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Anomalies in the kinetics of halogenation, hot-atom effects
Volume 188, number I,2
CHEMICAL PHYSICS LETTERS
3 January I992
Anomalies in the kinetics of halogenation, hot-atom effects Sidney W. Benson ’ Loker Hydrocarbon Research Institute, Vniversity Park, University ofSouthern California, Los Angeles, CA 90089-1661, USA
Received 22 August 1991
It is proposed that gross discrepancies reported in the literature for the kinetic rate constants for Cl t CIHL and Cl t C2D6may be a consequence of a hot Cl atom produced in the chain system. The exothermic step responsible for this is C2H5+Cl~+C2H,CI+Cl+25 kcal, followed by the slightly exothermic: CI+C2H,+HCI+CLH, t 3 kcal. Similar anomalies can arise in brominations while related effects may occur in solution halqenations.
The discovery that Cl atoms can participate in a chain reaction to destroy O3 in the stratosphere has stimulated interest in the determination of accurate rate constants for Cl atom reactions with hydrogencontaining substrates which could compete with ozone and trap Cl atoms as relatively inert HCl. We have in our own laboratories measured a number of such reactions in the past decade. In the last five years improvements in the technique of very low pressure reactor (VLPR) have made possible the measure of very rapid rate constants with very good precision and also high accuracy [ 1,2]. Of particular interest here are the studies of Cl+C2H6 [ 31 and CI+C*D, [4]. Rate constants for the former k,u have been measured in a number of other laboratories by a variety of techniques and in the last decade all agree to within 5% with our own measured value [ 31. Further study [ 51 of this reaction over an extended temperature range (203-343 K) have confirmed the room temperature measurement [ 31 and yielded an activation energy of 0.170 + 0.020 kcal/mol, again in excellent agreement with recent work. When combined with the equivalent data on Cl + CzD6 [ 2 1, k, D we have for the isotopic rate constant ratio at 300 K k E7.4kO.4. k ID
2 ’
(1)
Cl, 2
2CI ,
Cl+C,H& CzHs fCl$
HCI+CIH,, C2H5CI+Cl,
2CzH, -i, C4H,o. The initial mixture was CzH6 with enough excess of CzD6 to assure comparable rates of production of the stable products C2H,CI and CzD5Cl. Since the propagation reactions are both very fast and the quantum yield very large, the assumption of stationary state kinetics is justified and gives the isotopic rate constant ratio as k <
N UGH,CI)
k ID -
Distinguished Professor of Chemistry and Scientific Co-Director, Emeritus.
106
While there are no other measurements on the reaction of Cl+C,D, with which to compare, there have been two independent and direct measurements of the ratio using the technique of competitive chlorination. The earliest was by Chiltz et al. [ 61, who used a molecular leak from the reaction vessel to observe, mass spectrometrically, the competitive products CzH5C1 and &D&l. These products are produced in a photochemically induced chain reaction
VC2D5Cl)
(GDb)A, (C2H6)~v
’
(21
where Y(C2H5Cl) represents the yield of CzH&l, etc.
0009-2614/92/$ OS.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
Volume 188, number I,2
CHEMICAL PHYSICS LETTERS
At 300 K and over the entire course of the photochlorination they found a ratio of 2.68kO.10. A more recent determination [7] using similar techniques but measuring loss of reactants and done at higher pressures gave a ratio of 2.72 f 0.10 in excellent agreement. Both sets of competitive measurements disagree grossly with the direct determinations by VLPR, by a factor of 2.7, far outside the reported precision of all the measurements. Careful scrutiny of all the above papers gives no reason for doubting the reliability of the data or the methods used. Is there any way of reconciling such extremely large disagreement in the two sets of measurements? In the following we should like to suggest a possible reconciliation. At first glance it would seem that the product ratio technique should yield the most reliable measurement. However, the product ratio does not measure directly step 1 but rather step 2, the reaction of ethyl radicals with C12.The VLPR measurements measure step 1 in the absence of a chain or of CIZ.Why would step 2 be of interest? Step 1 is exothermic by 3 kcal but step 2 is exothermic by 25 kcal. Like many other exothermic, metathesis reactions, the energy release tends to be shared between the newly formed C-Cl bond and the departing Cl atom. We thus expect to see a translationally and/or electronically excited Cl atom emerge from step 2 #I. Step ( 1H) has a small activation energy of E,,=0.17 kcal/mol and hence one expects a larger activation energy for step ( 1D) on the basis of zeropoint energy differences between the ground and transition states. Based on either sets of results, AEl =EID- E, H will probably not exceed 1.2 kcal/ mol (RTln 7.4) since the A factors would not be expected to be very different for the two reactions, or if anything to be slightly larger for C2D6.A 1.2 kcal/ mol difference in activation energies corresponds to about 415 cm- ’ which is a reasonable difference in zero-point energies for the two isotopic systems. The A factors agree quite well with that calculated for a tight transition state with a bent Cl...H...C bond [9]. The A factor for step ( 1H ) is very large, corresponding to about one reactive collision in every five collisions with the 0.17 kcal activation energy. If the Cl I’ See references and discussion in ref. [ 81.
