3574
J. Phys. Chem. 1989, 93, 3514-3519
-
recently used a photoacoustic technique to measure AH in solution for t-C4H,0' C 6 H 5 0 H t-C4H90H + C6H50'; from their data, they have determined AHf0298(C6H50')g= 9.0 f 1 kcal. mol-'. In view of the uncertainty in AHf0298(allyl),,we prefer
+
(49) He, Y. 2.;Mallard, W. G.; Tsang, W. J. Phys. Chem. 1988, 92,2196.
to average the Mulder et al. value with that obtained from Colussi et al.'s data on C6H50Et decomposition. This gives AHfo298(C6H50')g = 9.2 kcalmol-' and, as we have outlined, the present work supports this lower value. Registry No. C6H50H,108-95-2.
Sodium Atom Reactions with the Bromochloromethanes: Branching Ratios and Relative Reaction Rates J. T. Jayne and P. Davidovits* Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02167 (Received: May 12, 1988; In Final Form: November 17, 1988)
Branching ratios into bromide and chloride products have been measured for the reaction of atomic sodium with BrCCI,, BrCHCI,, and BrCH2CI. The bromide to chloride product ratios are 0.55, 1.83, and 4.00, respectively. The experiments were performed in a linear flow tube apparatus at 340 K. Sodium halide reaction products were collected along the flow tube and analyzed titrimetrically. In a separate set of experiments the relative reactivities of the bromine and chlorine sites in each of the molecules as well as in CH3Br and CHCI, were determined by normalization to the reactivity of CCII. Semiempirical MNDO computer calculations were performed on the halomethane reactants to explore possible systematic correlations with the experimental data.
Introduction
The reactions of halomethanes with alkali-metal atoms have been studied These molecules are well suited for systematic studies such as, for example, the testing of the effect of electronegative substituents on reactivity. Most of the studies done in the past were designed to characterize the sequence of reactivities for a set of molecules. For example, Heller and Polanyi' drew attention to the fact that the marked progressive increase in reactivity on passing along the series CH3Cl to CC14 runs parallel to a decreasing force constant for the C-CI bond. Increased reactivity is also observed by substitution of an existing halogen by a less electronegative halogen.2 This too corresponds to a decreased force constant for the C-X bond. The results of
(1) Heller, W.; Polanyi, M. Trans. Faraday SOC.1936, 32, 633. (2) Haresnape, J. N.; Stevels, J. M.; Warhurst, E. Trans. Faraday SOC. 1940, 36, 465. (3) Kaufman, E. D.; Reed, J. F. J . Phys. Chem. 1963, 67, 896. (4) Heller, W. Trans. Faraday SOC.1937, 33, 1556. (5) CRC Handbook of Bimolecular and Termolecular Gas Reactions; Kerr, J. A., Ed.; Chemical Rubber Co.: Boca Raton, FL, 1981. ( 6 ) Lin, K. K.; Balling, L. C.; Wright, J. J. Chem. Phys. Lett. 1987, 133, 246. ( 7 ) Frommer, L.; Polanyi, M. Trans. Faraday SOC.1934, 30, 519. (8) Fairbrother, F.; Warhurst, E. Trans. Faraday SOC.1935, 31, 987. (9) Warhurst, E. Trans. Faraday SOC.1939, 35, 614. (IO) von Hartel, H.; Polanyi, M. Z . Phys. Chem., Abt. 5 1930, 11, 97. ( 1 1) von Hartel, H.; Meer, N.; Polanyi, M. Z . Phys. Chem., Ab?.5 1932, 19, 139. (12) Husain, D.; Marshall, P. Int. J . Chem. Kinet. 1986, 18, 83. (13) Husain, D.; Marshall, P. J. Chem. SOC.,Faraday Trans. 2 1985,81, 613. (14) Gowenlock, B. G.; Thomas, K. E. J . Chem. SOC.1965, 5068. ( 1 5 ) Reed, J. F.; Rabinovitch, B. S. J. Chem. Phys. 1957, 27, 988. (16) Husain, D.; Lee, Y. H. Int. J. Chem. Kiner. 1988, 20, 233. (17) Warhurst, E. Q. Rec. Chem. Soc. 1951, 5 , 44. ( 1 8) Trotman-Dickenson, A. F. Gus Kinerics; Buttenvorths: London, 1955; p 219. (19) Polanyi, M. Atomic Reactions; Williams and Norgate: London, 1932. (20) Hodgins, J. W.; Haines, R. L. Can. J . Chem. 1952, 30, 473.
