8403
J. Phys. Chem. 1992,96, 8403-8409 (21) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. P. J . Am. Chem. Soc. 1985,107,3902. (22) (a) N e h , S.F.; Blackstock, S. C.; Kim, Y. J . Am. Chem.Soc. 1987, 109,677. (b) Clark, T.; Nelsen, S. F. J . Am. Chem. Soc. 1988, 110, 868. (23)Varying AGO by h0.15eV has almost no effect on the best fit value of Iy. However, A, changes in a manner that keeps the sum of A, + AGO constant. Only the results obtained using the value of AGO in Table I11 are reported. (24) The optimal electron-transfer rate Constant is proportional to Iy2and refers to the situation where FCWDS is 1.0. (25) (a) Oevering, H.; Paddon-Row, M. N.; Heppener, M.; Oliver, A. M.; Cotsaris, E.; Verhoeven, J. W.; Hush, N. S. J . Am. Chem. Soc. 1987, 109, 3258. (b) Oevering, H.;Verhoeven, J. W.; Paddon-Row, M. N.; Warman, J. M. Tetrahedron 1989.45.4751. (c) Paddon-Row, M. N.; Oliver, A. M.; Warman, J. M.; Smit, K.J.; de Haas, M. P.; Ocvering, H.; Verhoeven, J. W. J. Phys. Chem. 1988,92,6958.(d) Lawson, J. M.; Craig, D. C.; Paddon-Row, M. N.; Kroon, J.; Verhoeven, J. W. Chem. Phys. Lett. 1989,164,120. (e) Warman, J. M.; Smit, K. J.; de Haas, M.P.; Jonker, S. A.; Paddon-Row, M. N.; Oliver, A. M.; Kroon, J.; Oevering, H.; Verhoeven, J. W. J. Phys. Chem. 1991,95,1979. ( f ) Oliver, A. M.; Craig, D. C.; Paddon-Row, M. N.; Kroon, J.; Verhoeven, J. W. Chem. Phys. k r t . 1988, 150, 366. (26)(a) Siders, P.; Cave, R. J.; Marcus, R. A. J. Chem. Phys. 1984, 91, 5613. (b) Cave, R. J.; Siders, P.;Marcus, R. A. J. Chem. Phys. 1986, 90, 1436. (c) Heiler, D.; McLendon, G.; Roplskyj, P. J. Am. Chem. Soc. 1987, 109,604. (d) Helms, A,; Hciler, D.; McLendon, G. J. Am. Chem. Soc. 1991, 113,4325. (e) Sakata, Y.; Nakashima, S.; Goto, Y.;Tatemitsu, H.; Misumi, S.; Asahi, T.; Hagihara, M.; Nishikawa, S.; Okada, T.; Mataga, N. J . Am.
Chem. Soc. 1989,111,8979.(f) Sakata, Y.; Tsue, H.; Goto, Y.;Misumi, S.; Asahi, T.; Nishikawa, S.; Okada, T.; Mataga, N. Chem. Loa. 1991, 1307. (27)Cotton, F. A. Chemical Applications of Group Theory, 2nd ed.; Wiley-Interscience: New York, 1971;p 280. (28)The importance of vibronic Coupling in symmetric electron-transfer DSA molecules has been discussed by Hush. (a) Hush, N. S . In Supramolecular Photochemisrry; Balzani, V., Reidel, D., Eds.; NATO AS1 Series C; Reidel: Dordrecht, The Netherlands, 1987;pp 53-72. (b) Reimers, J. R.; Hush, N. S.; Sammeth, D. M.; Calli, P. R. Chem. Phys. Letf. 1990,169,622. (29) (a) Yamagishi, A,; Iida, Y. Bull. Chem. Soc. Jpn. 1980, 53, 1340. (b) Yamagishi, A.; Watanabe, F.; Masui, T. J . Chem. Soc., Chem. Commun. 1977,274. (30) See: refs 1 b and 2d. (a) Miller, J. R.; Beitz, J. V . J. Chem. Phys. 1981,74,6746. (b) Beitz, J. V.; Miller, J. R. J. Chem. Phys. 1979,71,4579. (c) Calcaterra, L.T.; Closs, G. L.; Miller, J. R. J. Am. Chem. Soc. 1983,105, 670. (d) Guarr, T.; McGuire, M.; Strauch, S.; McLendon, G. J. Am. Chem. Soc. 1983,105,616.(e) Guarr, T.; McGuire, M. E. J . Am. Chem. Soc. 1985, 107,5104. (31) (a) Beratan, D. N.; Onuchic, J. N. Phorosynth. Res. 1989, 22, 173. (b) Onuchic, J. N.; Beratan, D. N. J. Chem. Phys. 1990, 92,722. (c) Beratan, D.N.; Onuchic, J. N.; Betts, J. N.; Bowler, B. E.; Gray, H. B. J . Am. Chem. Soc. 1990,112,7915.(d) Siddarth, P.; Marcus, R. A. J. Phys. Chem. 1990, 94,8430. (32)Gould, I. R.; Young, R. H.; Farid, S. In Photochemical Processes in Organized Molecular Systems; Tazuke, S., Honda, K., Eds.;Elsevier: Amsterdam, 1990 pp 19-40.
OH Radical Induced Oxidation of 2,3-Dimethyibutane in Alr Gerald Heimann and Peter Warneck* Max-Planck-institut fiir Chemie (Otto-Hahn-institut). Maim, Germany (Received: April 21, 1992; In Final Form July I , 1992)
The product distribution resulting from the OH induced oxidation of 2,3-dimethylbutane in air was measured and compared with predictions based on a general reaction mechanism. Relative rates derived for the abstraction of primary and tertiary hydrogen atoms by OH radicals from the parent compound are 17% and 83% respectively. The branching ratio for the alcohol versus alkoxyl radical producing pathways of the self-reaction of 2-propylperoxy radicals was determined to be (0.61 i 0.08):(0.39 f 0.08); the corresponding ratio for the self-reaction of primary 2,3-dimethylbutylperoxy radicals is (0.56 i 0.07):(0.44i 0.07). Large amounts of 2,3-dimethyl-2-hydroperoxybutane and small amounts of 2,3-dimethyl-Z-butanoI were found, the latter as a product of the cross combination reactions of 2,3-dimethyl-2-butylperoxywith 2-propylperoxy and 2 X cm3/(molecules), respectively, were and 2,3-dimethyl-l-butylperoxyradicals. Rate constants of 3.5 X estimated for these reactions with the help of computer simulations.