3 January I992
atom emerging from step 2 has about half of the exotherrnicity of the reaction, it is unlikely in a neat mixture of C,H, and C,D, to be thermalized in five or six collisions. Hence it will be relatively non-selective in its reactivity, in contrast to thermalized Cl atoms. We thus suggest that hot-atom effects may produce biased results in competitive chlorination experiments in neat mixtures. It should be fairly straightforward to test this suggestion by performing comparative chlorination in systems containing a large excess of buffer gas to thermalize the Cl atoms. So far such experiments have not been done for C2H6/ C2D, mixtures. They have been done for a series of chlorinated alkanes [ IO] using C2H6 or n-&H,, as reference standards. Interestingly enough these studies show systematic discrepancies for I-chlorobutane and I-chloropentane of about 3O*hwhen compared with earlier values for n-butane relative to ethane. Of equal interest is the fact that in ref. [7] the reference pairs were C2H6/CH4 and C2D,/CH,. However, their values for C,H5CI/C2H, disagrees with those of ref. [ 111 by a factor of 1.7, the values of ref. [ I I] being higher. The latter authors used CHI as a reference for their chloroethanes. While we have addressed chlorination reactions it should be noted that reactions of alkyl radicals with Br, are exothermic by about 24 kcal [ 91 and with I2 by about 18 kcal [ 91. In the case of Br, this is sufficient to produce electronically excited Br (‘PI ,,2) with 14 kcal left over for either translational and/or vibrational excitation in the products RBr+Br. In the case of I2 only translational excitation is expected. Such excitation if divided between RI+1 is unlikely to be of consequence since I atoms with 9 kcal do not have enough energy to break most C-H or C-I bonds. In the case of Br2 however, 12 kcal (assuming no electronic excitation and rough partition between RBr and Br) is sufficient to cause anomalous behavior in systems with tertiary C-H bonds and or allylic or benzylic C-H bonds. In fact very anomalous behavior has been commented on [ 121 in the thermal brominations of i-butane [ 131 and toluene [ 141 in the gas phase. These thermochemical consideration are equally relevant to liquid phase reactions when the substrate RH is present in high concentrations. The repulsion be tween the newly formed RX and the X atom 107
Volume 188,number I ,2
CHEMICAL PHYSICS LETTERS
should assist in diffusion from the solvent cage and render multiple chlorination of RX less likely by the newly liberated X atom. The repulsive energy will be transformed into a local hot spot in a time the order of 1O-100 fs. If we assume that the entire 28 kcal are shared with RX, X, and 8CH2C12solvent molecules on this time scale, the hot spot temperature will be of the order of 180-200” higher than the initial temperature. Using a formula for Newtonian cooling of a sphere [ i 5 ] we can estimate that the halflife of this hot spot will be of the order of 1 ps. This is long enough to initiate low activation energy reactions which might not otherwise occur. Ingold et al. [ 161 have recently examined a number of anomalies in the free radical chlorination of alkanes in solution phase, which they rationalized in part by invoking cage effects. It is possible that the hot-atom effects discussed here may also contribute to some of these anomalies. This work has been supported by a grant from the National Science Foundation (CHE-87 14647).