0022-3654/89/2093-3574$01.50/0
some of these experiments lead to the formulation of the BellEvans-Polanyi principle relating activation energies to heats of reactions which has been used for the prediction of reaction rates2' In most of the above studies total reaction rates have been measured. Branching ratios into product halides have been obtained in only one study for the reaction of sodium with the chlorofluoromethane^.^ In this paper we present results of experiments in which branching ratios into bromide and chloride products were measured for the reaction of sodium atoms with the bromochloromethanes BrCH2CI, BrCHCI2, and BrCCI3. In a separate set of experiments the relative reactivities of the bromine and chlorine sites in each of the molecules as well as in CH,Br and CHC13 were determined by normalization to the reactivity of cc14. The reactions of sodium with these molecules to form a bromide or chloride product are in all cases exoergic. The exoergicities are in the range of 1.4 to 1.7 eV depending on the specific reaction, being highest for the formation of NaBr from BrCC1, and lowest in the formation of NaCl from CHC13.22 Experimental Procedure and Results The experimental apparatus is shown in Figure 1. Sodium atoms are produced in a resistively heated oven and are entrained by a nitrogen carrier gas flow. The halomethane molecules are introduced into the flow through a perforated loop. The reaction products are formed, deposited along the flow tube, and collected for analysis. The resistively heated oven consists of a cylindrical stainless steel crucible 3-in. in length with I-in. 0.d. and 1/8-in.wall. The crucible was modeled after a design by Gole et aLZ3 A section (21) Dewar, M. J. S.; Dougherty, R. C . The PMO Theory of Organic Chemistry; Plenum Publishing: New York, 1975. (22) Vedenyev, V I.; Gurvich, L. V.; Kondratyev, V. W.; Medvedev, V. A,; Frankevich, Ye. L. Bond Energies, Ionization Potentials, and Electron A/finities; St. Martin's: New York, 1966. (23) Preuss, D. R.; Pace, S. A,: Gole, J. L. J. Chem. Phys. 1979, 71, 3553.
0 1989 American Chemical Society
N a Reactions with Bromochloromethanes of 1/8-in.-diameter stainless steel tube welded on the side of the crucible was used as the oven carrier gas inlet. Three layers of 90-mesh stainless steel wire screen were placed below the crucible lid to prevent possible entrainment of sodium aerosol particles by the carrier gas. The exit hole in the crucible lid was 1.3 mm in diameter. The lid was secured to the crucible by three 0-80 machine screws. The tungsten wire element was electrically insulated from the crucible by quartz tubing. The temperature of the oven was monitored by a type K chromel-alumel thermocouple. The whole assembly was thermally insulated with a layer of zirconia felt that was saturated with zirconia cement to minimize dust formation inside the vacuum chamber. The system required approximately 60 W of power to maintain the operating temperature of about 380 OC. The linear flow tube was 3.5 cm in diameter and 64 cm long and was oriented vertically. The system was evacuated from the top of the flow tube by a Sargent-Welch Model 1398 two-stage rotary pump. Nitrogen was used as the main carrier gas. The flow was split so that about 900 sccm passed through the oven and the remaining carrier flow, about 800 sccm, entered the flow tube through the carrier loop as shown in Figure 1. Under these conditions the flow velocity was approximately 1850 cm s-l. The total pressure in the flow tube was 1.2 Torr measured by a McLeod gauge (1 Torr = 133.33 N m-*). Methyl bromide is a gas. The other reactants are liquids at room temperature with vapor pressures in the range 50-200 Torr at 30 OC. Since the total pressure in the main flow tube is lower, only about 1 Torr, the reactants can be introduced into the flow tube directly without additional carrier gas. The liquid reactants were kept in a thermostated bath at 30 OC. The reactant gases entered the reaction zone through a 1/8-in.-diametermultiholed copper loop. The reactant loop was positioned approximately 1.5 cm above the center of the crucible lid. The gas flow rates were measured by using Matheson series 8100 electronic mass flow meters that were individually calibrated for each reactant by weight-loss measurements. Methyl bromide was purchased from Matheson Gas Co. The other reactants were obtained from Aldrich Chemical Co. The reported purity of these substances was better than 97%. Reactant number densities in the flow tube ranged between 4 X 1014and 2 X 10I6c ~ n depending -~ on the experimental conditions and the species studied. At the entrance to the reaction zone the sodium density was on the order of 1013cm-3 as determined by weight-loss measurements. The reactions were therefore pseudo-first-order with respect to sodium. The flow tube was lined with a Mylar sheet 0.13 mm thick that served as a surface for the deposition of the sodium salts. The Mylar sheet was 54 cm long, and it was positioned 10 cm above the reactant gas inlet. Each experimental run required about 3 h to collect a sufficient amount of salt for analysis. At the end of a run the sheet was removed and cut into sections. The first four sections were 10 cm long. The last section, at the downstream end of the flow tube, was 14 cm long. The five sections were each made ready for potentiometric analysis by washing them into 100-mL volumetric flasks. A 3-h run produced a quantity of product such that when washed into 100-mL flasks made solutions ranging from approximately 15 to 2 mM of halide ion as sampled from the beginning to the end of the collecting strip. Aliquots (10 mL) of each prepared solution were titrated against 0.006 M AgN03 for total halide ion content.24 A second 10-mL aliquot was titrated after treatment by a procedure developed by Proko p o in~ which ~ ~ the bromide ion is selectively oxidized to molecular bromine by a solution of 3% H 2 0 2in acetic acid. The bromine formed readily brominates the scavenger molecule, 8-hydroxyquinoline dissolved in acetic acid. In this way the chloride ions may be titrated separately. The difference between the two titrations yields the bromide ion content. The method was tested on a standard solution containing 0.827 mmol of chloride ion and (24) Skoog, D. A.; West, D.M.; Holler, F.J. Fundamentals of Analytical Chemistry, 5th ed.; Saunders College Publishing: New York, 1988; p 357. (25) Prokopov, T. S. Anal. Chem. 1970, 42, 1096.