RO
Iatroductim
A sound knowledge of chemical reaction mechanisms governing the low temperature oxidation of hydrocarbons in air is fundamental to any detailed considerationsof the fate of a hydrocarbon in the atmosphere. Previous laboratory studies of such gas-phase oxidation p r o c a m have primarily dealt with CI-C4 compounds, and from this work a general reaction mechanism has been worked out that should be applicable also to higher The oxidation of alkanes in the atmosphere is thought to start with hydrogen atom abstraction by reaction with OH radicals.’~~ The associated rate coefficients are known in many cases. The subsequent reactions involve alkylperoxy (ROJ and alkoxyl (RO) radicals as follows:
+ RH R +0 2 ROz + HOz ROz + NO OH
+
+
H20 R ROz
(1)
-+
(11)
+
R 0 2 + ROz
-
ROOH RO NO2 aldehydes/ketones
(111) (IV)
+
alcohols
+
-
+
2 R 0 02 ROOR + 02
-*
+ O2
(Va) (Vb) (VC)
+ Oz
-
HOZ+ aldehyde/ketone
RO
RO
(VI)
aldehyde + R’
-
HOR”
(VIII)
where R is the parent alkyl radical, R’ is an alkyl radical of lower carbon number, and HOR” is a hydroxyalkyl radical resulting from internal hydrogen atom abstraction by the alkoxyl group. In the polluted atmosphere, where the concentration of nitrogen oxides is fairly high, the reaction of peroxy radicals with NO (reaction IV) will dominate over all other reactions of ROZradicals. This contrasts with the background atmosphere in marine or sparsely inhabited continental regions, where the concentration of nitrogen oxides is so low that the interaction of ROz with HO2 and other RO, radicals (reactions 111and V) cannot be ignored.3.5,6 For methylproxy radicals it has been found7s8that the contribution of reaction Vc is small compared with reactions Va and Vb combined, and this condition is assumed to hold also in other cases. The partitioningbetween reactions Va and Vb as well as the degree of decomposition or rearrangement of the RO radical (reactions VI1 and VIII) must be determined separately for each hydrocarbon. Higher hydrocarbons allow the formation of several types of alkylperoxy radicals, which may be classified as primary,
0022-3654/92/2096-8403$03.00/00 1992 American Chemical Society
Heimann and Warneck
8404 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992
X
I OH102
2,3-dimethyl-2hydroperoxybutane GC column B teflon tube
>fO*
-/
)-(-OH 2,3-dimethyl-2-butanol
23
opni;ot;s;i\
- HO;
H0
quartz tube Penray mercury lamp
reoction bulb
Figure 2. Experimental arrangement for sample transfer from the reaction vessel to the gas chromatographic inlet systems.
3-methyl-2butanone
R% ,
2-propanol acetone
w
100%
- HO; Figure 1. General reaction mechanism for the oxidation of 2,3-dimethylbutane in air. secondary, or tertiary depending on the site of hydrogen abstraction. Very little information exists currently on the interaction of different peroxy radicals ( R 0 2 R'02) compared with their self-reactions (R02 R02).6 Both types of reaction are important, however, in laboratory studies of higher hydrocarbons as well as in the atmosphere. We have studied the distribution of products resulting from the oxidation of 2,3-dimethylbutane, where this problem occurs, and present our results below. The photodissociation of hydrogen peroxide served as a source of OH radicals. Figure 1 shows a general reaction scheme based on the preceding considerations. The goal was to quantify the expected products as far as possible in order to derive from the observed product distribution the relative importance of individual reaction pathways. Although absolute rate coefficients cannot be obtained by this procedure, it turned out that the data do provide some information on the magnitude of rate coefficients associated with reactions between different peroxy radicals.
+
+
Experimental Section
OH radicals were formed by the photodecomposition of hydrogen peroxide. The rate of formation of OH radicals was determined to be 1.4 X lo4 s-'. The reaction was carried out in a round glass bulb of about 2 dm3capacity, which was provided with two side-arms and Teflon-stoppered shut-off valves. A penray mercury lamp was mounted in a 14 mm i.d. quartz finger reaching into the bulb to about 2/3 of its diameter (see Figure 2). Heraeus M 235 type quartz was used in order to block radiation at wavelengths below 220 nm (mainly the 185-nm Hg resonance line). This prevented the formation of ozone from the photodissociation of oxygen while the intense group of lines at 254 nm and emissions at longer wavelengths were fully transmitted. Ozone formed inside the quartz tube was flushed out with a flow of
compressed nitrogen, which also served as coolant. The penray lamp required a few minutes of warmup time to reach a stable output. The reaction was started subsequently by placing the lit lamp into the quartz finger. A gas handling manifold was used for filling the glass bulb with the desired gas mixture. The bulb was first evacuated with a rotary pump, hydrogen peroxide was then admitted to a pressure in equilibrium with an 85% aqueous solution of Hz02kept in an ice bath, and finally a 0.1% (approximate) mixture of the hydrocarbon in synthetic air was added until the total pressure reached a value slightly exceeding atmospheric. Thereafter the valves were closed and the bulb was detached from the filling station and connected to the inlet valves of two gas chromatographs (GC) as shown in Figure 2. Short Teflon tubes of 0.5 mm i.d. were used to make the connection. Each tube was inserted in the bulb through a hole pierced into a Teflon-coated silicone rubber septum. The GC sample loops were glass-lined stainless steel tubes of ca. 0.2 mm3 volume. Sample transfer was accomplished by evacuating a 2-cm3 volume behind the loop and opening the necessary number of valves (nos. 2 and 3 or 4 in Figure 2) to withdraw an equivalent gas volume from the bulb. The hydrocarbon and its reaction products were separated on a capillary column and detected with a flame ionization detector. The CPSil5 column (50 m long, 0.32 mm i.d.) was coated with dimethylpolysiloxane (film thickness 0.12 pm). The nitrogen carrier gas flow was 3 cm3/min. The temperature program started with 30 "C for 3 min followed by a rise rate of 8 OC/min up to 65 O C ; thereafter the rise rate was 30 OC/min until the temperature reached 200 OC. Acetaldehyde, acetone, and 2-propanol were separated on a 30 m long, 0.53 mm i.d. GSQ column with a porous layer of styrene/divinylbenzene. In this case the carrier gas flow was 23 cm3/min, and the temperature program started with 90 OC for 1.6 min followed by a rise rate of 30 OC/min up to a temperature of 200 OC. Finally, a 1.8 m long, 3.2 mm i.d. stainless steel tube packed with graphitized molecular sieve SA was used to separate CO and COP The column temperature was kept constant at 110 "C, and the nitrogen carrier gas flow was 25 cm3/min. Behind the column a flow of 7 cm3/min of hydrogen was added and the gases were brought into contact with a nickel catalyst heated to 375 O C , which converted CO and COzpartly to methane so that these gases could be detected by flame ionization. Commercial samples of alcohols, aldehydes, and ketones were used as far as possible to identify products and calibrate the associated signals. In a few cases where authentic samples were unavailable and product peaks had to be identified by other procedures, similar compounds with an identical carbon number
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8405
OH Radical Induced Oxidation were used to estimate calibration factors.
