J . Chem. Soc., Furuduy Trans. I , 1982, 18, 3493-3498 Reaction of Pentafluoroethyl Radicals with Cyanogen Chloride B Y CECILIA M. DE VOHRINGER AND EDUARDO H. STARICCO* Departamento de Fisico Quimica, Facultad de Ciencias Quimicas, Universidad Nacional de Cordoba, Estafeta 32, 5000 Cordoba, Argentina Received 7th January, 1982 The reaction of C,F, radicals with cyanogen chloride was studied between 293 and 573 K, using perfluoroethyl iodide as the free-radical source. CJ,. The reactions involved are The main product, C2F,C1, is tormed via an addition reaction or C,F, + ClCN -+ C,F,Cl f CN C,F, + ClCN -+ C2F,Cl + CN kc C,F, + ClCN C,F,ClCN C2F5 + C2F5 -+ C,F,,. by abstraction of a chlorine atom by The Arrhenius plot shows pronounced curvature. The following rate constants were obtained for reactions (2) and (4) log [(s) /cm$ mol-4 s-11 = 6.91 -57.10 kJ mo1-'/2.303 RT log [($),/cmi mol-4 s-4 = 2.32-20.55 kJ mol-l/2.303 RT k!. i 1 where k , is the rate constant for C,F, combination.The results are compared with those for the reaction of CF, with CICN. Comparisons between the reactivities of CF, and C,F, radicals have been obtained mainly for hydrogen abstfactions in both polar and non-polar substrates. Whittle and coworkers studied the reaction CF, + RC1+ CF,C1 + R where RCl = chloromethane' or an aromatic halide,, but no results involving transfer of a chlorine atom have been reported for C,F, radicals. The reaction of CF, with ClCN has also been ~tudied.~ It was shown that CF3C1 formation is via an addition reaction or by the abstraction of C1 by CF,, leading to a strongly curved Arrhenius plot.We have now extended this study to the reaction of C,F, radicals with ClCN, with the purpose of establishing mechanistic analogies of both reactions and comparing the Arrhenius parameters with those reported previously. EXPERIMENTAL Perfluoroethyl iodide (PCR Chem. Co.) was purified by fractional distillation and stored in liquid air. No impurities were detected by i.r. spectroscopy and gas chromatography. Cyanogen chloride was prepared and puriiied as in ref. (3). Perfluoromethane (ICN Pharm. Inc.) was distilled twice, the middle fraction being retained. 34933494 REACTION OF C2F5 RADICALS WITH ClCN APPARATUS AND PROCEDURE The reaction was performed in a cylindrical quartz vessel. This cell was in an oven, the temperature of which was controlled to kO.4 K by a PTR R52 regulator (Lauda).Between 293 and 353 K, the temperature of the reaction cell was maintained by water circulation from a thermostat. The light from an Osram HBO 500 W high pressure mercury lamp was filtered by a Corning 7740 plate. In this way, only wavelengths > 3000 reached the reaction ~essel.~ The cell was connected to a high vacuum system, in which the section used for storing and measuring gases was free of both grease and mercury. After each run the total content of the reaction vessel was transferred quantitatively to a Varian 200 chromatograph equipped with a Gow Mac density balance. The analysis was performed on a 3 m copper column packed with silica gel. Perfluoroethyl iodide was used as the C,F, radical source.RESULTS When C,F,I was photolysed in the presence of ClCN, the only products were C,F,Cl, C4FI0, C,F,CN, ICN and I,. The first three products were identified by i.r. spectroscopy and by comparing their retention times in different chromatographic columns with