiophene was purified by recrystallization for three times from 95% (1) Work performed under the auspices of the Office of Basic Energy Sciences of the US. Department of Energy. (2) Faculty Research Participant, Argonne Center for Educational Affairs, 1978-79. (3) Department of Chemistry, University of North Dakota, Grand Forks, ND 58202. (4) S. Gronowitz, Adu. Heterocycl. Chem., 1 , 1 (1963). (5) R. Livingstone in "Rodds Chemistry of Carbon Compounds", Vol. IVA, 2nd ed, S. Coffey, Ed., Elsevier, New York, 1973, Chapter 3, p 219. (6)S. A. Safarov, Dokl. Akad. Nauk SSSR, 151, 890 (1963). (7) J. Lilie, 2. Naturforsch. B , 26, 197 (1971). (8) B. B. Saunders, P. C. Kaufman, and M. S. Matheson, J. Phys. Chem., 82, 142 (1978). (9) B. B. Saunders, J. Phys. Chem., 82, 151 (1978). (10) F. S. Dainton and D. B. Peterson, Proc. R. SOC.London, Ser. A , 267, 443 (1962).
ethanol. Other reagents included nitrous oxide (Liquid Carbonic 99% pure custom grade) and potassium hexacyanoferrate (Mallinckrodt AR) and were used without further purification. All solutions were prepared by using water triply distilled from a standard alkaline permanganate-acid dichromate system. Sample Preparation. On account of the volatility of benzothiophene, bubbling an aqueous solution of this compound with N20 leads not only to the elimination of dissolved oxygen but also a gradual reduction of the concentration of benzothiophene which is soluble in water only to the extent of ca. 1.1mmol/L at laboratory temperature. In order to obtain sufficient products for identification purposes, we performed some irradiations on suspensions of 3 mmol of benzothiophene in 1 L of the appropriate medium (water, M K,Fe(CN&, and/or M NaOH) bubbled with N20while stirring for 15 min. Homogeneous samples for the kinetic experiments were obtained by stirring for 48 h an excess amount of benzothiophene in aqueous M K,Fe(CN), solution, and bubbling the resultant suspension with N20, using a sintered glass diffuser. After 15 min, the flow of N 2 0 was reversed, causing the product solution to be filtered through the same diffuser out of the bubbling vessel. The bulk of each sample was then collected directly into an irradiation vessel which had been preflushed with N20,and a small portion was collected into a storage vessel for subsequent determination of benzothiophene concentration by HPLC analysis. Sample Irradiation. Irradiation of the samples was carried out at ambient temperature (ca. 30 "C) by using the ANL Radiation Chemistry 'Wo source. The dose rate selected was 4.9 X lo3 rd min-l (5.1 X 10l8 eV L-' s-l). Sample Analysis. General. Analysis of the irradiation products was based principally on HPLC using a Waters Associates liquid chromatograph assembly equipped with a stop-flow valve and a Beckman Model 25 microcell variable-wavelength UV-visible spectrophotometer both for peak detection and for spectral scanning of the individual product species. Identification of the specific products was based on comparison of the LC retention and UV absorption characteristics of these products with either authentic compounds or literature information,"-" sup-
0022-3654/81/2085-2098$01.25/0@ 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 14, 1987 2099
yRadiolysis of Benzo[ b ] thiophene Solutions
TABLE I: Characterization of Phenolic Products in Irradiated Benzothiophene Solution commund characterization benzo [ b Ithiophen-2( 3 H ) s n e benzo[ b ]thiophen-3(W)-one benzo[ b Ithiophen-3-ol benzo [ b Ithiophen-4-01
benzo[ blthiophen-5-01 benzo[ b lthiophen-6-01 benzo[ b lthiophen-7-01
2,2'-bibenzo[b]thiophen-~-ol