108
3 January 1992
References [ 1 ] 0. Dobis and S.W. Benson, Intern. J. Chem. Kinetics I9 (1987) 691. [2] S.S. Parmarand S.W. Benson, J. Phys. Chem. 92 (1988) 2652. [ 3 ] 0. Dobis and S.W. Benson, J. Am. Chem. Sot. 1I2 ( 1990) 1023. [ 4) S.S. Parmar and S.W. Benson, J. Am. Chem. Sot. I 11 (1989) 57. [S] 0. Dobis and S.W. Benson, J. Am. Chem. Sot. 113 (1991) 6377. [6] G. Chiltz, R. Eckling, P. Goldtinger, G. Huybrechts, H.S. Johnston, L. Meyers and G. Verbeke, J. Chem. Phys. 38 (1963) 1053. I71 E. Tschuikow-Roux, J. Niedzielski and F. Farah, Can. J. Chem. 63 (1985) 1093. [ 81 G.N. Robinson, G.M. Nathanson, R.E. Continetti and Y.T. Lee, J. Chem. Phys. 89 ( 1988) 6744. [9] S.W. Benson, Thermochemical kinetics, 2nd Ed. (Wiley, New York, 1976) [lo] T.J. Wallington, L.M. Skewes and W.O. Siegl, J. Phys. Chem. 93 (1989) 3649. [ I1 ] J. Niedzielski, E. Tschuikow-Roux and T. Yano, Intern. J. Chem. Kinetics 16 ( 1984) 621. 112) S.W. Bensonand J.H. Buss, J. Chem. Phys. 27 (1958) 301. [ 131 B.H. Eckstein, H.A. Scheraga and E.R. Van Artsdalen, J. Chem. Phys. 22 (1954) 28. [ 141 H.R. Anderson, H.A. Scheraga and E.R. Van Ansdalen, J. Chem. Phys. 21 (1953) 1258. [ 15 1S.W. Benson, Foundations of chemical kinetics, Reprint Ed. (Krieger, New York, 1982) p. 435; J. Chem. Phys. 22 ( 1954) 46. [ 161 K.U. ingold, J. Lusztyk and K.D. Raner, Accounts Chem. Res. 23 ( 1990) 2 19.
CHEMICAL PHYSICS LETTERS
3 January I992
Anomalies in the kinetics of halogenation, hot-atom effects Sidney W. Benson ’ Loker Hydrocarbon Research Institute, Vniversity Park, University ofSouthern California, Los Angeles, CA 90089-1661, USA
Received 22 August 1991
It is proposed that gross discrepancies reported in the literature for the kinetic rate constants for Cl t CIHL and Cl t C2D6may be a consequence of a hot Cl atom produced in the chain system. The exothermic step responsible for this is C2H5+Cl~+C2H,CI+Cl+25 kcal, followed by the slightly exothermic: CI+C2H,+HCI+CLH, t 3 kcal. Similar anomalies can arise in brominations while related effects may occur in solution halqenations.
The discovery that Cl atoms can participate in a chain reaction to destroy O3 in the stratosphere has stimulated interest in the determination of accurate rate constants for Cl atom reactions with hydrogencontaining substrates which could compete with ozone and trap Cl atoms as relatively inert HCl. We have in our own laboratories measured a number of such reactions in the past decade. In the last five years improvements in the technique of very low pressure reactor (VLPR) have made possible the measure of very rapid rate constants with very good precision and also high accuracy [ 1,2]. Of particular interest here are the studies of Cl+C2H6 [ 31 and CI+C*D, [4]. Rate constants for the former k,u have been measured in a number of other laboratories by a variety of techniques and in the last decade all agree to within 5% with our own measured value [ 31. Further study [ 51 of this reaction over an extended temperature range (203-343 K) have confirmed the room temperature measurement [ 31 and yielded an activation energy of 0.170 + 0.020 kcal/mol, again in excellent agreement with recent work. When combined with the equivalent data on Cl + CzD6 [ 2 1, k, D we have for the isotopic rate constant ratio at 300 K k E7.4kO.4. k ID
2 ’
(1)
Cl, 2
2CI ,
Cl+C,H& CzHs fCl$
HCI+CIH,, C2H5CI+Cl,
2CzH, -i, C4H,o. The initial mixture was CzH6 with enough excess of CzD6 to assure comparable rates of production of the stable products C2H,CI and CzD5Cl. Since the propagation reactions are both very fast and the quantum yield very large, the assumption of stationary state kinetics is justified and gives the isotopic rate constant ratio as k <
N UGH,CI)
k ID -
Distinguished Professor of Chemistry and Scientific Co-Director, Emeritus.
