Radiolysis of Aqueous Ethanol in the Presence of CO

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Radiolysis of Aqueous Ethanol in the Presence of CO Hyoung-Ryun Park * and Nikola Getoff Institut für Theoretische Chemie und Strahlenchemie der Universität Wien und Ludwig-Boltzmann-Institut für

1 Radiolysis of Aqueous Ethanol in the Presence of CO Hyoung-Ryun Park * and Nikola Getoff Institut für Theoretische Chemie und Strahlenchemie der Universität Wien und Ludwig-Boltzmann-Institut für Strahlenchemie und Strahlenbiologie, Währingerstraße 8, A-090 Wien, Austria Z. Naturforsch. 4 a, (992); received June 29, 992 First, the following rate constants (in d m m o l " s " ) were reported: k(eäq+ C 2 H 5 O H ) = (.2 ± 0. ) x 0, lc(h + C 2 H 5 O H ) = (2.8 ± 0.2) x 0 and fc(oh + C 2 H 5 O H ) = ( ) x 0* The major radiolytic products resulting from aqueous 0~ 2, and 0 mol d m " ethanol were studied in the absence and presence of C O as a function of dose at ph 2 and. Probable reaction mechanisms are presented. Introduction The radiolysis of airfree as well as of oxygenated aqueous ethanol has been extensively studied in respect to various points of view, e.g. [-]. In deoxygenated aqueous solutions (ph > ) the solvated electron (e~q) is rather unreactive towards ethanol (k < 400 dm m o l - s " [4]). However, aqueous ethanol is attacked by H-atoms with k=. x 0 dm m o l " s " [5] and by OH-radicals with /c =.9 x 09 dm mol" s" [6] in neutral and acidic solutions. The O H radicals withdrawn an H atom preferentially (85%) from the a-c-atom, % from C-2-position and 2% from the OH-group of the ethanol molecules [], In airfree solution, the C H C H O H species can dimerize to 2,butandiol or disproportionate to acetaldehyde and ethanol [, ]. In addition G(H 2 ) = 4.05 and hydrogen peroxide (G = 0.6) have been determined [8]. It has been also reported that in the presence of oxygen (ph =.2) the yield of hydrogen is strongly reduced up to G(H 2 ) = 0.6, and no 2,-butandiol was found. However, the yield of H and acetaldehyde is increased, namely G ( H ) = 4.5 and G(CH CHO) = 2.6, respectively [9]. Thereby the discovery of the rapid elimination of H 0 2 / 0 2 from a-hydroxylalkylperoxyl radicals brought a significant step for better understanding of the reaction mechanisms [0-2]. Finally, by means of pulse- and flash-conductometric measure- * On leave from Department of Chemistry, C h o n n a m National University, Kwang-ju, 500-5, South Korea. Reprint requests to Prof. Dr. N. Getoff, Institut für Theoretische Chemie und Strahlenchemie, Universität Wien, Währingerstraße 8, A-090 Wien, Österreich. ments it was possible to elucidate the ethanol oxydation process and the formation of the Final products (acetaldehyde, acetic acid, formic acid and peroxide) []. The aim of the present work was to investigate the radiation induced chemical behavior of the ethanol/ C O system in aqueous solutions (ph = 2 and ). Thereby ethanol concentrations of 0" 2, and 0 mol d m " were used. In the last case the effect of direct radiolysis is also expected. The analyses were focussed mainly on the products resulting from the CO-participation in the chemical transformation of ethanol. In addition to the studies under steady-state conditions also some pulse radiolysis experiments were performed. Experimental Preparation of solutions. The solutions were prepared by using at least triply distilled water (quarz vessels) and p.a. chemicals (Merck). The aqueous ethanol solutions were first deoxygenated by purging high purity argon ( min) and subsequently saturating with C O at room temperature (40 min, 0 " m o l d m " CO). Irradiation source. A "Gammacell 220" served as 60 Co-y-irradiation source (dose rate: 0 G y m i n " ) * for steady-state experiments. Pulse radiolysis studies were also performed (0.4 ps pulses of MeV electrons [4, 5], Analysis. The formaldehyde yield was determined spectrophotometrically by means of the Hantz-method * Gy = 00 rad - Joule/kg = Ws/kg / 92 / $ 0.0/0. - Please order a reprint rather than making your own copy.