comparison of HPLC retention with authentic compound comparison of HPLC retention with authentic compound comparison of HPLC retention with authentic compound UV k,,(CHCI,): 256 nm ( E = 4640), 267 nm ( e = 3760), 287 nm (shoulder), 297 nm ( E = 4480), 3 s7 nm ( E = 5360) IR(CC1,): 3400 cm-' (vs), 1580 cm-' (s), 1430 cm-I (m), 1383 cm-' (m), 9 3 5 cm-' (s), 750 cm-' (m) UV kmax(CHC1,): 259 nm ( E = 6900), 267 nm ( E = 3600), 300 nm ( E = 2930), 306 nm ( e = 3200), 312 nm ( E = 2900) mass spectra : M' = 150 UV kmax(CHCI,): 266 nm ( E = SOOO), 274 nm (e = 7650), 296 nm ( e = 1070), 302 nm ( E = 8 0 0 ) , 306 nm ( E = 800) mass spectra: M' = 150 UV kmax(CHC1,): 256 nm ( E = 6580), 264 nm ( E = 5760), 287 nm (shoulder), 297 nm ( E = 7330). 307 nm ( E = 7340) mass spectra: M' = 150 UV hmax(CHC1,): 264 nm, 298 nm, 306 nm, 312 nm IR(KBr): 3400 cm-' mass spectra: m / e 282, 264, 233, 175 ( 6 6 ) , 134 (100)
plemented, when necessary, by mass spectral data obtained with a modified time-of-flight mass spectrometer. Product IdentificationlMechanisticExperiments. The irradiation products were isolated from each 1-L irradiated suspension in the following manner. The pH of the system was adjusted to about 9-9.5, and the solution was extracted once with 300 mL and twice with 100 mL each of ether. The combined ether extracts were then washed twice with water, dried over MgS04, and the neutral products recovered by evaporation of the ether under reduced pressure. The residual aqueous phase was then acidified, subsequent to which the phenolic products were extracted similarly with one 300-mL and two 100-mL portions of ether. These portions were then combined in turn, washed twice with saturated sodium chloride solution, and similarly dried and evaporated. Both the neutral and the phenolic products were then analyzed by HPLC, the former on a 2.1 X 600 mm column packed with Waters Associates p-Bondapak C18and using methanol as the mobile phase, and the latter on a 2.1 X 600 mm column packed with Waters Associates p-Porasil and using chloroform as the mobile phase. Kinetic Experiments. The concentrations of benzothiophene in both the unirradiated and the irradiated solutions were determined by direct HPLC analysis of the aqueous solutions using two Waters Associates p-Porasil columns (3.9 X 300 mm each) connected in series and as the mobile phase. isooctane (2,2,4-trimethylpentane) Quantitative measurement was afforded by calibration with standard solutions analyzed spectrophotometrically at 280 nm (t = 1739 L mol-' cm-'). On account of the deleterious effects of water on the performance of the p-Porasil packings, the columns were reconditioned after each fifth analysis by successive flushings of eight or more column volumes of methylene chloride, methanol, methylene chloride, dry chloroform, and isooctane. Reproducible results were obtained by this procedure. For the analysis of the irradiation products, each 50-mL aliquot was acidified with 1 mL of 3 N HC1 and extracted successively with one 15-mL and three 5-mL portions of (11)R. P. Dickinson and B. Iddon, J. Chem. SOC.C, 1926 (1970). (12)C. J. Baxter, R. F. C. Brown, and G . L. McMullen, Aust. J . Chem., 27, 2605 (1974). (13)M.R. Padhye and S. R. Desai, Trans. Faraday SOC.,49, 1386 (1953). (14)(a) L.J. Pandya, D. S. Rao,and B. D. Til&, J. Sei. Ind. Res. Sect. B , 18,516(1959);Chem. Abstr., 54,17391d (1960).(b) A.V.Sunthankar and B. D. Tilak, Proc. Indian Acad. Sci., Sect. A , 33, 35-41 (1950).
100
80 +-
60 40 20 RETENTION VOLUME IN mL
Figure 1. Typical liquid chromatogram of the irradiation products in a benz~thbphene-N,O-K,Fe(CN)~ system. (1:2 CHC13:I-C8H,8,two 3.9 X 300 mm j&xasil columns in series). (A) CH2CI, benzothiophene; (B) neutral products: (C) a-naphthol internal standard; (D) 4hydroxybenzothiophene; (E) 7-hydroxybenzothiophene; (F) 0hydroxybenzothiophene; (G) 5-hydroxybenzothiophene.