RHultS The vapor pressure in equilibrium with the concentrated HzOz solution used here was 40 Pa at 0 OC. The composition of the vapor pressure in this binary system (H202/H20)can be estimated from previous measurements for such solutions9at temperatures between 30 and 60 O C . From these data one obtains the ratio of the partial pressures of water vapor and H202at various temperatures. By extrapolation of the ratio to 0 O C we find a volume fraction for Hz02of 3 1 f 1%. This is equivalent to a mixing ratio in the gas phase of 120-130 ppm under our experimental conditions. Rate of OH Radical Formation. Carbon monoxide and 2propanol were used as scavengers of OH radicals to determine their rate of formation due to photolysis of H202. In the first case the gas mixture employed was 2% of CO in air. The rate coefficients for the reactions of OH radicals with CO and H202are and 1.7 X cm3/(molecules), respectively.1oWith 2.3 X these data we calculate that 95% of the OH radicals react with CO to produce C02. The evolution of COz was linear with time for periods up to 1 h. The average rate of C 0 2 formation as determined from five runs (35 data points) performed on different days was (9.5 f 0.2) X 10" m01ecules/(cm3s). The corresponding rise in the C 0 2 mixing ratio during 1 h was about 135 ppm, which is equivalent to the decomposition of roughly half of the amount of H202initially present. However, the observation of a linear C 0 2 increase of that magnitude requires that the H202concentration remained nearly constant. Accordingly, H 2 0 2lost by photodecomposition must have been replaced by other chemical processes. This condition is inherent in the well-known mechanism of CO oxidation H202 + hv 20H OH + CO (+OJ C02 + HO2 HOZ + HOz H202 + 0 2 which shows that HZOzis fully recovered by the self-reaction of H 0 2 radicals. After making allowance for the incomplete scavenging action of CO, the average OH production rate is (1 .O f 0.2) x 10l2 molecules/(cm3 s). The gas-phase reaction of OH radicals with 2-propanol was carried out with a mixture of 0.1% of 2-propanol in air. The rate coefficient for the reaction is 5.63 X cm3/(molecule s).Io By comparison with the rate coefficient for the reaction of OH with H202we calculate that 96% of the OH radicals were scavenged by 2-propanol under these conditions. Only acetone and acetaldehyde were identified as products. The observed acetaldehyde/acetone product ratio of 0.084 f 0.018 is in good agreement with the value 0.06 calculated with the estimation method by Atkinson," was independent of reaction time, and is similar to that found for the same reaction in aqueous solution (0.053)." Formaldehyde, which in aqueous solution occurs in amounts nearly equal to those of acetaldehyde, could not be determined with the gas chromatographic technique used here. However, it should be present in equal amounts to preserve the carbon balance. Accordingly, the sum of the concentrations of acetaldehyde and acetone should be equivalent to the total OH radical yield. The rise in product concentration was linear with time for about 30 min. Thereafter the extent of the conversion became too large to maintain the assumption of a constant concentration of 2-propanol, and the scavenging rate declined. Again, under these circumstances, the total product concentration rose to about half the initial HzOz concentration, but as in the case of CO as scavenger, one HO2 radical is formed in the oxidation of 2-propanol for each OH that has reacted, so that H202 is largely reconstituted. The average rate for the formation of acetone and acetaldehyde combined was (9.96 f 0.79) X 10" molecules/(cm3 s) as determined from four runs (40data pints).When allowance was made for the incomplete scavenging action of 2-propanol, the average OH production rate was (1.06 f 0.08) X 1OlZmolecules/(cm3 s), in good agreement with the value given above for CO as OH radical scavenger. 4
-
+
1
10
I4
6
.b
,
I
I
I
0
2
4
6
8
10
12
14
retention time [minutes]
Figure 3. Gas chromatograms before and after irradiation of 2,3-dimethylbutane and H202 in air: (1) acetone, (2) 2-propano1, (3) impurity, (4) 2,3-dimethylbutane, ( 5 ) 2,3-dimethyl-2-butanol, (6) 2,3-dimethylbutanal, (7) 2,3-dimethyl-l-butanol, ( 8 ) impurity, ( 9 ) unknown, (10) 2,3-dimethyl-2-hydroperoxybutane.
