J. Phys. Chem. 1983, 87, 1209-1213
1209
Rate Constants for the Reactions of Diolefins with OH Radicals in the Gas Phase. Estimate of the Rate Constants from Those for Monooiefins Tomohlro Ohta Department of Environmental sciences. Tokyo Metropolitan Research Laboratory of Public Health, Hyakunincho, Shinjukuku, Tokyo, 160 Japan (Received: September 9, 1982)
Rate constants for the reactions of 19 gaseous diolefins with OH radicals were determined by relative rate determination methods. Hydroxyl radicals were generated by photolysis of H2Oz at 2537 8, in a mixture of two olefins in the millitorr pressure range at 24 f 2 "C and 1-atm pressure of 02/Nz. Analysis of the rate constants indicated that overall rate constants of diolefins were almost the sum of the contributions from each carbon-carbon double-bond group to the the overall reaction with OH radicals and that the contributions were predictable from the rate constants for the reactions of the corresponding monoolefins with OH, based on the procedure derived in this work.
Introduction It is widely accepted that OH radicals are important in atmospheric chemistry and combustion chemistry. From the viewpoint of ozone formation potentiality in polluted atmosphere, the more reactive a hydrocarbon toward OH, the more important its role in oxidizing NO to NO2,which is photolyzed to form ozone. Since dialkenes, except for some allenes, are assumed to have larger rate constants for the reactions than simple monoolefins and very few of the rate constants are known, it is worthwhile to know how large the rate constank for the reactions of diolefins with OH are. Meanwhile, some empirical findings on the rate constants of reactions of alkanes with simple radicals have been reported. Huie and Herron' derived general rate expressions for the rate constants for the reactions of O(3P) with alkanes. On the basis of the accumulated data of the rate constants for the reactions of alkanes with OH radicals, Greiner2derived an expression to give an overall expression fitting the rate constants for the alkanes, except for CHI and C2Hs, by summing up contributions to the overall rate constant from primary, secondary, and tertiary C-H bonds. Revised parameters for the expression were proposed later by Darnall, Atkinson, and P i t t ~ .And ~ more general expressions for hydrogen-abstraction reactions by OH have been developed for a larger temperature range by Heick1en.I Hendry and Kenley5 have considered substituent effects and developed a formulation to compute room-temperature rate constants for general compounds by the summation of three terms accounting for the reactivity toward OH: abstraction of all reactive hydrogen atoms, addition to all carbon-carbon double bonds, and addition to aromatic rings. In their hypothetical expression, they suggested that the rate constant for addition to all double bonds in the molecule was expressed by simply summing up the reactivity of each double bond when heteroatoms were not present in the molecule. In relation to this, it is interesting to examine experimentally whether an additivity rule exists in the reaction of unsaturated hydrocarbons by which we can get overall rate constants for diolefins by adding contributions from corresponding double-bond groups and, further, whether we can predict the rate constants from those for the reactions (1) R. E. Huie and J. T. Herron, h o g . React. Kinet.,8, 1 (1975). (2) N. R. Greiner, J. Chem. Phys., 63, 1070 (1970). (3) K. R. Darnall, R. Atkinson, and J. N. Pitts, Jr., J. Phys. Chem.,
82. 1.581 (1978). _ _ . ~ _ _ _ \
(4)J. Heicklen, Znt. J. Chem. Kinet., 13,651 (1981). ( 5 ) D. G. Hendry and R. A. Kenley, "Atmospheric Reaction Products of Organic Compounds", EPA Report 560/12-79-001, 1979. 0022-3654/83/2087-1209$01 S O f O
of monoolefins with OH, which are available from extensive studies on the reactions reviewed by Atkinson et ale6 For these aims, we measured the rate constants for the reactions of 19 diolefins in the millitorr pressure range with OH radicals by a relative rate determination method at room temperature and 1 atm, and obtained a new empirical prediction rule.