+
ether. One milliliter of 1% (wt/vol) ethanol solution of cy-naphthol was then added as an internal standard to the combined ether extract, and the mixture was evaporated to dryness. The residues dissolved slowly in methylene chloride, and the solutions so obtained were transferred to 10-mL volumetric flasks and diluted to mark. Good resolutions and absorption spectra for the individual products were obtained by using two 3.9 X 300 mm pPorasil columns connected in series, and 1:2 chloroform:isooctane as the mobile phase.
Results and Discussion The following aspects are reported in this publication: (1)identification of the radiolytically produced species and examination of the mechanisms of their formation, and (2) kinetic analysis of the disappearance of the parent benzothiophene molecules as well as the formation and destruction of the products of the radiolytic processes. Product Identification/MechanisticStudies. A typical liquid chromatogram of the irradiation products in a benzothiophene-N20-K3Fe(CN)6 system is shown in Figure 1, and the analytical characteristics of the phenolic products are given in Table I. On the basis of comparisons with authentic compounds or literature information, the products have been identified to be the hydroxylated benzothiophenes, the corresponding thioketonic and thiolactonic tautomers, and 2,2'-bibenzo[ blthiophene and a hydroxy derivative. In addition, in neutral solutions containing potassium hexacyanoferrate, a reddish com-
2100
The Journal of Physical Chemistry, Vol. 85,No. 14, 1981
Feng et
TABLE 11: Relative Yields of Radiolytically Produced HydroxybenzothiopheneP site of hydroxylation system
c-2
c-3
0.32 0.33 0.24 0.21
1 1 1 1
C-4
c-5
81.
apparent G values
C-6
c-7
totalb
dimer
1.3 0.2 0.09 0.04 2.5 0.29 0.26 2.4 0.36 0.07 2.1 Since these runs were carried to rather high (I At a total dose of 1.27 X lo2'eV g-l (2.83 X l O I 9 eV g-' min-I for 4 5 min). Total hydroxybenzothiophene. conversions, yields and G values are not initial values. N*O N,O, pH 12 N,O, K,Fe(CN), N,O, K,Fe(CN),, pH 1 2
0.18 0.64 0.54
pound with apparently high polarity is also produced but the low yield of this species did not permit its characterization. Benzothiophene-N20 Solutions. The products consist principally of 3-hydroxybenzothiophene and its ketonic tautomer together with a lesser amount of the corresponding 2-substituted tautomeric mixture. No phenolic 4-, 5-, 6-, or 7-hydroxybenzothiophene could be detected in the product mixture, but small amounts of the dimeric 2,2'-bibenzo[ blthiophene and a hydroxy derivative are also produced. Ben~othiophene-N~O-K~Fe(CN)~ Solutions. All six isomeric hydroxylated benzothiophenes are produced, but dimer formation appears to be completely suppressed. As in the case of solutions in the absence of potassium hexacyanoferrate, the 2- and 3-lactonic and ketonic tautomers are relatively unstable. HPLC analyses show that these species disappear upon standing as does the phenolic 7hydroxybenzothiophene which is known to be less stable than its i ~ 0 m e r s . lThe ~ ~ lactonic and ketonic tautomers are, therefore, seen to behave similarly to their simple thiophene analogues which convert easily to sparingly soluble polymers. Benzothiophene-N20in Alkaline Solutions. Radiolyses of benzothiophene-N20 in alkaline solutions have been carried out both in the presence and in the absence of added potassium hexacyanoferrate. The formation of all six hydroxylated isomers was observed. Futhermore, consistent with the product distribution in irradiated neutral solutions, the 3-substituted tautomeric mixture is again produced with the highest yield. On account of the very limited solubility of benzothiophene and the buildup of the hydroxylated products during radiolysis the net rates of formation of these products are expected to decrease significantly with increasing total dose as would be expected from competition kinetics. The apparent G values of the hydroxylated products should, therefore, be functions which depend particularly sensitively on total dose. The relative yields of these products should nevertheless provide useful information relative to the nature of the chemical processes involved. These are therefore listed in Table 11. Mechanism of Hydroxybenzothiophene Formation. In conventional electrophilic processes, benzothiophene undergoes substitution reactions in the heterocyclic ring rather than the benzene ring-an observation which is consistent with the fact that thiophene is significantly more reactive than benzene or its derivatives in many chemical reaction^.