those of authentic samples. ICN and I, were characterized by their absorptions in the U.V. or visible. Despite thorough searching, no addition products of ClCN were found by i.r. spectroscopy or chromatography on suitable columns, at any degree of conversion. The reaction was studied in the temperature range 293-573 K and the ratio RC,Y,/l&F,q was determined at different concentrations of ClCN. The experimental results are given in table I . RC2F5C1/RL4F10 [ClCN] = kexptl at a given temperature was independent of a 5-fold variation in ClCN pressure and of a 2-fold variation in C,F51 pressure.Neither the percentage conversion nor pressure of inert gas affected kexptl. The possibility of a dark reaction between ClCN and C2F5 was tested for as follows. First, 70 Torr* of ClCN plus 60 Torr C,F,I were left in the reaction vessel for 20 h at 323 K. No products were detected by gas chromatography. Then 50 Torr of ClCN and 60 Torr of C,F,I were left for 20 min at 533 K with a similar result. However, for reaction times longer than ca. 2 h at 533 K the main products appeared owing to thermal decomposition of C,F,I. kexptl at each temperature was calculated from the slope of a plot of R~2F,Cl/&4F10 against [ClCN]. Consumption of reactants was usually negligible, but in those temperatures where the conversion was of a low percentage, an average value of ClCN was used.An Arrhenius plot of kexptl is shown in fig. 1. DISCUSSION When C,F,I is photolysed alone, the important reactions are C,F51 + hv --+ C,F, + I k, C2F5 + C2F5 --+ C4F10 I+I+M+I,+M. In the presence of cyanogen chloride two reactions could occur C2F5 + ClCN --+ C,F,CN + Cl C2F5 + ClCN -+ C,F,Cl+ CN. * 1 Torr = (101 325/760) Pa.C. M. DE VOHRINGER AND E. H. STARICCO TABLE ~.-PHOTOLYSIS OF C,F,I WITH CICNa 3495 reactant pressure/Torr photolysis conversion T / K time/s C,FJ ClCN R c ~ F ~ ~ RC~F~CN (%I kexptl 293 32 400 58.9 61.4 324 25 200 60.1 13.0 324 25 200 60.6 60.7 353 21 690 61.5 32.2 373 25 260 63.9 59.3 373 24000 127.7 59.9 383 43 860 54.3 16.6 383 25 200 66.3 60.4 400 28 920 68.5 70.7 413 51 900 49.1 14.7 413 16200 71.6 73.8 433 14400 71.2 52.1 443 8 100 68.5 52.6 457 10 800 63.4 47.7 473 14 400 70.9 9.8 473 10 800 69.5 41.0 503 10 800 76.6 16.5 533 7 200 84.8 4.0 533 1 200 62.3 5.4 533 1 200 63.8 5.5 533 7 200 85.0 17.4 573 1980 90.9 10.3 0.746 0.339 1.548 1.662 1.885 1.787 0.929 3.187 2.423 1.400 8.103 7.324 13.51 11.52 7.79 23.07 32.29 26.13 40.54 39.27 84.57 231.5 2.080 2.301 2.394 2.619 0.448 0.420 0.803 0.798 0.184 0.448 0.673 0.529 0.914 0.485 1.653 0.945 2.282 10.36 10.65 9.52 5.59 27.4 0.279 0.209 0.965 1.129 1.353 1.156 0.879 2.820 2.373 0.721 7.378 6.105 12.07 11.17 7.49 21.95 32.00 26.09 40.51 39.03 83.45 230.2 0.07 0.13 0.13 0.25 0.19 0.17 0.59 0.32 0.25 1.27 0.46 0.55 0.57 0.74 3.43 1.81 6.85 16.95 2.98 2.84 12.33 17.30 0.049 0.110 0.105 0.222 0.349 0.339 0.472 0.446 0.630 1.16 1.09 1.65 2.3 1 3.12 5.86 5.45 13.3 23.1 24.4 24.5b 22.9 52.7 a Volume of reaction vessel: 105 cm3 (between 293 and 353 K) and 100 cm3 (between 373 and 573 K).Rates of formation of products, R, in units of lOI3 mol cmP3 s-I.kexptl = RCzF5C1/R&Flo [CICN] in units of cmz mol-i s-i. 300 Torr of CF, added. 1.6 1.8 2.0 2.2 2 . L 2.6 2.8 3.0 3.2 3.L 103 KIT FIG. I .-Arrhenius plot for reaction of C,F, radicals with CICN. (--), kexpt, = k,/kt +k,/k%, from eqn (11) and (111); (---), k,/kL from eqn (11); (-.