Experimental Section Photolysis of H202(go%, Mitsubishi Gas Co.) was used as the source of OH radicals. About 1torr of H202vapor was first introduced into a quartz cylindrical reaction cell with 4-cm diameter and 15-cm length, whose inside wall had been treated with 25% H2S04to decrease wall reactions.' Then, a certain amount of mixture of two reactants in diluent gas, 02,prepared beforehand and kept overnight, was introduced into the cell; the pressure was measured with an MKS-Baratron to obtain final concentrations of the olefins in the millitorr pressure range and a final concentration of O2 on the order of 10 torr. Nitrogen was added to pressurize the cell up to 760 torr. The mixture in the cell was kept for several minutes to get homogeneity of the gases. A germicide lamp (National GL-15), whose output is greatest at 2537 A, irradiated the cell at 24 f 2 "C; 1mL of the gas was sampled after successive exposures of 10-sduration each with a gas-tight pressure lock syringe (Precision Sampling Co.) and injected into a gas-chromatographic column with a flame ionization detector to monitor the concentrations of e standard substance and the other olefin of interest. Only in the case of the largest molecule among the compounds studied, 2,5-dimethyl2,4-hexadiene, was conditioning of the inside wall of the syringe with sample gas necessary to get good reproducibility. Special care was taken to avoid the interference of products with reactant peaks by using three different columns, Porapak Q, R, and T, which have different polarities. Ten runs with different initial concentrations of olefins and O2 were carried out to determine the rate constant for any olefin relative to the standard substance. Propylene, 1,3-butadiene, and trans-2-butene were from Takachiho Kagaku Co. Twenty diolefins and the other two monoolefins were obtained from Tokyo Kasei Co. and were of the best grade available. They were propadiene, 1,2-butadiene,2-methyl-173-butadiene,3-methyl-1,2-bu(6) R. Atkinson, K. R. Darnall, A. L. Lloyd, A. M. Winer, and J. N. Pitts, Jr., Adu. Photochem., 11, 375 (1979). (7) W. C. Schumb, C. N. Satlerfield, and R. L. Wentworth, "Hydrogen Peroxide", Reinhold, New York, 1955, ACS Monogr., pp 221-7.
0 1983 American Chemical Society
1210
The Journal of Physical Chemistty, Vol. 87,No. 7, 1983
Ohta
TABLE I: Example of t h e Measurement of t h e Relative Rate Constant for OH Radicalsa initial concn H202, torr 1.00 0.90 0.93 1.04 1.10 0.92 0.87 0.93 0.95 0.95
3c,
10 20 30 40 50 IRRADIATION TIME,sec
Figure 1. Some examples used for the determination of the relative rate constants for the reaction of OH with diolefins. Initial conditions for H,02,CIHB, and diolefin were respectively as follows: for triangles, 0.8, 7.38, 17.8 (1,3-butadlene); for closed circles, 0.9, 4.68, 18.1 (1,541exadiene);for open circles, 0.9, 4.79, 8.30 (1,4-hexadene); unit for the first is torr and for the rest is mtorr.
tadiene, 2,3-dimethyl-1,3-butadiene, 1,2-pentadiene, 1,3pentadiene (100% cis), lP-pentadiene, 2-methyl-1,4-pentadiene, 3-methyl-l,3-pentadiene, kmethyl-1,3-pentadiene, 1,3-hexadiene (97% trans and 2% cis), 1,4-hexadiene (94% trans), 1,5-hexadiene, 2,4-hexadiene (55% trans, trans, 35% trans, cis, 10% cis, cis), 2-methyl-1,5-hexadiene, 2,5-dimethyl-1,5-hexadiene, 2,5-dimethyl-2,4-hexadiene, l,&cyclohexadine, 1,6cyclohexadiene, 2-methyl-2-butene, and trans-2-penetene. The chemicals were degassed and used without further purification.