^ It is interesting to note, therefore, that in the presence of potassium hexacyanoferrate, hydroxylation of the benzene ring occurs to an extent comparable to that of the thiophene ring. In solutions containing only benzothiophene and N20, on the other hand, no benzene ring hydroxylation can be detected. A plausible reaction scheme must, therefore, be able to account for these observations as well as the apparent preference for C-3 substitution. We note that the reactions of OH radicals with either benzene or thiophene are both very fast,15and there is no
0.02 0.24 0.69
apparent reason why the formation of the fused ring system should materially affect the rates of such processes. We believe, therefore, the reaction process is best described by the addition of OH radicals to all carbon atoms of benzothiophene a t comparable, though not necesssarily identical, rates. In the presence of an oxidizing agent such as K,Fe(CN)6, therefore, these adducts would be efficiently trapped and converted quantitatively into the corresponding hydroxybenzothiophenes in agreement with the results of our experiment. In the absence of such oxidants, however, these hydroxylated radical adducts can undergo a variety of other reactions either forming dimeric products or regenerating the parent hydrocarbon. Product Distribution in Irradiated Neutral Benzothiophene-N20 Solutions. On account of the importance of benzene as an aromatic molecule, the reaction between the OH radical and benzene has been studied by a large number of investigators.l6'l' It is well established that phenol formation does not result from bimolecular disproportionation of the hydroxycyclohexadienyl adducts, but proceeds via a dimeric intermediate which is converted (usually acid-catalyzed) to either biphenyl or phenol and For the thiopheneN20system, on the other hand, Saunders et al. suggested that the formation of the hydroxylated products may conceivably be formed from a bimolecular disproportionation process although the role of intermediates such as I also cannot be ruled out.' 2 q
H
o
H
J
?
?
-
S
H
o
H
S
I
Quantitatively, Mantaka and co-workers reported that, for the benzene-N20 system, G(dimer)/G(phenol) = 2.1.20 For the thiopheneN20 system, Saunders and co-workers found the corrzsponding G (dimer)/ G (hydroxylated products) ratio to be 1.4 f 0.3.' In contrast, for the benzothiophene-N20 system, we obtained G(dimer)/G(hydroxylated products) = 0.15. On the basis of studies of several aromatics under a variety of conditions, Walling and Johnson suggested that the hydroxycyclohexadienyl radicals formed by the addition of OH radicals to benzene derivatives can suffer a rapid, reversible, and acid-catalyzed collapse to radical ion I1 which can, among other reactions, undergo competitive reduction back to the starting material.21 Similarly, Sehested and co-workers also showed that, a t low pH's, (15) Farhataziz and A. B. Ross,Natl. Stand. Ref. Data Ser., Natl. Bur. Stand.. No. 59 (1977). ~~(16)'G. W. Klein and R. H. Schuler, Rodiat. Phys. Chem., 11, 167 (1978), and references cited therein. (17) K. Bhatia, Radiat. Res., 59, 537 (1974), and references cited therein. (18) V. S. Shikharev and N. A. Vysotskaya, Khim. Vys. Energ., 2,249 (1968). (19) K . Bhatia, J. Chromatogr. Sci., 13, 84 (1975). (20) A. Mantaka, D. G. Marketos, and G. Stein, J. Phys. Chem., 75, 3886 (1971). (21) C . Walling and R. A. Johnson, J.Am. Chem. SOC.,97,363 (1975). I
The Journal of Physical Chemlstty, Vol. 85, No. 14, 1981 2101
yRadiolysis of Benzo[ 61thiophene Solutlons
TABLE 111: Effect of pH on Product Distribution in Benzothiophene Radiolysis no K,Fe(CN), neutral pH 12
distribution % thiophene
TI the products of OH addition of methylated benzenes and biphenyls can undergo a water elimination reaction to form the corresponding (radical) c a t i ~ n . ~ Indeed, ~ - ~ ~ on the basis of a pulse radiolysis study of an N20-saturated solution of naphthalene, a v o s and Sehested concluded that the reaction for the formation and disappearance of the hydroxyl radical adducts of naphthalene formed by ionizing radiation is
m
IIIa
Ip