2 986 H.-R. Park and N. Getoff Radiolysis of Aqueous Ethanol in the Presence of C O (described in [6]). The colored complex has e = 8000 dm m o l - c m - at / = 42 nm. The yield of total aldehydes (formaldehyde, acetaldehyde and glyoxal) was measured using the method of Banks et al. []. Aliquot parts of the irradiated solutions were treated with 2,4-dinitrophenylhydrazine in order to convert the aldehydes into the corresponding hydrazones. The last ones were extracted with benzene, and the obtained extract was then treated by distillation under reduced pressure in order to remove the solvent. Subsequently the residue was disolved in dioxane and the aldehydes were separated by thinlayer chromatography (Silicagel plates, Merck) using toluene: benzene: CC4 = : 2 : as solvent (running time ca. hour). The colored spots corresponding to formaldehyde (R{ = 0.4), acetaldehyde (Rf = 0.42) and glyoxal (Rf = 0.22) were individually extracted in 5 ml absolute ethanol, and after centrifugation of the solid phase the optical density for the determination of formaldehyde and acetaldehyde was measured at A = 50 nm and for that of glyoxal at X = 40 nm. The product yields were determined by means of calibration curves. The formaldehyde yield (determined by the Hantz-method) was substracted from the total aldehyde yield to obtain that of acetaldehyde. The glyoxalic acid was analyzed spectrophotometrically (/ = 50 nm, e = 020 dm mol" c m " [8]). The total yield of carboxylic acid was determined by titration with 5 x 0" mol d m " NaOH. Results and Discussion Pulse Radiolysis Experiments The previously reported values of the rate constant for the reaction aq + C 2 H 5 O H -> C2H50- + H D e t e r m i n a t i o n o f / c ( H + C 2 H 5 O H ) at ph = 2 This rate konstant has been previously determined by two groups: k2 =. x 0 dm m o P s _ [5] and k2 = 2. x 0 dm m o l - s" [2]. For completeness of our work we have also performed some pulse radiolysis experiments in this respect using x 0 " 5 mol d m - M n 0 4 as a competitive reagent for Hatoms towards ethanol ( 0-2 to 5 x 0~ 2 mol d m - ) in airfree solutions (ph = 2) [22], C2H5OH + H * C2H4OH + H2, (2) (k2 to be determined) * Mn04~ + H+, Mn04+H ic = 2.6 x 0 0 () dm m o P s" [22], Applying low doses ( ~ 5 G y / 0. 4 p s pulse), the change of the M n 0 4 absorption was followed at x = 545 nm. Several determinations resulted in k2 (2.8 ± 0.2) x 0 dm m o l - s _, which is in very good agreement with the previous published uata. D e t e r m i n a t i o n of /c(oh + C 2 H 5 O H ) at ph = 9 D e t e r m i n a t i o n of fc(ea"q + C 2 H 5 O H ) at ph = 9 e After matrix-correction (solution without ethanol) using fc(e~q+h20)= 6 dm m o P s _ [20], a rate constant, k = {.2 ± 0.) x 0 dm m o P s _ was obtained as a mean value of 0 determinations. This value does not agree with the two above ones. It might be mentioned that the rate constant for the e~-attack aq on both alcohols is reported by Hickel and Schmidt [4] to be the same. They have been performed by flash photolysis in basic solutions. No plausible explanation can be given for this discrepancy at present. () are rather divergent, namely: k(q~q + C 2 H 5 O H ) < 05 dm mol" s _ at ph = 2 [9], kl =. x 04 dm mol" s" [20] and kl < 400 dm mol" s" [4], This fact motivated us to undertake new measurements using a highly purified ethanol (distillation under argon and taking the middle fraction only), triple distilled water and p.a. Ba(OH) 2 for adjusting the ph of the solution. By applying 0.5 to 4 mol d m " aqueous ethanol and doses of 5-6 Gy per 0.4 ps pulse, the pseudo-first order decay of e~q was followed at 20 nm. There are a number of studies done in this respect ranging from ph to with k-values from.6 x 09 to 2.2 x 09 dm m o l - s " [2], The selected value is /c =.9x 09 dm m o P s _. In the present work this subject matter was reinvestigated using 2.5 to 5 x 0" 5 mol dm ~ ferrocyanide as competitive reagent in 5 x 0-4 mol d m " ethanol saturated with N 2 0 (2.8 x 0" 2 mol d m - at ph = 9), C2H5OH + OH * C2H40H + H20, (4) (/c4 to be determined) Fe(CN)4- + OH * Fe(CN)- + O H -, (5) k5= x l 0 l o d m m o P s" [24], The pseudo-first order kinetics of the ferrocyanide absorption change at 420 nm serviced as basis for the