It should be noted that a fairly large scatter occurred in the OH production rates from run to run, indicating that the H 2 0 2 vapor pressure did not always reach the optimal value. This observation is reflected in the standard deviation for the average rates given above. Reaction of OH with 2,3-Dimethylbutane. The rate constant for the reaction of OH with 2,3-dimethylbutane is close to 6 X cm3/(molecule s)l2sZ2 and fairly independent of temperature. Figure 3 shows a typical chromatogram obtained when 2,3-dimethylbutane (0.1% in air) was reacted with OH radicals for about 15 min. Most of the products were identified by their retention times in comparison to those of commercial samples. These included acetone, 2-propanol, the primary C6 alcohol, and the corresponding aldehyde. The tertiary C6alcohol was not available commercially. Its retention time was estimated by interpolation from a plot of retention times against boiling point temperatures of several branched and straight chain alcohols. The plot gave a straight line resulting in an unambiguous determination of the retention time for 2,3dimethyl-2-butanol. More problematic was the identification of peak no. 10 (see Figure 3), whose strength showed it to be a major product. We suspected 2,3-dimethyl-2hydroperoxybutane, but had no authentic sample for confirmation. However, since alkylhydroperoxides arise from the interaction of alkylperoxy radicals with HOz, we studied the response of the signal to changes in the steady-state concentration of HOZ. For this purpose, sufficient CO (3.1%) was added to the reaction mixture to convert about 75% of the OH radicals toward HOz by reaction with CO, while the remaining 25% were left to react with 2,3-dimethylbutane (0.04%). This procedure raised the concentration of H02 and lowered that of alkylperoxy radicals. Figure 4 shows the results of the experiment. The addition of CO caused a considerable reduction in the production of acetone, whereas the intensity of the peak assigned to the hydroperoxide was doubled. This response was taken as confirmation of the assignment. Several small peaks apparent in Figure 3 remained unidentified. From their relative intensities we estimate that the association products contribute less than 5% to the total product concentration. The largest of the unidentified peaks is no. 9, which may be due to 2-propylhydroperoxide or an isomerization product resulting from the 2,3-dimethyl-l-butoxyl radical. Product distributions observed in five experiments are shown in Table I. Averaged product distributions were calculated from the individual data and are included in the table. Ratios of
8406 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992
Heimann and Warneck
TABLE I: Percentage Yields of Products Resulthg from the Owidation of 2.3-Dlwthylbet.w (0.1% in Air, 1@Pa, 297 K) run time acetone 2-PrOH 2,3-(Me2)butanal 2,3-(Me),-l-BuOH 2,3-(M~)~-2-Bu00H2,3-(Me),-2-BuOH unidentified tert-H no. (min) (W) (9%) 6) (96) (4%) (a) (%) abstr (%) 1 I 46.32 1.02 8.42 4.21 30.88 1.4 1.75 12.8 . . 1.21 1.96 3.1 1 15 48.79 30.79 0.69 1.38 84.6 8.96 8.36 3.58 2 1 46.56 26.21 1.79 4.48 83.5 15 41.31 8.48 8.36 3.58 26.21 1.79 4.48 83.5 1.58 3 11 50.01 9.10 3.03 27.21 1.51 1.52 83.0 8.20 3.52 15 48.01 1.03 29.31 2.34 1.52 83.8 8.46 3.01 6.92 25 50.01 27.69 2.31 1.54 83.9 7.84 3.92 5.88 4 I 41.18 35.38 3.92 1.96 84.6 9.90 2.97 8.91 15 48.51 24.15 2.91 1.98 81.9 8.77 3.51 1.02 5 I 40.35 35.08 3.51 1.I5 83.9 3.48 6.96 9.56 14 46.09 30.43 1.74 1.74 82.2 9.90 3.47 1.43 27.73 1.98 25 48.01 81.5 1.49
av
46.11
1.45
8.71
3.42
TABLE Ik Product Ratios for the Oxidation of 2,1Dimethylbutane, Comparison of O k w e d a d Calculated Values observed calculated [acetone]/ [2-PrOH] 6.34 f 0.67 6.63 [2,3-(Me)2-2-BuOH]/ [2-PrOH] 0.30 0.15 0.29 2.60 i 0.39 2.62 [2,3-(Me)2-butanal]/[2,3-(Me)z-l-BuOH] 0.63 f 0.12 0.38 [2,3-(Me),-2-BuOOH]/ [acetone]
.-t0:
2.21
2.05
83.2
of secondary and tertiary peroxy radicals, respectively, will be governed by the equation d[R202I/dt = 2k2[R302I2 - (k3 + k4)[R3021[R2021 + k3[R3021[R2021 - 2(ks + k6)[R202I2 0 (1) We subdivide by k6[R~02]~ and set Y = [R3021/[R202]to obtain (2k2/k6)p = (k4/k6)Y+ 2(kS + k6)/k6 The product ratios can then be written in the form [acetone]/[2-PrOH] = (2k2/k6)r + (2k3 + k4)Y/k6 + (2k5 + k6)/k6 =
hyhperoxide
without CO with CO, x 10
29.30
-
14
(2)
x (3)
[2,3-Me2-2-BuOH]/[2-PrOHl = (k4/k6)Y = 2 (4) With the further assumption that k3 k4 the above equations can be combined to yield x = 42 4k5/k6 3 (5)
lo
+
k5/k6 =
’
L--& --- 2 0
2
4
6 8 10 12 14 irradiation time [minutes]
2-propond OC.(W
2-popanol
16
18
20
Figure 4. Evolution with time of acetone, propanol, and 2,3-dimethyl2-hydroperoxybutane in the presence and absence of CO.