The OH adducts decay by second-order kinetics with 212 = 1.35 f 0.10 x 109.26 The preceding observations, coupled with the higher acidity of the C-2 and C-3 OH adducts on account of the electronegativity of the sulfur atom which thus might act as virtual sources of H+, suggest a similar mechanism to account for the product distribution in irradiated neutral solutions of benzothiophene and N20. Bimolecular reactions between the less acidic cyclohexadienyl radicals in a neutral medium, on the other hand, would be expected to result only in the formation of dimeric species. Only C-2 and C-3 substituted hydroxybenzothiophenes can therefore be produced, and statistically would be expected to be in much higher yield than the dimers:
substitution % benzene substitution % benzene substitution/ % thiophene substitution % dimeric products %other products
10-3 M K3Fe(CN), neutral pH 12
86
73
46
40
0
18
53
56
0
0.25
1.2
1.4
14
0
0
0
0
9
1
4
Preferential Formation of 3-Hydroxylated Products. The preferential formation of 3-hydroxylated benzothiophene and its tautomer contrasts strikingly with the preponderance of 2-substitution processes in both classical and radiation-induced reactions involving thiophene. Nevertheless, our results are completely consistent with the electrophilic characteristics of the OH radical and the knowledge that benzothiophene is best represented as a resonance hybrid of V and the charged structures XI and
-@$@P
XI
xu
XII.6 (V being the most important, and XI1 the least). Also, by virtue of the preservation of its resonance structure, the C-3 adduct is expected to have a slightly higher stability than the C-2 adduct, and consequently, a higher rate of its formation.
m m
m
m tm;
Ix
P t HC+@&fGH
/ ‘ T
+H
*
t
X
a OH
i.e., an overall reaction given by PItm--+P*IX (orX1
and similarly PII + ~ - - P + I X (or X)
However PI t ILl-
DIMERS
(22)H. C. Christensen, K. Sehested, and E. J. Hart, J. Phys. Chern., 77, 983 (1973).
(23)K.Sehested, H.Corfitzen, and E. J. Hart, J. Phys. Chern., 79,310 (1975). (24)K. Sehested and E. J. Hart, J. Phys. Chern., 79, 1639 (1975). (25)K.Sehested, J. Holchman, and E. J. Hart, J.Phys. Chern., 82,651 (1978). (26) N. Zevos and K. Sehested, J. Phys. Chem., 82,138 (1978).
The Effect of PH. A comparison of the relative extents of substitution at the benzene and thiophene rings in our various product identification/mechanistic experiments are summarized in Table 111. In view of the instability of the lactonic and ketonic tautomers of 2- and 3hydroxybenzothiophene, the relative extents of benzene substitution to thiophene substitution in the presence of added potassium hexacyanoferrateat both neutral and pH 12 should be considered to be comparable within the limits of experimental error. For systems which do not contain added potassium hexacyanoferrate, however, the formation of the phenolic products in basic solution is a clear contrast to the total absence of such species in irradiated neutral solutions. A number of postulates might be advanced, e.g., (1)the effect of pH on the redox potential of the hexacyanoferrate-hexacyanoferrite couple, (2) contribution of HOZformed by the radiolytically produced H202at pH 12, or (3) the increased OH yield at high pHs as the result of the two-step reaction:
2102
The Journal of Physical Chemistry, Vol. 85, No. 14, 1981
H
+ OH-
-
eaq- + N 2 0 + H 2 0
eaq-+ HzO
-
Nz + OH
+ OH-
However, these postulates either cannot be expected to alter the course of reaction in the directions observed (postulate l),or cannot be expected to materially affect the yields of the intermediates and can therefore, at best, modify only marginally the product distribution (postulates 2 and 3). On the other hand, we note that pK, of OH = 11.9 f 0.2.27 Over half of the OH radicals are thus present as 0- at pH 12. Inasmuch as benzothiophene would probably react more readily with the electrophilic OH radicals than with the nucleophilic species 0-,possible explanations are (1)stabilization of the phenoxide ion due to the increased delocalization of the electron, (2) basecatalyzed formation of phenolic products from the dimeric intermediates, and (3) enhancement of formation of M and X as the result of the removal of H+ at pH 12. Kinetic Analysis of Neutral Benzothiophene-N20K3Fe(CM6Solutions. Under the conditions of our experiment, the principal intermediate species produced by the radiolysis of water is the hydroxyl radical formed as the result of (1) the primary radiolytic process
--
H20
H
Feng et al.