-.-), k,/k$ from eqn (111).3496 REACTION OF C2F5 RADICALS WITH ClCN If reaction (1) occurs, the C,F,Cl could arise from C2F5 + Cl + C,F,Cl.However, experiments carried out in our laboratory5 have shown that chlorine atoms readily add to ClCN giving compounds with a CN double bond. Furthermore, reaction of C1 with ClCN would produce less C,F,Cl than the amount expested if every chlorine atom formed in reaction (1) is consumed by reaction with C,F5. Accordingly, the C,F,CN/C,F,Cl ratio would always be > 1. We disregard the CN abstraction, reaction (l), because: (a) no addition product could be detected and (b) the C,F,CN/C,F,Cl ratio was never > 1 (and is indeed < 1 at low temperatures). Therefore, it seems probable that the reaction between C2F5 radicals and ClCN involves chlorine-atom transfer. As was found in the CF, + ClCN reaction, the curvature in the Arrhenius plot can be ascribed to two reactions with different A factors and activation energies which produce C,F,Cl, one of which is the genuine chlorine abstraction C2F5 + ClCN -P C,F,Cl + CN C2F5 + ClCN + C2F5ClCN (2) (3) C,F,ClCN + C,F5Cl+ CN.(4) If these are the only sources of C,F,Cl, we have where R represents the rate of formation of the products. At a given temperature, RCIFSC1/ficIF1O depends only on the concentration of ClCN, in agreement with data in table 1. At high temperatures the abstraction process becomes much more important than the addition, giving kexptl z k,/kt. On the other hand, at low temperatures, the contribution of reaction (2) to the production of C,F,Cl would be negligible. Probably k-, is much less than k, under the same conditions leading to kexIltl E k,/ki at low temperatures.The Arrhenius parameters for reactions (2) and (3) were obtained by the Prony method6 for the sum of two exponentials. The following equations were obtained log [($)/cmi mol-4 s(] = 6.91 -57.10 kJ mol-l/2.303 RT kc log [($)/cmi m o b s 2 = 2.32-20.55 kJ mol-l/2.303 RT. -1 (111) The experimental and calculated values using eqn (11) and (111) in eqn (I) are compared in table 2. The agreement is acceptable, with a deviation of 10%. This is shown in fig. 1, together with log (k,/kk) and log (k,/ki) from eqn (11) and (111). The curve represents the experimental points. For the reaction of CF, with ClCN, the Arrhenius plot was fitted taking into account the reversibility of the addition, reaction (-3), and a difference E-,-E4 of 8.37 kJ mol-1 was obtained by an iterative method.Similar calculations carried out in this work did not lead to such a result. A good fit was achieved by using Arrhenius parameters very close to those already obtained from eqn (11) and (111). This suggests that the ratio k-,/k, in eqn (I) is < 1 for the complete temperature range.C. M. DE VOHRINGER AND E. H. STARICCO 3497 TABLE 2 .-OBSERVED AND CALCULATED RATE CONSTANTSa 293 324 353 373 383 400 413 0.047 0.108 0.220 0.362 0.466 0.721 1.02 kexptl 0.047 0.107 0.227 0.348 0.453 0.638 1.09 T / K kcale. T / K kcalc. 433 443 457 473 503 533 573 1.75 2.29 3.35 5.13 11.0 22.5 52.8 kexptl 1.70 2.30 3.34 5.63 12.6 23.2 51.8 a Calculated from eqn (11) and (111); in units of cmf mol-i s-5. TABLE 3.