Results and Discussion None of the studied diolefins except 1,3-cyclohexadiene was decayed by irradiation in the absence of H202 under our conditions. 1,3Xyclohexadiene absorbs 2537-w radiation strongly ( E = 3360 L mol-' cm-').* The rate constant for this compound was not determined in this work. For the other compounds, plots of the logarithm of concentration vs. irradiation time showed straight lines under our experimental conditions. Some examples are shown in Figure 1. The major reactions occurring in the reaction cell are reactions 1-3, and eq 4 gives a relative ratio of the rate H202+ hv ---* 20H (1)
+A OH + B OH
ka
kb
products
(2)
products
(3)
1,3-B, mtorr 6.19 3.24 1.66 11.5 6.33 3.41 1.94 6.25 3.50 1.52
cis-1,3PD, mtorr 16.1 8.44 4.32 13.0 7.17 3.87 2.20 10.7 5.98 2.59
temp, "C 22.0 24.3 24.6 25.2 25.3 21.0 21.8 23.9 24.3 24.4
0 2 ,
torr 30.3 15.8 8.0 63.0 34.6 18.7 10.6 29.3 16.4 7.1
~c&I,~-PD/ 1,38
1.59 1.43 1.51 1.51 1.46 1.58 1.52 1.55 1.48 1.47 1 . 5 1 i 0.05, (av i U )
a 1,3-B = 1,3-butadiene. cis-1,3-PD = cis-1,3-pentadiene.
TABLE 11: Summarized Data f o r Relative Rate Constants and Absolute Rate Constants for the Reactions of OH with Diolefins and Monoolefins at 297 i 2 K 1012k, cm3 molecule-' compd
re1 rate
S-
'
~~
1,3-butadiene trans-l,3-hexadiene trans-1,4-hexadiene 1,5-hexadiene 2,4-hexadiene (mixture of cis and trans) cis-l,3-pentadiene 2-methyl-1,3-butadiene
1.00 1.68 i 1.35 i 0.93 i 2.01 i
0.05 0.08 0.02 0.08
1 . 5 1 t 0.05 1.48 i 0.04 2,3-dimethyl-l,3-butadiene 1.82 i 0.08 propylene 1.00 1,4-pentadiene 2.01 r 0.05 1,5-hexadiene 2.33 t 0.13 trans-l,.l-hexadiene 3.43 i 0.16 cis-1,3-pentadiene 1.00 2-methyl-l,4-pentadiene 0.78 i 0.08 3-methyl-l,3-pentadiene 1 . 3 5 i 0.08 4-methyl-1,a-pentadiene 1.30 i 0.04 trans-2-butene 0.59 I0.03 1 , 5-hexadiene 1.00 2-methyl-1,5-hexadiene 1.55 i 0.07 2,5-dimethyl-1,5-hexadiene 1.94 i 0.03 1,4-cyclohexadiene 1.60 i 0.05 2-methyl-1,5-hexadiene 1.00 2,5-dimethyl-2,4-hexadiene 2.19 i 0.10 1,3-butadiene 1.00 propadiene 0.15 t 0.02 1,3-butadiene 0.39 i 0.03 1,2-pentadiene 0.53 t 0.02 3-methyl-1,2-butadiene 0.85 I0.03 trans-2-pentene 1.00 i 0.03 Reference 10.
Reference 9.
68.5a 115 i 3 92.5 i 5.5 63.5 f 1.4 138 i 5 103 i 3 101 i 3 125 5 25.1b 50.6 1 1 . 3 58.5 f 3.3 86.1 t 4.0 103' 79.9 i 8.2 139 i 8 134 t 4 61.0 i 3.1 61.6c 95.5 i 4.3 120 i 1.8 9 8 . 6 i 3.1 95.5c 209 i 10 68.5 1 0 . 3 f 1.1 26.9 -L 2 . 1 36.3 f 1 . 3 58.0 t 1.9 68.5 i 2 . 1
This work.
constants k, to kb. The right-hand side of eq 4 corresponds to the ratio of the slopes of the plots of the logarithm of concentration vs. irradiation time. A typical example used for the determination of the relative rate constants of cis-1,3-pentadiene with 1,3-butadiene is shown in Table I. The ratios for 10 runs were independent of the initial concentrations of the two diolefins, 02,and HzOz and showed small deviations near the average value. The rate constants for OH reactions with eight conjugated dienes, six diolefins with isolated double bonds, two monoolefins, four allenes, and 1,4-cyclohexadiene were
determined by this method. The results are summarized in Table 11. The room-temperature rate constants for diolefins increase with the number of substituents on the double bond, in a fashion analogous to the reaction of olefin with OH and O(3P). The relative rate determination method with a static system is usually thought to be inferior to absolute rate methods, such as discharge flow or flash photolysis techniques, because of probable side reactions involving unstable species. Therefore, in order to have confidence
(8) S. W. Orchard and B.A. Thrush, R o c . R . Sac. London, Ser. A , 337, 243 (1974).