GZ d[OH]/dt = -- [OHl(ka[Al + kb([B] + [ c ] + ..*)) lOONo - GI --lOONo [OHl(ka[Al + kb([AlO - [AI)) (1) -d[A] /dt = k,[A] [OH]
( 2)
where G is the yield of OH radicals per 100 eV of absorbed radiation energy, Z is the dose rate in electronvolts per liter per second, No is Avogadro's number, [A], [B], [C] etc. are the molar concentrations of, respectively, benzothiophene, the hydroxylated benzothiophenes, and their irradiation products. Under the conditions of our experiment, G = G(OH) + G(e,;) = 2.75 + 2.7 = 5.45,20I = 5.1 X 10l8eV L-l s-l, and GZ/(lOONo) = 4.53 x mol L-' s-l. Using the steady-state approximation for the concentration of the OH radicals we obtain [OH] =
GI/ (1OONo) ka[Al + kb[Bl
--
G I / (1OONo) ka[Al + kb([&l - [AI) GZ/(lOONo)
-
kb[AO1 + (ka - kb)[Al
(3)
+ OH + ea; + H2Oz + H2
and (2) the subsequent conversion of the radiolytically produced electrons into additional OH radicals by N20 eaq- + NzO + H 2 0
-
N2 + OH
+ OH-
The effective 100-eV yields (G values) of the OH and H radicals are therefore, in neutral medium, respectively 5.45 and 0.55.20 On account of the low dose rates in y-radiolysis experiments, however, interactions of these primary radicals are entirely negligible, and both the H and the OH radicals therefore react with benzothiophene to form adduct radicals at any of the available sites of the benzene and thiophene rings of the molecule. In the presence of reagents such as the hexacyanoferrate ion, however, these adduct radicals should be quantitatively oxidized, the hydrogen adducts therefore reverting to the original benzothiophene molecule, and the hydroxyl adducts forming a variety of isomeric products having the empirical formula CaH5SOH. The reaction of OH radicals with benzene has been studied by a large number of investigators, and the rate constants reported for pH 7 ranged from 3.7 X lo9 to 8.2 X 109.15 The rate constant for the reaction of OH with thiophene has been determined by Lilie7 to be 3.3 X lo9, and revised to 8.2 x log by Saunders et a1.8 As a first approximation based on statistical considerations, therefore, adduct formation by the addition of OH radicals to the benzene ring of the benzothiophene molecule should occur to a comparable extent to additions to the heterocyclic portion of the same molecule, and an average rate constant, k,, could be used to designate the overall reaction rate constant of the reactions between the OH radicals with benzothiophene. Similarly, it should also be reasonable to assign an average rate constant, kb, for the reactions between the hydroxyl radicals and the various hydroxylated products of benzothiophene by analogy of the similarity of the rate constants of OH radicals with the three isomeric dihydro~ybenzenes.'~We can, therefore, write (27) J. Rabani and M. S. Matheson, J . Am. Chem. SOC.,86, 3175 (1964).
giving the solution IAl
For the special case where k , = kb, eq 5 is reduced to
i.e., the familiar first-order disappearance equation. Similarly, at low conversions (i.e., when t is small
and a simple first-order disappearance kinetics should again be observed as expected. At much higher conversions (i.e., higher integrated doses), on the other hand, [A]/ [A,,] becomes negligible compared to unity, and the second term of the right-hand side of eq 5 approaches (k,/kb - 1). A semilog plot of [A]/[Ao] vs. dose should therefore have an initial slope equalling -CZ/(l00No[A,,]), but would approach at higher doses an asymptote given by
i.e., a straight line intercepting theln ([A]/[Ao]) axis a t (k,/kb - 11,and with a slope differing from the initial value by a factor ( k , / k b ) . Such a plot is shown in Figure 2, which shows a reasonably good first-order fit up to approximately 70% disappearance of benzothiophene, and a shift to a different slope at still higher extents of reaction. Least-squares analysis of the experimental data leads to the result (ka/kb) = 0.5 f 0.1, Le., the OH radicals react with the hydroxylated benzothiophenes at a rate approximately twice that with the parent benzothiophene molecule. Although there appears to be no direct comparison
The Journal of Physical Chemistty, Vol. 85, No. 14, 1981 2103
y4adiolysis of Benzo[ b] thiophene Solutions
04
oL2Y 006
0040
IO
20
30
40
60
50
IRRADIATION DOSE IN 102’eV C’
Figure 2. Radiation-induced disappearance of benzothiophene: (-) calculated from eq 5 by use of k,lkb = 0.5; (----) extrapolation of results at low and high doses experimental data (dose rate 5.1 X 10” eV L-‘ s-‘, initial concentrations in mM: [C8H,S] 1; [N,O] 2 X 10; [K,Fe(CN),] = 1.0).