products that will be used in evaluating the data are summarized in Table 11. According to the mechanism shown in Figure 1, acetone, 2-propanol, 2,3-dimethyl-2-hydroperoxybutane,and 2,3-dimethyl-2-butanol result from the attack of OH radicals on the tertiary hydrogen atoms of the hydrocarbon, whereas the primary (26 alcohol and the corresponding aldehyde are formed by abstraction of a primary hydrogen atom. The relative prob ability of H-atom abstraction thus should be reflected in the product distribution. In calculating this probability, one must take into account that each tertiary 2,3dimethylbutoxyl radical breaks up to form twice as many molecules of acetone and/or 2-pmpan01, so that the sum of both must be halved. The last column of Table I gives the percentage of tertiary hydrogen abstraction thus calculated for each individual product distribution. In the calculation, the unidentified product (peak no. 9 in Figure 3) was assumed to arise from tertiary H-atom abstraction. This leads to an average value of 83 1%. Accordingly, the reaction forms predominantly tertiary 2,3-dimethylbutylproxyand subsequently secondary 2-propylperoxy radicals. As will be shown in more detail further below, the primary 2,3-dimethylbutylperoxyradicals react mainly with each other so that their interaction with the secondary and tertiary peroxy radicals may be neglected to a first approximation. As a consequence, the two branches of the oxidation mechanism shown in Figure 1 are nearly independent. This allows us to evaluate the branching ratio k5/k6 for the self-reaction of 2-propylperoxy radicals from the observed product ratios acetone/2qropanol and 2,3-dimethyl-2-butanol/2-propanol as follows. Assuming steady-state conditions, the concentrations [R202]and [R302]
*
&(x-4 2 - 3)
+
0.54 f 0.19
The numerical value shown was derived by averaging over the individual data of X - 4 2 contained in Table I. The fraction of the self-reaction of 2-propylperoxy radicals leading to the formation of 2-propanol is k6/(k5 + k6) = 0.65 f 0.08. This value must be considered an upper limit, because a part of 2,3-dimethyl-2-butanol may be formed by the interaction of primary with tertiary proxy radicals. If the formation of 2,3-dimethyl2-butanol were due entirely to this latter reaction, k4 0 and the term with Z would varnish in the above equations. Under these conditions k6/(k5+ k6) = 0.55 f 0.08 and this value represents a lower limit. The true value will lie between both limits. In a similar manner we may take the product ratio [2,3-Me2-butanal]/ [2,3-Me2-1-butanol] = W (6) to derive the branching ratio for the self-reaction of primary 2,3-dimethylbutylperoxy radicals kg/klo = ( W -1)/2 = 0.80 f 0.20; kio/(k, klo) = 0.56 f 0.07
-
+
Here again, the true value for klO/(k,+ klo) will be slightly higher, because of our neglect of the interaction of primary with m n d a r y and tertiary peroxy radicals. Moddl c.leul.tions. The Harwell FACSIMILE computer program13 was used to calculate the evolution of products in accordance with the mechanism shown in Figure 1. The iudividual reactions and the aspociated rate coefficientsare compiled in Table 111. Rate coefficients for the self-reactions of several primary, secondary, and tertiary peroxy radicals are approximately known from previous work, and their values served as anchor points in the selection of rate coefficients for the cross combination reactions between different types of peroxy radicals. Recommended values for the self-reactions of methylperoxy, ethylperoxy, and 1propylperoxy radicals at 298 K are 3.6 X 0.86 X lo-”, and cm3/(molecule s), respectively.1° We have adopted for 3x the self-reaction of 2,3-dimethyl-l-butylperoxyradicals a rate coefficient of 1 X cm3/(molecule s). For the self-reaction
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8407
OH Radical Induced Oxidation
TABLE III: Rerctio~ma d Rite Coefficients Used for Computer Simulations reaction H,O,
+
OH
ref
kc
+ OH
see text 1.1 x 10-10 1.7 X 10-I2 1.6 X 5.15 X 1.05 X 10-l2 5.0 X 5.0 X 5.0 X 10-l2 2.1 x 10-17 1.3 x 105 s-1 1.5 X lo-" 1.4 x 10-17 2.1 x 1 0 4 7 4.0 X 10-l6 6.0 X 10-l6 7.6 x 10-15 8.0 x 10-17 1.2 x 10-16 7.6 X IO-" 4.0 x 1 0 4 4 6.0 x 10-14 4.0 x 1 0 4 5 3.0 x 1 0 4 5 3.0 x 10-15
b 10
10 10 10 10
a a a
14 2 10
b b 10, b 10, b 2
b b 2
10, 20, b 10, 20, b a a a
OAssumed. bThis study, best fit. CUnitscm3/(molecule s) unless otherwise indicated.
of 2-propylperoxy radicals the recommended rate coefficient is 1 X cm3/(molecule s).I0 The self-reaction of 2,3-dimethyl-2-butylperoxyradicals must be even slower, since the value for tertiary butylperoxy radicalsI4 is 2.5 X lo-',, and we have 9 assumed a similar value to be applicable here. 2.3-dimethylbutanal 3E, 8 The measured product ratio Z = [2,3-M%-2-BuOH]/[2-PrOH] 7provides a guide toward the selection of rate coefficients for the hydroperoxide 6 - 2.3-dimethyl- 1-butanol cross combination reactions between primary and m n d a r y peroxy radicals on the one hand and tertiary peroxy radicals on the other. If we assume that the tertiary alcohol is produced exclusively by reaction 4, that is the interaction between secondary and tertiary peroxy radicals, we can use eq 2 and the known values of k2,k5, and k6 to estimate Y = [R302]/[R202]= 7,which together with eq 4 suggests k4 Ik $ / Y = 2.6 X lo-', or ( k , k4) 5 6 X lo-" 0 10 20 r " / ( m ~ l e ~ ~s)l.e If, as an alternative, we assume that the tertiary irradiation time [minutes] alcohol arises exclusively from reaction 8, that is the interaction Figure 5. Comparison of calculated and observed product evolution with between primary and tertiary peroxy radicals, then time. z = (ks/k6)Y[RiO21/ [RzOz] (7) From steady-state considerations it can be shown that with the period