(9) R. Atkinson and J. N. Pitts, Jr., J. Chem. Phys., 63, 3591 (1975). (10) R. Atkinson, R. A. Perry, and J. N. Pitts, Jr., J. Chem. Phys., 67, 3170 (1977).
The Journal of Physical Chemistry, Vol. 87, No. 7, 1983
Reactions of Diolefins with OH Radicals
T = 297 10'lh, cm3 molecule-' s-'
TABLE 111: Data for the Experiments Made for the Check of the Relative Rate Determination Method at compd
re1 rate
std substance
0.43 f 0.15 f 1.27 f 1.04 f 0.93 t 2.33 f 1.35 t 3.43 f
this work
0.01
t
1211
2K
lit.
1.0 25.9 t 2.5b 0.02 1.1 9.3 t 0.93c 0.04 87.3 t 8.Bd 2.6 0.04 2.5 62.4f 0.02 1.4 0.13 3.3 5.5 0.08 0.16 4.0 10IZk= 68.5 cm3 molecule-' s-' from ref 10. Calculated from the Arrhenius parameters in ref 9 for T = 295 K. Reference 10. Reference 11. e 10"k = 61.6 cm3 molecule-' s-' determined in this work. Reference 12. 10"k = 25.1 from ref 9 at T = 298 K. propylene propadiene 2-methyl-2-butene cyclohexene 1,5-hexadiene 1,5-hexadiene 1,4-hexadiene 1,4-hexadiene
1,3-butadienea 1,3-butadiene 1,3-butadiene 1 ,5-hexadienee 1,3-butadiene propyleneg 1,3-butadiene propylene
in the data determined by this method, we determined the relative rate constants for the reactions of propylene, propadiene, and 2-methyl-2-butene with OH using 1,3butadiene as a standard substance, and they showed good agreement with literature values determined by different methods (Table 111). The absolute rate constant for cyclohexene was obtained from the competitive reactions with 1,5-hexadiene, whose rate constant was itself determined from the relative rate with 1,3-butadiene in this work. The obtained value, 64.1 X 10-l2cm3molecule-l s-l, showed good agreement with the literature value, 62.4 X 10-l2cm3molecule-' s-l at 303 K, of Wu, Japar, and Niki.12 In addition, rate constants for the reaction of OH with 1,5-hexadiene and 1P-hexadiene were determined by using two different standard olefins in separate runs. The results shown in Table I11 agreed very well with each other. The average of the two values determined was used for later discussions: klb-hexadene = 61.6 X 10-l2, kl,*-heXadiene = 89.3 X 10-l2cm3molecule-' s-l. The agreement in these checks indicated that side reactions such as those involving peroxy, alkyl, and alkoxy radicals did not have much effect on the measurement of the rate constant by the method employed here, although addition reactions of these radicals to double bonds or hydrogen abstractions from weak C-H bonds-for instance, allylic hydrogen and doubly allylic hydrogen-are usually important in polymerization or autooxidation in the condensed phase. In spite of the fact that the precise rate constants for the addition reactions of peroxy radicals and alkoxy radicals13 and the self-reaction of these radicald4 have not been well established experimentallyin general, we feel it is safe to assume that unstable OH adducts with excess vibrational energy decompose to smaller fragments to produce stable products without reacting significantly with parent molecules and that hydrogen-abstracted intermediates, if any, react with oxygen to proceed to formation of stable products. Four sets of three diolefins (A-D) are shown in Figure 2. A is an examplc for unconjugated diolefins and the others are for conjugated diolefins. Each set has two diolefins which have two equivalent double-bond groups, respectively, and a third diolefin which has two different double bonds. For the first two compounds, each double-bond group is estimated to make an equal contribution to the overall rate constant. Taking A as an example, from the data of 1,5-hexadieneand 2,5-dimethyl-1,5-hexadiene, we can assign the rate constants 30.5 X 10-l2 and 60.0 x 10-l2 cm3 molecule-' s-l for the contributions from C= C-C- and C==C(C)-C- groups, respectively. Summing (11)R.Atkinson and J. N. Pitta, Jr., J . Chem. Phya.,68,2992(1978). (12)C.H.Wu,S. M. Japer, and H. Niki, J. Enuiron. Sci. Health, Part A, 11, 191 (1976). (13)S. W.Benson, J. Am. Chem. Soc., 87,972 (1965). (14)S.W.Benson, J.Phys. Chem., 85,3375 (1981).