-vARIATION OF ( & z ~ 5 C ~ / & z F 5 C I ) = A WITH TEMPERATURE 29 3 324 353 373 383 400 0.44 0.56 0.75 0.72 0.92 0.96 0.06 0.06 0.06 0.05 0.03 0.04 TIK A 41 3 433 443 457 473 533 573 0.87 0.83 0.94 0.98 0.97 0.99 0.94 TIK %a 0.07 0.12 0.05 0.02 0.03 0.0 1 0.0 1 A a Standard deviation.If the ratio of pre-exponential factors &/A, were about the same as in the CF,+ClCN system, the difference E-,-E4 would be at least 16 kJ mol-l. The CN radicals formed by reactions (2) and (4) could be removed by reaction with I,, C,F,I or by recombination with C2F5 CN+I, -+ ICN+I ( 5 ) (6) (7) Due to analytical limitations, a quantitative determination of cyanogen iodide could not be performed. Nevertheless, as can be seen in table 1, the RC2F5CN/RC2P5CI ratio was constant at each temperature, within experimental error.The variation of this ratio with temperature is shown in table 3. Near 433 K, RCzFSCN/RCzF,CI approaches unity and maintains this value up to the highest temperature studied. Cyanogen iodide was not found above 433 K. Thermal decomposition7 of ICN could explain this feature since the steady-state concentration of CN increases as the temperature increases, making the production of C,F,CN important. When hexafluoroacetone was the radical source in the CF, + ClCN reaction,, the CF,CN/CF,CI ratio was always close to unity (between 408 and 473 K). This provides more evidence for good CN trapping by I, or C,F,I, at least in the low-temperature range. A value of k , is required in order to put k , and k, on an absolute basis. Skorobogatov et a1.8 obtained k, = 3.0 x lo1, cm3 m o t 1 s-l independent of tempera- ture for C2F5 combination.However, we assume that k , for C,F, is the same as k , for CF, radicals, for which Ayscough’s value of k , = 2.3 x lo1, cm3 mol-1 s-l has usually been used.lo We also assume that Ec = 0. There is ample evidence that E, = 0 for most combination reactions89l0 (but see also Ogawa et all1), in which case our values of E would be correct. Acceptance of Skorobogatov’s value of k , would require adjustment of our A factors. CN + C,F,I -+ ICN + C,F, CN + C2F5 --+ C,F,CN.3498 REACTION OF C2F5 RADICALS WITH ClCN The Arrhenius parameters for the addition and abstraction reactions from eqn (11) and (111) using k , = 2.3 x lo1, cm3 mol-l s-l are, respectively, A , = 3.9 x lo1, cm3 mol-1 s-l, A , = 1.0 x lo9 cm3 mol-1 s-l, E, = 57.1 kJ mol-l E3 = 20.6 kJ mot1.The values can be compared with those for the CF,+ClCN reaction reported previously3 A, = 1.3 x 1013 cm3 mol-1 s-l, A , = 2.7 x 1O1O cm3 mol-1 s-l, E, = 48.6 kJ mo1-1 E3 = 26.6 kJ mol-l. Although it seems probable that there is some self-compensation of Arrhenius parameters for the addition as well as for the abstraction reaction, the difference in reactivities between CF, and C2F5 radicals become apparent when we consider the rate constants. For example, at 295 K, where addition is the main process, ICcp,+cIcN = 0.100 cmt mol-4 S-4 kC,F,+CICN = 0.047 cml mol-4 s-i. Chamberlain and Whittle12 have reported a similar difference in reactivity when CF, or C2F, radicals add to benzene. At 573 K kCF,+CICN = 102 cmi mol-+ S-4 kC2F6+CICN = 51.8 c d mol-4 s-4 and chlorine abstraction from ClCN by CF, radicals is about twice as fast as that by C2F5 radicals.Further work on the reactions of other radicals with ClCN could provide additional data for a more complete comparison of the Arrhenius parameters for chlorine abstraction, and it could also be determined if the reaction scheme is general for reactions of different radicals with ClCN. 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