3 98 H.-R. Park and N. Getoff Radiolysis of Aqueous Ethanol in the Presence of CO calculation of the fc 4 -value. The mean value of several measurements was /c 4 = ( ) x 0 9 dm mol" s" which is rather similar to the above mentioned selected value. D- 0x- CHjCHO HCHO (HCO); Steady-state Radiolysis and Product Analysis The formation of the final products is a result of the competition reactions of the primary species of water radiolysis (e~ q, H, and OH) with both substrates, CO and ethanol. The dose dependence of the major radiolytic products resulting from a 0~ 2 moldm~ C 2 H 5 OH/0 - mol dm - CO system at ph 2 and is presented in Figure. The yields of acetaldehyde, formaldehyde and glyoxal are essentially higher at ph 2 compared to those observed at ph (Fig. insert). A similar effect has previously been observed for aqueous methanol/co system [25], The yield-dose curves are tending to a saturation, indicating the occurrence of a product radiolysis at doses higher than 0. kgy. The formation reaction of the main products in acid solution (ph = 2) can be summed up as follows: e~ + aq H ix aq CO + H 2 HCO CO + OH COOH 2 COOH H (6) k 6 = 2. x 0 0 dm mor s" [9], HCO () k =.8 x 0 8 dm mol - s~ [26], 2CH CHOH CO + HCHO (formaldehyde) (8 a) (CHO) 2 (glyoxal) (8 b) COOH (9) /c 9 = 8. x 0 8 dm mop s" [2], C0 2 + H + pk =.4 [28], C0 2 + HCOOH (COOH) 2, (0) (a) (lib) Fig.. Formation of products from 0 mol dm - aqueous ethanol saturated with CO at ph 2 and as a function of dose. Insert: initial G-values (G ; ) of the major products. (0 - mol dm - ) 96% of the OH radicals and ca. 50% of the H atoms are leading mainly to the formation of the CHCHOH radicals, reactions (2) and (4). The resulting species can undergo several reactions: CH CH 2 OH + CH CHO (4 a) (acetaldehyde, G =.25), CH CH(OH)CH(OH)CH (2,-butandiol, G = 0.6), (4 b) CH CHOH + HCO CH CH(OH)CHO (5) (a-hydroxypropionic aldehyde, G = 0.6). Some experiments in basic solutions of 0 _2 mol dm _ ethanol and 0 ~ mol dm ~ CO system showed the previously observed formation of formate [26, 2, 0, ]: COOH + HCO-> HCOO.CHO, 2 CO: (2) (C0 2 ) 2 (oxalate), () k l =.26 xlo 9 dm mop's" [29], Only traces of formic acid as well as oxalic and glyoxalic acid could be found, indicating that reactions (9) to () can be neglected. In diluted aqueous ethanol (0-2 mol dm - ) in the presence of CO CO + e: co: (6) (/c 6 = x 0 9 dm mol - s _ [26]). CO aq is identical with the HCOOH can give rise of a chain reaction: HCOOH-+ OH; HCOO'+OH HCOO~ + e; species which () CO, - -(- H.O (8) (/c 8 = 2.5 x 0 dm mol - s _ [2]),