29.2 i 10.3 t 87.1 t 64.1 * 63.5 t 58.5 t 92.5 t 86.1 t
r
c c
r
r1.
r
95.5,
c
= 30.5
c
- C -
c
-
Y
c 60.0 = c
90.5b
101,
c=c-c=c96.8 34.3
134,
62.5
c34.3 = c - c86.5= C - c
121
Figure 2. Addltivi rule observed in the rate constants for the reaction of diolefins with OH radicals. (a) Rate constants are in units of lo-'* cm3 molecule-' s-'. (b) Underllned numbers are calculated values.
up the two assigned values gives 90.5 X cm3molecule-' s-l, which is almost equal to the rate constant 95.5 X 10-l2 cm3molecule-l s-l for 2-methyl-1,5-hexadiene,consisting of the two double-bond groups C=C-Cand C=C(C)-c--. Even for conjugated dienes in which T electrons between two adjacent double bonds are fairly dense, a similar thing was found. The data for 1,3-butadiene and 2,3-dimethyl-1,3-butadienegive 34.3 X and 62.5 X cm3 molecule-' s-l for the contributions from C=C- and C= C(C)-, groups, respectively, as is shown in B in Figure 2. The sum of these is 96.8 X 10-l2cm3 molecule-' s-l, which showed good agreement with the observed data, 101 X cm3molecule-' s-l for 2-methyl-l,3-butadiene, consisting A similar adof the two groups C=C- and C=C(C)-. ditivity rule is applicable for the cases of 1,3-pentadiene (C) with 4-methyl-l,3-pentadiene (D), as is shown in Figure 2, and the agreement is fairly good: the summed value for 1,3-pentadiene is 103 X and the observed value is 103 X cm3 molecule-' s-l; the summed value for 4methyl-1,3-pentadiene is 121 X and the observed value is 134 X cm3molecule-' s-l. Following the above discussion, we can predict the rate constant with OH, when we already know the rate constants for two of three diolefins belonging to one set. Moreover, the finding indicates that we can assign the contribution from each double-bond group to the overall rate constants by partitioning t h e data obtained in this work. The assigned value is shown below each double bond in the left half of Figure 3 for unconjugated dienes. In addition, the chemical formulas which correspond to mon o o l e f i that are derived by replacing C=C- groups with hydrogen atoms are shown in the same row on the right
1212
obsda 50.6, 61.0,
The Journal of Physical Chemistty, Vol. 87, No. 7, 1983
Ohta obsda
c25.3 = c - c - c25.3 =c c30.5 = c - c - c - c30.5 = c
25.1
c - c - c32.4 = cc
115,
c 34.3 = c - c = c - c - cc 80.7
125,
c = ! - c \= c
P
62.5 r
79.9,
L
50.7
c - t =50.7c e
62.9
c - c87.1=! - c'
108
c=c-t=c-c 34.3
Figure 3. Relations between assigned contribution from double bond in unconjugated dienes and the rate constants for moncdefins. (a) Rate constants are in units of lo-'* cm3 molecule-' s-'. (b) Reference 9. (c) Average of the data in ref 9 and 15. (d) Trans form. (e) Trans form, determined in this work. (f) Reference 10.