of about 4 min, which is caused by the initially slow build-up neglect of cross combination reactions of tertiary 2,3-dimethylbutylperoxy radicals until their concentration reaches a steady state. During this initial period the relative ( [ R I ~ ~ I / [ R ~ O l(1 ~ I -) ~a ) / 4 ( k 5 + ks)/(k9 + h o ) (8) rate of formation of tertiary hydroperoxide is enhanced compared where a is the fraction of tertiary hydrogen atom abstraction from with that of acetone, and this is reflected to some extent in the 2,3-dimethylbutane. This leads to [R1O2]/[RZO2]= 0.04 and experimental data. Table I1 includes product ratios calculated k8 5 6.5 X or k7 + k8 I1.5 X cm3/(molecule s). with the reaction scheme and the rate coefficients shown in Table Armed with this information the values for k3 k4 and k, k8 I11 to demonstrate that they compare well with the experimental were adjusted until the ratios X = [acetone]/[2-PrOH], Z = values. The formation of 2,3-dimethyl-2-butanol occurs primarily [2,3Mez-2-BuOH]/ [2-PrOH], and W = [2,3-Me2butanal]/ by the interaction of 2,3dimethylbutylperoxy with 2-propylperoxy [2,3-Me2-1-BuOH]agreed with the observed values. This proradicals. The contribution from the interaction with primary cedure led to values for k3 + k4 and k7 k8 of 3.5 X and 2,3aimethylbutylperoxy radicals is about 20% and would decrease 2X cm3/(molecule s), respectively. The rate coefficient for further, if k7 + k8 I 2 X cm3/(molecules). Figure 6 shows the cross combination reaction between 2,3-dimethyl-l-peroxy and the dependence of the product ratio X = [acetone]/ [2-PrOH] on 2-propylperoxy radicals could not be determined because this the magnitude of the rate coefficient k3 + k4. This figure deminteraction does not lead to a unique product that can be distinonstrates most clearly that the rate coefficient for the cross guished from the others. We have adopted for the rate coefficient combination reaction between secondary and tertiary peroxy cm3/(molecule associated with this reaction a value of 1 X cm3/(molecules). radicals must be smaller than about 5 X s), intermediate in magnitude between the rate constants for the self-reactions of the peroxy radicals involved. The true value may Discussion be smaller, although probably not less than about 2 X cm3/(molecules). Trial runs using the latter value did not lead Greine@ was the first to present an empirical formula for the cm3/(molecule to results much different from those with 1 X rate of hydrogen abstraction from hydrocarbons reacting with OH. His method of calculation makes allowance for the difference in SI. Figure 5 compares the calculated and the observed evolution averaged rate constants for primary, secondary, and tertiary of products with time. The calculations indicate an induction hydrogen atoms and leads to a ratio for the abstraction of tertiary
-
+
+
+
+
8408 The Journal of Physical Chemistry, Vol. 96, No. 21, 199‘2 17 i l6
1
experimental value: 6,34 f 0,7
/I
15 14 -f
t
n
I
Heimann and Warneck TABLE I V SIlarmary of Rate Collstanta for Cross Reactlolls betwoen Peroxy Radialsu radicals k m ref (CH~~CO + Z~ u ) - C S H I I O Z 3.0 x 1 0 4 4 200 CH$(O)Oz+ CH3O2 (CHo13C02 + CH302 (CH3)zCH02 (CH3)zCHC(CH3)2(02) (CH3)2CHCH(CHI)CH202 + (CH~)~CHC(CH~)Z(O~)
+
11
10
1.3 X lo-” 3.1 X lo-” 3.5 X lo-’’ 2.0 X
24 25 this study this study
“Value at 373 K.
- 16.4
- 16.8
log k + ,,
- 16
- 15.6
[cm s/molec s]
Figure 6. Calculated and observed range of values for the acetone/2propanol product ratio as a function of k3 k,.
+
to primary hydrogen atoms from 2,3-dimethylbutane of 88: 12. Atkinson et al.” have suggested a slightly different method, based on group additivity rules, for the estimation of relative abstraction as rate rates. Their set of data includes kprim= 1.44 X constant for primary H-atom abstraction from a methyl group, k,,,, = 1.83 X for abstraction of a tertiary H atom and a group factor of 1.29 to be applied to the rate of tertiary H abstraction. With these data we calculate a ratio of 86.4:13.6. The experimental determination gave 83:17. This ratio is close to but not quite in agreement with prediction. The experimental ratio would be even less favorable, namely 8 1:19, had we assumed the unidentified product (peak number 9 in Figure 4) to arise from primary rather than tertiary H abstraction. However, a ratio of 83:17 is still compatible with the known total rate coefficient for the reaction OH 2,3-dimethylbutane, provided one accepts for the abstraction of a primary hydrogen atom a rate constant of about 6.2 X cm3/(molecule The combination of cm3/(molecule s). The the two values leads to k,,, = 5.1 X reported total rate coefficients fall into the range (4.3-7.4) X 10-l2 cm3/(molecule s).12*22 The branching ratio for the self-reaction of 2-propylperoxy radicals derived here from the ratio of acetone and 2-propanol lies in the range 0.54 > k5/k6> 0.82, with a probable value of k5/k6= 0.64 or k6/(k5 k6) = 0.61 f 0.08. Kirsch et ai.’’ and Cowley et aI.l9 have used steady-state photolysis of 2,2-azopropane in the presence of oxygen and added nitrogen to study the selfreaction of 2-propylperoxy radicals. From the amounts of acetone and 2-propanol observed they derived a value of k5/k6= 1.39 at 302 K,which is equivalent to k6/(k5+ k6) = 0.42. Their result suggests that the pathway generating 2-propxyl radicals is favored over that giving stable products. We have carefully checked our calibration procedures for acetone and 2-propanol, whose ratio largely determines k6/(k5 k6),and have found no reasons for distrusting our data. A 60% conversion toward the alcohol and 40% toward alkoxyl radials would be similar to that usually observed for the self-reaction of primary alkylperoxy radicals.I0 The value determined for the