4 988 H.-R. Park and N. Getoff Radiolysis of Aqueous Ethanol in the Presence of C O,0 kgy Fig. 2. Formation of acetaldehyde from aqueous ethanol as a function of dose. I. Series: mol dm ethanol ethan airfree, (a) ph 2, (b) ph ; saturated with CO, (c) ph 2 and (d) ph. - II. Series: 0 mol d m ethanol airfree, (A) ph 2, (B) ph ; saturated with CO, (C) ph 2 and (D) ph. (curve b in Figure 2). The same ph-effect is observed in the ethanol solutions containing CO (compare curves c and d in Figure 2). In the case of 0 mol d m - ethanol (second series in Fig. 2) the highest yield of acetaldehyde was obtained in airfree solution at ph 2, followed by that at ph (curves A and B in Figure 2). In the presence of CO the yields at ph 2 and are even lower. The G,-values of acetaldehyde of both series in absence and presence of CO are calculated and presented in Table. In the first case the presence of CO is contributing to the formation of acetaldehyde (compare e.g. the G-values at ph 2 in absence and presence of CO). Based on this fact it is obvious that direct radiolysis of alcohol is involved in the process, in course of which C - C bond cleavage takes place leading to the formation of C H and C H 2 O H radicals among H-atoms, C H 2 O H and other species [], Hence in airfree ethanol solution reactions (4 a) and (4 b) in addition to reactions (20a) and (20b) are playing an important role: 2 CH2OH CHOH + HCHO > (CH 2 OH) 2 (20b) 9 Table. Formation of acetaldehyde from and 0 mol dm aqueous ethanol at ph 2 and in the absences and presence of CO. C2H5OH [mol d m - ] 0 Curve Nr. in Fig. 2 ph CO [mol d m - ] Z..0 b c d 2 A B C D 2 C F L C H O H + CO: G, ~ ".9 _ lo" 0~ C H C H ( O H ) C O O ~ (9) (a-hydroxypropionate ion). This reaction pathway was not further studied. The formation of acetaldehyde in mol d m - as well as 0 mol d m - ethanol in the absence and presence of C O at ph 2 and was investigated as a function of dose. The results are presented in Figure 2. In the first series of experiments: airfree mol d m - ethanol it is obvious that the acetaldehyde yield at ph 2 (curve a) is much higher than that at ph (20a) /c20 = 2.8 x 0 dm mol" s " []. In solution saturated with CO, reaction () is also involved and the resulting HCO radicals can combine with C H and C H 2 O H species in competition to reactions (8 a) and (8 b): r>u Ti Tnv^vy C H, O H + HCO v^ i IV kj K^i) CH2(OH)CHO (glycolaldehyde). (22) Only traces of glycolaldehyde were detected, but its yield was not determined. Using 0 mol d m - ethanol in deoxygenated solutions, the yield of acetaldehyde is strongly increased, G = 5.4 at ph 2 and G = 2.8 at ph, respectively, being observed. This fact can be explained by the reaction of various transients (R), e.g. H, C H, C H 2 O H etc. with ethanol in addition to H, O H leading finally to resulting in C H C H O H species, (2). The last ones are disappearing by reactions (4a) and (4b), CH,CHOH + R CH,CHOH. (2) In aqueous 0 mol d m - ethanol saturated with CO the yield of acetaldehyde is strongly decreased in comparison to the airfree solutions (Fig. 2, Table ).

5 kgy Fig.. Formation of formaldehyde from aqueous ethanol as a function of dose. /. Series: mol dm"* ethanol airfree, (a) ph 2, (b) ph : saturated with CO, (c) ph 2 and (d) ph. II. Series: 0 mol dm ~ ethanol airfree, (A) ph 2, (B) ph ; saturated with CO, (C) ph 2 and (D) ph. Insert: calculated (/.-values. kgy Fig. 4. Formation of glyoxal as a function of dose using 0.0, and 0 mol dm aqueous ethanol saturated with CO at ph 2 and. Insert: calculated (,-values.