TABLE IV: Rate Constants for Unconjugated Diolefins Calculated from Those of Monoolefins
1,4-pentadiene 1,5-hexadiene 1,4-hexadiene (trans) 2-methyl-1,4-pentadiene 2-methyl-l,5-hexadiene 2,5-dimethyl-l,5-hexadiene
C d 84.9
C
139,
rate constanta
c - c68.5 = c - c L
62.5
I-
c25.3 = c - c - c54.6 =c
compd
P
c-iE=ce
L
xl.24 31.1
c - c 25.1 = cb
C-C=Cb
TABLE V: Rate Constants for Conjugated Diolefins Calculated from Those of Monoolefins rate constanta
calcd
[oIb
50.6 61.0 89.3 79.9 95.5 120
50.2 64.8 86.1 75.8 89.6 114
0.99 1.06 0.96 0.95 0.94 0.95
Rate constants are in units of lo-'' cm3 molecule-' s-'. Ratio of calculated rate constant t o observed one.
side of Figure 3 along with the rate constants for reactions with OH radicals. The assigned values for double-bond groups and the rate constants for corresponding monoolefins agree very well. This means that a double bond in a diolefin which has two isolated double bonds does not affect significantly the other double bond in the molecule in terms of the reactivity toward OH addition. This is probably because the intervening methylene group prohibits the interaction of the P electron belonging to the other double bond. Applying the relations found here, we could predict the rate constant for this kind of diolefin by summing up the constants of corresponding monoolefins that were derived by replacing one of the double-bond groups with hydrogen. For example, the corresponding olefins for 2,5-dimethyl1,Bhexadiene are two 2-methyl-1-butenes. The s u m of the constants for two 2-methyl-1-butenes is 2(57 X 10-'2),'2 which shows good agreement with the experimentally observed value 120 X cm3 molecule-l s-l. The values predicted by this method for other unconjugated diolefins are compared with observed values in Table IV. The agreement is farily good within several percent. In a similar way, for conjugated dienes, a modified method for the prediction of the overall rate constant is derived. On the basis of the discussion for Figure 2, we can assign the contribution from each double-bond group to the observed rate constants, which are shown on the left side of Figure 4. In the right half of Figure 4, monoolefins that are derived by replacing a double-bond group in a diolefin with a methyl group are shown with the rate constants. These values appear very different in a given row. But, if we multiply the rate constants for monoolefins by 1.24, which was determined to give the best fit for all of the compounds measured in this work, the new values show very good coincidence with the partitioned value in
calcd
[oIb
68.5 101 125 103 139 134 115 138 2,5-dimethyl-2,4-hexadiene 209
62.2 94.0 126 107 139 139 116 143 216
0.91 0.93 1.01 1.04 1.00 1.04 1.01 1.04 1.03
1,3-butadiene 2 -meth y 1- 1 , 3-butad i en e 2,3-d imet h yl- 1,3-butadiene 1,3-pentadiene (cis) 3-methyl-1,3-pentadiene 4-methyl-1,3-pentadiene 1,3-hexadiene (trans) 2,4-hexadiene ( mixturec)
a
[cli
obsd
compd
[Cli
obsd
104.7
Figure 4. Relations between the assigned contribution from double bond in conjugated dienes and the rate constants for monoolefins. (a) Rate constants are in units of lo-'' cm3 molecule-' s-'. (b) Reference 9. (c) Trans form. (d) Trans form, determined in this work. (e) Reference 9. (f) Determined in this work.