106
While there are no other measurements on the reaction of Cl+C,D, with which to compare, there have been two independent and direct measurements of the ratio using the technique of competitive chlorination. The earliest was by Chiltz et al. [ 61, who used a molecular leak from the reaction vessel to observe, mass spectrometrically, the competitive products CzH5C1 and &D&l. These products are produced in a photochemically induced chain reaction
VC2D5Cl)
(GDb)A, (C2H6)~v
’
(21
where Y(C2H5Cl) represents the yield of CzH&l, etc.
0009-2614/92/$ OS.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
Volume 188, number I,2
CHEMICAL PHYSICS LETTERS
At 300 K and over the entire course of the photochlorination they found a ratio of 2.68kO.10. A more recent determination [7] using similar techniques but measuring loss of reactants and done at higher pressures gave a ratio of 2.72 f 0.10 in excellent agreement. Both sets of competitive measurements disagree grossly with the direct determinations by VLPR, by a factor of 2.7, far outside the reported precision of all the measurements. Careful scrutiny of all the above papers gives no reason for doubting the reliability of the data or the methods used. Is there any way of reconciling such extremely large disagreement in the two sets of measurements? In the following we should like to suggest a possible reconciliation. At first glance it would seem that the product ratio technique should yield the most reliable measurement. However, the product ratio does not measure directly step 1 but rather step 2, the reaction of ethyl radicals with C12.The VLPR measurements measure step 1 in the absence of a chain or of CIZ.Why would step 2 be of interest? Step 1 is exothermic by 3 kcal but step 2 is exothermic by 25 kcal. Like many other exothermic, metathesis reactions, the energy release tends to be shared between the newly formed C-Cl bond and the departing Cl atom. We thus expect to see a translationally and/or electronically excited Cl atom emerge from step 2 #I. Step ( 1H) has a small activation energy of E,,=0.17 kcal/mol and hence one expects a larger activation energy for step ( 1D) on the basis of zeropoint energy differences between the ground and transition states. Based on either sets of results, AEl =EID- E, H will probably not exceed 1.2 kcal/ mol (RTln 7.4) since the A factors would not be expected to be very different for the two reactions, or if anything to be slightly larger for C2D6.A 1.2 kcal/ mol difference in activation energies corresponds to about 415 cm- ’ which is a reasonable difference in zero-point energies for the two isotopic systems. The A factors agree quite well with that calculated for a tight transition state with a bent Cl...H...C bond [9]. The A factor for step ( 1H ) is very large, corresponding to about one reactive collision in every five collisions with the 0.17 kcal activation energy. If the Cl I’ See references and discussion in ref. [ 81.