branchmg ratio of the self-reaction of primary 2,3-dimethylbutylperoxy radicals is k9/klo= 0.8 f 0.2, leading to k l 0 / ( k g klo) = 0.56 f 0.07. This probability for the alcohol producing channel of the reaction accordingly is 5695, that for the alkoxyl radical producing channel 44%. In the calculations we have assumed a ratio of 60:40 in order to compensate the influence of the cross combination reaction of primary with tertiary peroxy radicals, which can produce only tertiary but not primary alcohol. Nevertheless, a 60:40 branching ratio would agree well with that observed for most other primary peroxy radicals whose self-reactions have so far been studied. These include methylperoxy with a ratio of 65:35,’ and neopentylperoxy with a ratio of 60:40,20but not ethylperoxy, for which a branching ratio of 37:63 has been reported.I0 There is no obvious reason why ethylperoxy should behave differently. The comparatively small values for the rate coefficients derived for the cross combination reactions of tertiary with primary and
+
s).12315916
+
+
+
secondary peroxy radicals, respectively, are surprising. In a previous study6 it was assumed that the rate coefficients for such reactions should have values lying midway on a logarithmic scale between those for the self-reactions of the two types of radicals involved. Thus, for the c m combination of primary with tertiary peroxy radicals the expected arithmetic mean value would have been about cm3/(molecules), whereas the expectation for the cross combination of secondary with tertiary peroxy radicals would have been cm’/(molecule s). The values obtained here are about 2 X and 3.5 X lo-’’ cm3/(molecule s), respectively. There are no other reports on these reactions in the literature. Osbome and Waddington?’ in a study of the oxidation of isobutane, have compared the observed product distribution with model calculations and obtained a best fit for a value of about 3.1 X cm3/(molecules) for the cross combination reaction between methyl and tertiary butylperoxy radicals. This value agrees better with expectation. Table IV lists the values for cross reaction rate coefficients described so far.23 Finally we must discuss the disparity between calculated and observed yields for tertiary alkylhydroperoxide apparent from Table 11. We consider it unlikely that the experimentalvalue is too high due to calibration errors, because this would lead to a subtantial reduction of the tertiary/primary hydrogen abstraction ratio, which is already somewhat lower than predicted. On the other hand it should be noted that if 2,3-dimethyl-2-hydroperoxybutane were formed exclusively by reaction of H 0 2 with tertiary peroxy radicals, the yield would be limited by the extent of H 0 2formation. From the individual branching ratios involved in the overall reaction mechanism, one can estimate a ratio of [2,3-Me2-2-BuOOH]/[acetone] = 0.3 1 once steady-state conditions are achieved. This value is increased to some extent by the preferential formation of tertiary hydroperoxide during the initial stage of the reaction when tertiary butylperoxy radicals have not yet reached stationary concentrations. Nevertheless it will be clear that the various results can be reconciled only if there exists another reaction causing the production of additional tertiary hydroperoxide. The most likely reaction would be an interaction of tertiary butylperoxy radicals with hydrogen peroxide. A rate coefficient of 6 X cm3/(molecule s) would be required to produce the extra 2,3-dimethyl-2-hydroperoxybutane.Inasmuch as this type of reaction is essentially thermoneutral, the magnitude of the rate coefficient required appears reasonable.
Acknowledgment. This study was performed as part of EUROTRAC subproject LACTOZ and has received financial s u p port from the German Ministry for Research and Technology. R-by NO. H2, 1333-74-0; CH3CH(CH3)CH(CH,)CH,, 79-29-8; OH, 3352-57-6; CH3CH(CH3)C(CH1)200H, 28888-35-9; CH,CH(CH3)C(CH3),0H, 594-60-5; 2,3-dimethylbutyldioxy, 102925-89-3; 1,1,2trimethylpropyldioxy, 243 10-68-7; 2-propyldioxy, 4399-86-4.
References and Notes (1) Atkinson, R.; Lloyd, A. C. J . Phys. Chem. ReJ Dura 1984, 13, 3 15-444.
(2) Atkinson, R. Atmos. Enuiron. 1990, 24A, 1-41. ( 3 ) Warneck, P. Chemistry of the Natural Atmosphere; Academic Press: San Diego, CA, 1988. (4) Carter, W. P. L.; Atkinson, R. J . Atmos. Chem. 1985, 3, 377-405. (5) Logan, J. A. J . Geophys. Res. 1983, 88, 10785-10807. (6) Madronich, S.; Calvert, J. G. J . Geophys. Res. 1990,95, 5697-5715. (7) Baulch, D. L.; Cox, R. A,; Hampson, R. F.; Kerr, J. A.; Troe, J.; Watson. R. T. J . Phvs. Chem. Ref Dura 1984, 13, 1259-1380. (8) Kurylo, M. J.{Wallington, ‘I? J. Chem. Phys. Lett 1987,138,543-547.
J. Phys. Chem. 1992,96, 8409-8413 (9) Kotowski, A. Gmelin’s Handbook of Inorganic Chemistry; Verlag Chemie: Weinheim, 1966; Vol. 03-Part 7, pp 2205-2206. (10) Atkinson, R.; Bauth, D. L.; Cox, R. A.; Hampson, R. F.; Kerr, J. A.; Troe, J. J . Phys. Chem. Re/. Dofa 1989, 18, 881-1097. (11) Deister, U.; Warneck, P.; Wurzinger, C. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 594-599. (12) Atkinson, R. Chem. Reo. 1986, 86, 69-201. (13) Curtis, A. R.; Sweetenham, W. P. FACSIMILEICHECKMA7 User’s Manual, Acre-R-12805; His Majesty’s Stationary Office: London, 1988. (14) Anastasi, C.; Smith, I. W. M.; Parkes, P. A. J . Chem. Soc., Faraday Trans. 11978, 74, 1693-1701. (15) Greiner, N. R. J . Chem. Phys. 1970, 53, 107C-1076. (16) Atkinson, R.; Carter, W. P. C.; Aschmann, S. M.; Winer, A. M.; Pitts, J. N. Inr. J. Chem. Kiner. 1984, 16, 469-481. (17) Atkinson, R. Int. J . Chem. Kinet. 1987, 19, 799-828.
8409
(18) Kirsch, L. J.; Parkes, D. A.; Waddington, D. J.; Woolley, J. J. Chem. Soc., Faraday Trans. 1 1918, 74, 2293-2300.