-
I
I
I
IO
20
30
1
INTEGRATED DOSE I N 102’eV 4? C
-
I
50
Figure 3. Effect of integrated dose on total product yield in benzothiophene radiolysis: (-) calculated from eq l l by using GI/ (lOON,[A,,]) = 4.2 X lo4 s-l and k,lkb = 0.5; (0) experimental data (dose rate 5.1 X 10” eV L-’ s- ’, lnithi concentrations in mM: [C,H,S] 1, [N,O] 2 X 10, and [K,Fe(cN),] = 1.0).
-
-
of rate constants of OH radical addition to benzothiophene and its various hydroxylated derivatives, the enhanced rate is consistent with the electrophilic nature of the OH radicalqZ8A similar effect has been observed with benzene and phenol; the best values of the rate constants for hydroxyl radical addition at pH 7 are (in lo9 L mol-’ s-l,) 7.8 f 1.1and 14 f 3, r e s p e c t i ~ e l y . ~ ~ ~ ~ ~ (28)P. O’Neill, S.Steeken, H. Van der Linde, and D. Schulte-Frohlinde, Radiat. Phys. Chem., 12, 13 (1978). (29)P. Neta and L. M. Dorfman, Adu. Chern. Ser. 81, 222 (1968). (30)E. J. Land and M. Ebert, Trans. Faraday SOC.,63,1181(1967).
Product Buildup Kinetics. The formation of hydroxybenzothiophene is described by the kinetic equation d[Bl/dt = (ka[Al - kb[BI)[OHI ka[Al - kb[Bl =- G I (9) 100N0 kb[&bl + ( k a - kb)[AI As indicated in Figure 2, up to approximately 70% of the disappearance of benzothiophene is described satisfactorily by the simple first-order relationship given by eq 7 . On the other hand, at still higher conversions, the error introduced by this approximation would also be small both on account of the diminishing values of [A], and on account of the relative magnitudes of ka and kb. Equation 9 becomes therefore d[Bl -- dt GI Ita[&] ~xP(-GI/(~OONO[AOI))~ - kb[B] 100N0 kb[Aol + (ka - kb)[Aol ~XP(-GI/(~OONO[A,I))~ (10) which has the solution GIt/ (100No[Ao1) -[BI - [A01 (kb/ka) exp(GIt/100No[&l) - ( k b / k a ) + 1 -GIt/(100No[AoI) (kb/ka)(exp(Glt/lOONo[Aol) - 1) -k 1 (11) Using the experimental slope GI/lOONo[&] = 4.2 X lo4 s-l, and k,/kb evaluated from the benzothiophene disappearance data, the dependence of [B]/ [&I (and therefore [B]/[B],,) on time (and dose) can be readily calculated from eq 11 and compared with experimental data. This is illustrated in Figure 3. Good agreements are indeed observed, supporting our model involving quantitative conversion of the adducts into the hydroxybenzothiophenes. Furthermore, since the initial 100-eV yield of the latter equals d[B]/ldt,,, eq 10 readily reduces to G(B),, = d[B]/Idt,+ = G E G(0H) Le., the initial G value for the hydroxybenzothiophenes is 5.45.
Acknowledgment. We thank Dr. H. A. Kaufman of the Mobil Chemical Co. for the generous supply of 4-benzothienyl-N-methylcarbamate (Mobam) needed for the synthesis of 4-hydroxybenzothiophene. We also thank our colleagues Dr. Martin Studier for obtaining the mass spectral data reported herein, Dr. Charles Jonah for the use of his least-squares data analysis program, and Drs. Gerhard Closs and Barbara Saunders for their comments and advice. One of us (Paul Y. Feng) also expresses his appreciation to the Argonne Center for Educational Affah and to the Marquette University Sabbatical Program supporting this research.