Rate constants are in units of cm3 molecule-' s-'. Ratio of calculated rate constant t o observed one, See text. a
diolefins (Figure 4). The necessity of the multiplication factor may be mainly attributed to the resonance effect that facilitates the OH addition to conjugated dienes. Thus, we could predict rate constants even for conjugated diolefins from the data for monoolefins. Taking 2,3-dimethyl-l,&butadiene as an example, we get two isobutenes as the corresponding monoolefins. Addition of 1.24(50.7 X 10-12)7 twice gives 126 X cm3molecule-l s-l and that agrees very well with the observed value, 125 X cm3 molecule-l s-l. The observed and calculated values for the conjugated dienes are shown in Table V. This method is also applicable to isomeric mixtures of 2,khexadiene which consist of 55% trans, trans, 35% trans, cis, and 10% cis, cis. The corresponding monoolefin for the compound is 2-butene. The rate constant for trans-2-butene was determined to be (61.0 f 3.1) X in this work; the relative ratio to cis-1,3-pentadiene was 0.59 f 0.03. The constant for cis-2-butene was reported to be 53.7 X by Atkinson et alS9and 42.6 X cm3molecule-l s-l by Ravishankara et al.ls Taking the average value for the two data and considering the isomeric ratio, we could calculate the rate constant for 2,4-hexadiene (mixture) to be 143 X 10-l2, which is in good agreement with the observed data, 138 X lo-'* cm3 molecule-l s-l. Conversely, the finding in this work is useful to examine the reliability of the rate constants for monoolefins. From the observed value, 115 X cm3 molecule-l s-l for trans-1,3-hexacm3 molecule-'^-^ for the diene, we assigned 80.7 X contribution from -C=C-C-C (Figure 4). The corresponding monoolefin for the group is trans-2-pentene,and (15) R. A. Ravishankara, S. Wagner, S. Fisher, G. Smith, R. Schiff, R. T. Watson, G. Tesi, and D. D. Davis, Znt. J. Chem. Kinet., 10,783 (1978).
J. Phys. Chem. 1983, 87, 1213-1219
we can predict ita rate constant to be near 65 X cm3 molecule-'s-'. A larger value for 2-pentene (mixture of cis, trans), 90.1 X cm3 molecule-' s-' a t 298 K, was reported in 1971 by Morris et al.le To check the discrepancy, we measured the relative rate constant against l,&butadiene and obtained 1.00 f 0.03, and the absolute cm3 molecule-' s-l. The rate constant was 68.5 X value showed good agreement with the expected value. The additivity rule was not observed for cyclohexene and 1,4-cyclohexadiene. Rate constants for allenes did not follow the rule, either. In conclusion, our results verified the hypothesis of Hendry and Kenley and showed that the reactivity of diolefins, except for allenes and cyclic diolefins, can be expressed by summing up the contributions from each carbon-carbon double bond. And our estimation method for the reactivity of each double bond from the rate constant for the corresponding monoolefin is proved to be much more accurate and more widely applicable than the (16)E.D. Morris, Jr., and H. Niki, J. Phys. Chem., 75, 3640 (1971).
1213
method of Hendry and K e n l e ~since , ~ they proposed fixed rate constants for unit double bonds and ignored conjugation effects. Acknowledgment. I thank Mitsubishi Gas Corp. for supplying 90% H202. I also thank Dr. H. Akimoto and Professors 0. Kajimoto and S. Sato for helpful discussions. I am also grateful to Dr. I. Mizoguchi for his interest and encouragement. Registry No. OH, 3352-57-6;1,3-butadiene,106-99-0;trans1,3-hexadiene,20237-34-7;trans-1,4-hexadiene,7319-00-8; 1,5hexadiene, 592-42-7; cis,cis-2,4-hexadiene, 6108-61-8; trans,5194-50-3; trans-2,4-hexadiene,5194-51-4;trans,cis-2,4-hexadiene, cis-1,3-pentadiene,1574-41-0;2-methyl-1,3-butadiene,78-79-5; 2,3-dimethyl-1,3-butadiene, 513-81-5; 1,4-pentadiene,591-93-5; 2-methyl-1,4-pentadiene, 763-30-4; 3-methyl-1,3-pentadiene, 4549-74-0; 4-methyl-l,3-pentadiene,926-56-7; trans-2-butene, 624-64-6; 2-methyl-l,5-hexadiene,4049-81-4; 2,5-dimethyl-1,5hexadiene, 627-58-7; 1,4-cyclohexadiene, 628-41-1; 2,5-dimethyl-2,4-hexadiene,764-13-6;propadiene, 463-49-0;1,2-butadiene, 590-19-2;l,e-pentadiene,591-95-7;3-methyl-l,2-butadiene, 598-25-4; trans-2-pentene, 646-04-8; propylene, 115-07-1; 2methyl-2-butene,513-35-9;cyclohexene, 110-83-8.