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3575 TABLE I: Branching Ratios for BrCH2CI in the Reaction with Sodium dist along
[NaBr]/[NaCl]
flow tube, cm
run 1
run 2
run 3
10-20 20-30 30-40 40-50 50-64
4.13 4.13 4.13 3.98 4.1 1 av 4.10
3.92 4.13 4.1 1 4.02 3.75 av 3.99
3.98 4.12 4.04 3.84 3.65 av 3.94
grand av = 4.00 f 0.15
TABLE 11: Branching Ratios for BrCHCI2 in the Reaction with Sodium - .-.
dist along
[NaBr]/[NaCl]
flow tube, Em
run 1
run 2
10-20 20-30 30-40 40-50 50-64
1.73 1.70 1.81 1.88 1.87 av 1.80
1.92 1.79 1.83 1.95 1.82 av 1.86
grand av = 1.83 f 0.08
TABLE 111: Branching Ratios for BrCC13 in the Reaction with Sodium dist along
[NaBr]/[NaCI]
flow tube. cm
run 1
run 2
run 3
10-20 20-30 30-40 40-50 50-64
0.54 0.55 0.56 0.49 0.56 av 0.54
0.55 0.58 0.55 0.59 0.58 av 0.57
0.53 0.50 0.55 0.54 0.54 av 0.53
grand av = 0.55 f 0.03
0.371 mmol of bromide ion. The analytical method yielded the correct concentration to within 0.7%. For solutions containing less than 0.1 mmol of halide ion, typical of solutions from the last 24 cm of the collection strip, the error in the analysis was as large as 5%. From published rate constants by Haresnape et aL2 (see Table V) it can be shown that under our experimental conditions the reactions are in all cases completed in less than lo4 s. This corresponds to a distance of less than 0.2 cm along the flow tube. Since the Mylar sheet is placed 10 cm above the reactant inlet, products are collected in a region where the flow is depleted of Na. This was tested by monitoring, in every analysis, the pH of the product solutions. In each case the solutions were neutral. When the conditions were purposely offset to extend the reaction region into the collection zone, the solutions were found to be basic, consistent with the presence of NaOH formed by the reaction of atomic sodium and water. The reaction conditions were offset by appropriately reducing the reactant density. The absence of sodium in the collection zone precludes contributions to products by wall reactions. An additional test was performed, but only with BrCH2C1, to ascertain that the products collected on the Mylar lining of the flow tube wall were due to gas-phase rather than to wall reactions. In this test sodium was vaporized without addition of a reactant gas. After a period typical of an experimental run, visible quantities of sodium were deposited on the flow tube wall. The sodium source was then shut off, and BrCH2C1 was introduced into the flow tube for approximately 30 min. No detectable quantities of NaCl or NaBr were measured by the outlined procedure. The sodium and its carrier gas enter the reaction zone at an elevated temperature and combined there with a room-temperature gas flow consisting of additional carrier gas and the halomethane. At that mixing point the temperature, as measured by a 10mil-diameter chromel-alumel thermocouple, was 340 K. The ability of the thermocouple to correctly measure the temperature
3516
The Journal of Physical Chemistry, Val. 93. No. 9, 1989
Jayne and Davidovits
to pYmp
3'00
I
I
0.00 0.00
1 .oo 2.00 3.00 4 3 Reactant Ratio [CHJBr]/[BrCHCI,] Figure 3. Product ratio measured in the reaction of Na + CH,Br + BrCHC12. Density of BrCHCI, varied between 7.1 X IOl4 and 1.0 X IO" cm". Density of CH,Br varied between 2.6 X IO" and 5.6 X IO" Slope of the line is the ratio of the reaction rate coefficients kalBrlkCI.
tc.rr1.r r m l a " l ~ 9" I"lB1
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Figure 1. Schematic diagram of apparatus used for studying reactions of sodium with the halomethanes.
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