3 January I992
atom emerging from step 2 has about half of the exotherrnicity of the reaction, it is unlikely in a neat mixture of C,H, and C,D, to be thermalized in five or six collisions. Hence it will be relatively non-selective in its reactivity, in contrast to thermalized Cl atoms. We thus suggest that hot-atom effects may produce biased results in competitive chlorination experiments in neat mixtures. It should be fairly straightforward to test this suggestion by performing comparative chlorination in systems containing a large excess of buffer gas to thermalize the Cl atoms. So far such experiments have not been done for C2H6/ C2D, mixtures. They have been done for a series of chlorinated alkanes [ IO] using C2H6 or n-&H,, as reference standards. Interestingly enough these studies show systematic discrepancies for I-chlorobutane and I-chloropentane of about 3O*hwhen compared with earlier values for n-butane relative to ethane. Of equal interest is the fact that in ref. [7] the reference pairs were C2H6/CH4 and C2D,/CH,. However, their values for C,H5CI/C2H, disagrees with those of ref. [ 111 by a factor of 1.7, the values of ref. [ I I] being higher. The latter authors used CHI as a reference for their chloroethanes. While we have addressed chlorination reactions it should be noted that reactions of alkyl radicals with Br, are exothermic by about 24 kcal [ 91 and with I2 by about 18 kcal [ 91. In the case of Br, this is sufficient to produce electronically excited Br (‘PI ,,2) with 14 kcal left over for either translational and/or vibrational excitation in the products RBr+Br. In the case of I2 only translational excitation is expected. Such excitation if divided between RI+1 is unlikely to be of consequence since I atoms with 9 kcal do not have enough energy to break most C-H or C-I bonds. In the case of Br2 however, 12 kcal (assuming no electronic excitation and rough partition between RBr and Br) is sufficient to cause anomalous behavior in systems with tertiary C-H bonds and or allylic or benzylic C-H bonds. In fact very anomalous behavior has been commented on [ 121 in the thermal brominations of i-butane [ 131 and toluene [ 141 in the gas phase. These thermochemical consideration are equally relevant to liquid phase reactions when the substrate RH is present in high concentrations. The repulsion be tween the newly formed RX and the X atom 107
Volume 188,number I ,2
CHEMICAL PHYSICS LETTERS
should assist in diffusion from the solvent cage and render multiple chlorination of RX less likely by the newly liberated X atom. The repulsive energy will be transformed into a local hot spot in a time the order of 1O-100 fs. If we assume that the entire 28 kcal are shared with RX, X, and 8CH2C12solvent molecules on this time scale, the hot spot temperature will be of the order of 180-200” higher than the initial temperature. Using a formula for Newtonian cooling of a sphere [ i 5 ] we can estimate that the halflife of this hot spot will be of the order of 1 ps. This is long enough to initiate low activation energy reactions which might not otherwise occur. Ingold et al. [ 161 have recently examined a number of anomalies in the free radical chlorination of alkanes in solution phase, which they rationalized in part by invoking cage effects. It is possible that the hot-atom effects discussed here may also contribute to some of these anomalies. This work has been supported by a grant from the National Science Foundation (CHE-87 14647).
108
3 January 1992
References [ 1 ] 0. Dobis and S.W. Benson, Intern. J. Chem. Kinetics I9 (1987) 691. [2] S.S. Parmarand S.W. Benson, J. Phys. Chem. 92 (1988) 2652. [ 3 ] 0. Dobis and S.W. Benson, J. Am. Chem. Sot. 1I2 ( 1990) 1023. [ 4) S.S. Parmar and S.W. Benson, J. Am. Chem. Sot. I 11 (1989) 57. [S] 0. Dobis and S.W. Benson, J. Am. Chem. Sot. 113 (1991) 6377. [6] G. Chiltz, R. Eckling, P. Goldtinger, G. Huybrechts, H.S. Johnston, L. Meyers and G. Verbeke, J. Chem. Phys. 38 (1963) 1053. I71 E. Tschuikow-Roux, J. Niedzielski and F. Farah, Can. J. Chem. 63 (1985) 1093. [ 81 G.N. Robinson, G.M. Nathanson, R.E. Continetti and Y.T. Lee, J. Chem. Phys. 89 ( 1988) 6744. [9] S.W. Benson, Thermochemical kinetics, 2nd Ed. (Wiley, New York, 1976) [lo] T.J. Wallington, L.M. Skewes and W.O. Siegl, J. Phys. Chem. 93 (1989) 3649. [ I1 ] J. Niedzielski, E. Tschuikow-Roux and T. Yano, Intern. J. Chem. Kinetics 16 ( 1984) 621. 112) S.W. Bensonand J.H. Buss, J. Chem. Phys. 27 (1958) 301. [ 131 B.H. Eckstein, H.A. Scheraga and E.R. Van Artsdalen, J. Chem. Phys. 22 (1954) 28. [ 141 H.R. Anderson, H.A. Scheraga and E.R. Van Ansdalen, J. Chem. Phys. 21 (1953) 1258. [ 15 1S.W. Benson, Foundations of chemical kinetics, Reprint Ed. (Krieger, New York, 1982) p. 435; J. Chem. Phys. 22 ( 1954) 46. [ 161 K.U. ingold, J. Lusztyk and K.D. Raner, Accounts Chem. Res. 23 ( 1990) 2 19.