6 990 H.-R. Park and N. Getoff Radiolysis of Aqueous Ethanol in the Presence of C O This observation can be explained by the fact that CO is consuming part of the H-atoms resulting in HCO radicals, reaction (), which can give rise in the formation of formaldehyde, reaction (8 a) and glyoxal, reaction (8 b). The yields of formaldehyde using and 0 mol d m - ethanol in absence and presence of CO are presented in Fig. as function of dose. They are about one order of magnitude lower than those of acetaldehyde obtained under more similar experimental conditions. It is remarkable that in this the G(HCHO)-values at ph 2 are lower than those obtained at ph, which is in contradiction with the results observed in 0" 2 mol d m - ethanol (see Table in Figure ). The same ph-effect is also observed for the glyoxal yields resulting from 0" 2, and 0 mol d m - ethanol in the presence of 0" mol d m " CO at ph 2 and (Fig. 4), namely, in diluted aqueous ethanol at ph 2 the yield of glyoxal is essentially higher than that at ph. But, using or 0 mol d m " ethanol the ph influence is just opposite: the yields in neutral solutions are higher. Also, with increasing the ethanol concentration the glyoxal yield is strongly reduced. This effect is explainable by the following fact: With increasing ethanol concentration the formation of Hatoms is facilitated as a consequence of the direct radiolysis effect. However, the H-atoms are preferentially scavanged by the ethanol and reaction () (formation of HCO radicals) is suppressed. Hence, mainly the yield of C H C H O H is essentially increased, see reaction (2), resulting in acetaldehyde and 2,butandiol. The rate constants of the reactions of eä q, H-atoms and OH radicals with aqueous ethanol were redetermined (see Abstract). The formation of the major products resulting from steady state radiolysis of aqueous 0" 2 to 0 mol d m " ethanol in the absence and presence of CO at ph 2 and was studied. Using 0" 2 mol d m " ethanol, the yields of the final products are depending on the primary products of water radiolysis (OH, H, and e~q). Their yields are higher in acid solutions than those in neutral media. In concentrated ethanol solutions ( and 0 mol d m - ) in the absence and presence of C O the effect of the direct radiolysis of substrate is playing an essential role. In this case the formation mainly of the C H C H O H radicals is favoured, whereas the yield of HCO is suppressed. Hence, the distribution of the final products, studied as a function of dose, is different compared to that in diluted solution. [] L. Kevan, Actions Chimiques et Biologiques des Radiations (M. Haissinsky, ed.), ser., p. 5. Masson et Cie., Paris 969. [2] G. R. Freeman, Actions Chimiques et Biologiques des Radiations (M. Haissinsky, ed.), ser. 4, p.. Masson et Cie., Paris 969. [] J. W. T. Spinks and R. J. Woods, Introduction to Radiation Chemistry, rd ed., J. Wiley & Sons Inc., New York 990. [4] B. Hickel and K. H. Schmidt, J. Phys. Chem. 4, 240 (90). [5] B. Smaller, E. C. Avery, and J. R. Remko, J. Chem. Phys. 55, 244 (9). [6] B. S. Wolfenden and R. L. Willson, J. Chem. Soc. Perkin Trans. II, 805 (982). [] K. D. Asmus, H. Möckel, and A. Henglein, J. Phys. Chem., 28 (9). [8] J. T. Allan, E. M. Hayon, and J. Weiss, J. Chem. Soc. London 959, 9. [9] G. G. Jayson, G. Scholes, and J. Weiss, J. Chem. Soc. London 95, 58. [0] Y. Han, J. Rabani, and A. Henglein, J. Phys. Chem. 80, 558 (96). [] E. Bothe, D. Schulte-Frohlinde, and C. von Sonntag, J. Chem. Soc., Perkin Trans. II, 46 (98). [2] E. Bothe, M. N. Schuchmann, D. Schulte-Frohlinde, and C. von Sonntag, Photochem. Photobiol. 28, 69 (98). [] E. Bothe, M. N. Schuchmann, D. Schulte-Frohlinde, and C. von Sonntag, Z. Naturforsch. 8b, 22 (98). [4] N. Getoff and F. Schwörer, Radiat. Res. 4, (90). [5] N. Getoff, A. Ritter, and F. Schwörer, Radiat. Phys. Chem. 28, 4 (986). [6] T. Nash, Biochem. J. 55, 48 (958). [] T. Banks, C. Vaughn, and L. M. Marshalle, Analyt. Chem. 2, 48 (955). [8] M. Pesez and J. Bartos, Bull. Soc. Chim. France, 48 (960). [9] L. M. Dorfman and I. A. Taub, J. Amer. Chem. Soc. 85, 20 (96). [20] M. Anbar and P. Neta, Int. J. Appl. Radiat. Isotop. 8, 49 (96). Conclusion Acknowledgement One of us (H. R. Park) thanks the Federal Ministry for Science and Research in Austria for the financial support of a postdoctoral fellowship.

7 99 H.-R. Park and N. Getoff Radiolysis of Aqueous Ethanol in the Presence of CO [2] R. W. Fessenden and N. C. Verma, Faraday Disc. Chem. Soc. 6, 04 (9). [22] J. H. Baxendale, J. P. Keene, and D. A. Stoff, Pulse Radiolysis (M. Ebert, J. P. Keene, A. J. Swallow, and J. H. Baxendale, eds.), Academic Press, London 965, p. 0. [2] Farhataziz and A. B. Ross, Selected Specific Rates of Reactions of Transients from Water in Aqueous Solution III. Hydroxyl Radical and Perhydroxyl Radical and Their Radical Ions. Nat. Bur. of Standards, Washington 9. [24] A. J. Elliot and A. S. Simsons, Radiat. Phys. Chem. 24, 229 (984). [25] N. Getolf and D. Seitner, Z. Phys. Chem. N.F. 5, 2 (966). [26] N. Getoff and H. P. Lehmann, Int. J. Radiat. Phys. Chem. 2 (2), 9 (90). [2] Y. Raef, J. K. Thomas, and S. Gordon, Radiation Res. Suppl. 4, 4 (964). [28] G. V. Buxton and R. M. Sellers, J. C. S. Faraday Trans. I, 69, 555 (9). [29] N. Getoff, Intern. J. Energy Environm. economics, in print. [0] D. Seitner and N. Getoff, Z. Phys. Chem. 66, 22 (969). [] H. R. Park and N. Getoff, Z. Naturforsch. 4a, 40 (988). [2] J. K. Thomas, Faraday Soc. 6, 02 (965). [] N. Getoff, A. Ritter, and F. Schwörer, J. C. S. Faraday Trans. I, 9, 289 (98).

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