(19) Cowley, L. T.;Waddington, D. J.; Woolley, A. J . Chem. Soc., Faraday. Trans, 1 1982, 78, 2535-2546. (20) Lightfoot, P. D.; Roussel, P.; Veyret, B.; Lesclaux, R. J. Chem. SOL, Faraday Trans. 1 199Q,86, 2927-2936. (21) Osbome, D. A.; Waddington, D. J. J . Chem. Soc., Perkin Trans. 2 1984, 1861-1867. (22) Atkinson, R. J . Phys. Chem. Re/. Data 1989, Monograph No. 1 . (23) Lightfoot, P. D.; Cox, R. A.; Crowley, J. N.; Destrieu, M.; Hayman, G.D.; Jenkin, M. E.; Moortgat, G. K.; Zabel, F. Armm. Enoiron. 1992,26A, 1805-1 964. (24) Moortgat, G. K.; Veyret, B.; Lesclaux, R. J . Phys. Chem. 1989,93, 2362-2368. (25) Osbourne, D. A.; Waddington, D. J. J . Chem. Soc., Perkin Trans. 2 1984, 1861-1867.
Outer-Sphere Electron-Transfer Oxidation of 10,l O‘-Dimethyl-9,9’, I O , 1O’-tetrahydrcM,9’-biacridine Shunichi Fukuzumi* and Yoshihiro Tokuda Department of Applied Chemistry, Faculty of Engineering, Osaka University, Sulta, Osaka 565, Japan (Received: April 28, 1992; In Final Form: July 14, 1992)
10,1(Y-Dimethyl-9,9’,10,1(Y-tetrahydro-9,9’-biacridine [(AcrH)z] acts as a unique two-electron donor in the electron-transfer oxidation with various organic oxidants. The rate-determiningstep is electron transfer from (AcrH), to oxidants, followed by facile cleavage of the C(9)-C bond of (AcrH)*’+to yield the acridinyl radical (AcrH’) and IO-methylacridinium ion (AcrH+). The second electron transfer from AcrH’ to oxidants is much faster than the initial electron transfer from (AcrH), to oxidants. On the other hand, the corresponding monomer, 10-methyl-9,lO-dihydroacridine(AcrH2),acts as a normal hydride (two electrons and proton) donor in the reactions with oxidants. Rates of electron-transferreactions from (AcrH), to various inorganic and organic one-electron oxidants depend solely on the one-electron-reduction potentials of the oxidants irrespective of the size of the oxidants, indicating that (AcrH)z acts as a novel two-electron outer-sphere electron-transfer reagent. The one-electron-oxidationpotential of (AcrH), (vs SCE)has been evaluated as 0.62 V, which is less positive than that of the corresponding monomer (0.80 V).
Introduction The outer-sphere mechanism for electron-transfer reactions has been extremely useful and successful in predicting the electrontransfer rates of a number of inorganic redox reagents in terms of the Marcus simple and classical theory.’ This concept requires no sptcific knowledge of the transition-state structure, since rates of the outer-sphere electron-transfer reactions having nonbonded transition states can be described only in terms of the thermodynamic parameters of each independent reductant and oxidant. In contrast, transition-state structures play important roles in mechanistic thinking in organic redox chemistry, since bonding is so important in transition states of organic redox reactions. In addition, electron-transfer oxidation of organic reductants usually involve the cleavage of C-H bonds, since radical cations are much stronger acids than the neutral parent molecules. Thus, welldocumented cases of outer-sphere electron-transfer reactions involving organic redox reagents are rare as Eberson and Shaik have recently pointed We report herein that 10,10’-dimethyl-9,9/,lO,lO‘-tetrahydro9,9’-biacridine [ ( A C ~ H )acts ~ ] as a two-electron donor accompanied by the cleavage of the C(9)-C bond instead of the C(9)-H bond in the outer-sphere electron-transfer reactions with various inorganic and organic one-electron oxidants! The donor properties of the dimer are compared with those of the corresponding mo(AcrH2).’ The second nomer, 10-methyl-9,lO-dihydroacridine electron transfer from ACTH’,formed by the C-C bond cleavage upon the initial electron-transfer oxidation, to oxidants to yield 10-methylacridiniumion (AcrH+) may be much faster than the initial electron transfer, judging from the low one-electron oxivs SCE = -0.43 V).sq6 In such dation potential of AcrH’ (gx ) ~ be the a case, initial electron-transfer oxidation of ( A c ~ H may
rate-determining step for the over-all two-electron oxidation of ( A C ~ Hto) ~two AcrH’. Thus, ( A C ~ His) suitable ~ to investigate the electron-transfer reactions with a variety of oxidants which can cover wide range of steric and electronic effects. To the bcst of our knowledge, such a unique donor property of (ACTH)*may be the first example of an organic reductant acting as a pure two-electron donor in the outer-sphere electron-transfer oxidation. Experimental Section
Materials. lO,lO‘-DimethyL9,9’-biacridine [(AcrH),] was prepared by the one-electron reduction of 10-methylacridinium perchlorate by hexamethylditin? 10-Methylacridiniumiodide was prepared by the reaction of acridine with methyl iodide in acetone, and it was converted to the perchlorate salt (AcrH+C104-)by the addition of magnesium perchlorate to the iodide salt, and purified by recrystallization from methanol? Triphenylmethyl perchlorate (Ph3C+C104-)was prepared according to the literature? (Triphenylmethy1)methanolwas obtained commercially. Cobalt(I1) tetraphenylporphyrin (CoTPP) was prepared as given in the literature? The CoTPP was oxidized by dioxygen in the prew~ce of HCI in methanol to obtain (tetraphenylporphinato)abalt(III) chloride ( c o T p p c I ) , l o which was purified by recrystallization from methanol. The perchlorate salt of CoTPP’ was obtained by the metathesis of the chloride salt with AgC104 and recrystallized from toluene.” Quinones, ferrocene derivatives, 7,7,8,8-tetracyano-p-quinodimethane were obtained commercially. Tris(2,2’-bipyridine)ruthenium(II) dichloride hexahydrate, [Ru(bpy),]Cl2.6H20 was prepared and purified by the literature procedure.I2 Tris( 1,lo-phenanthroline)iron( 111) hexafluorophosphate, [Fe(phen),](PFs), WBS prepared by oxidizing a solution of the iron(1I) complex with ceric sulfate in an aqueous solution
0022-365419212096-8409$03.00/0 0 1992 American Chemical Society