Cesium and Rubidium Ion Equillbria in Illite Clay E. Brouwer,t B. Baeyens, A. Maes, and A. Cremers' Centrum voor Opperviaktescheikunde en Coib-&le Scheikunde, Kathoiieke Universiteit Leuven, de Croyiaan 42, 8-3030 Leuven (Heveriee), Belgium (Received: M r c h 1, 1982; I n Final Form: November 4, 1982)
The ion-exchange selectivity of cesium and rubidium ions is studied in illite clay, saturated with either calcium, strontium, barium, sodium or potassium ions. The cesium- and rubidium-exchange levels studied vary between 50% and vanishingly low values. It is shown that the equilibria can be described in terms of three kinds of sites-corresponding to 0.5%,3%, and 96.5% of the exchange capacity-each of which shows a characteristic selectivity coefficient. The site representing 0.5% of the capacity exhibits an exceedingly high selectivity for cesium ions (33 and 40 kJ/equiv with respect to sodium and calcium ions) and discriminates strongly between cesium and rubidium (6 kJ/equiv). The equilibria on this site are thermodynamically reversible, as shown from three kinds of evidence: (1)the Cs-Rb trace selectivity difference, obtained from the equilibria in Ca, Sr, and Ba clay, agrees with the value obtained in the Na or K clay; (2) the same result is obtained from direct Cs-Rb trace mixture selectivity studies in Sr and Na clay; (3) the K-Na selectivity difference for this site as calculated from trace Cs equilibria agrees with the result obtained from Rb equilibria (11kJ/equiv). Finally, it is shown that the driving force for the high selectivity on this site is exclusively enthalpic.
Introduction The study of the selectivity of ions such as potassium and cesium in clay minerals has been a matter of interest for a long time. It is well-known that, depending on clay mineral ~tructure,l-~ ions with low hydration energy are very selectively retained by clays, a process which is sometimes loosely referred to as an irreversible one or as one of fixation. The interest in the behavior of K+ relates, of course, to soil-K+ availability4 whereas the treatment of radioactive effluents promoted the interest in C S + . ~ More V ~ recently, the problem of risk evaluation in geological disposal of high-level radioactive provided a new incentive for such studies. Illite clay, which often forms a substantial part of the exchange complex of shales which are considered as potential repositories for such waste, is well-known for its On leave from the Department of Soils and Fertilizers, Landbouwhogeschool Wageningen, The Netherlands. 0022-3654/83/2087-1213$01.50/0
selective sorption of cesium ions. Its sorption properties are considered to result from the involvement of two or more exchange sites2i5J0of decreasing selectivity. In surveys of the literature on the subject, however, it appears that some problems have not been resolved satisfactorily, a t least not from a purely quantitative point of view. Perhaps the most obvious and important one is the question of whether the highly selective sorption of B. L. Sawhney, Clays Clay Miner., 18,47 (1970). B. L. Sawhney, Clays Clay Miner., 20, 93 (1972). D. D.Eberl, Clays Clay Miner., 28, 161 (1980). M. E. Sumner and G. H. Bolt, Soil Sci. SOC.Am. h o c . , 26, 541 (1962). (5) D. G. Jacobs and T. Tamura, Trans. Int. Congr. Soil Sci., 7th, 1960,2, 206 (1960). (6)T.Tamura and D. G. Jacobs, Health Phys., 2, 391 (1960). (7)S.Komarneni and D. M. Rov. Clays Clay Miner.. 28, 142 (1980). (8)S. Komarneni, J. Inorg. Nu& Chem., 4i,397 (1979). (9)The National Research Council, "Geological Criteria for Repositorim for High-Level Radioactive Wastes", National Academy of Sciences, Washineton. DC. 1978. (IO) 6. H:Bolt, M. E. Sumner, and A. Kamphorst, Soil Sci. Soc. Am. Proc., 27,294 (1963).
0 1983 American Chemical Society
