J . Phys. Chem. 1985,89, 551-552
55 1
Gas-Phase Elimination Kinetics of w-Phenylalkyl Chlorides. Participation of the Phenyl Ring Gabriel Chuchani,* Alexandra Rotinov, and Ignacio Mardn Centro de Qdmica, Instituto Venezolano de Investigaciones Cient$cas, Caracas 101 0- A , Venezuela (Received: April 4, 1984) The kinetics of the gas-phase dehydrochlorination of several phenylalkyl chlorides were determined in a static system, seasoned with allyl bromide, and in the presence of the free radical inhibitor propene. The working temperature and pressure ranges were 398.8-480.6 OC and 66-202 torr, respectively. The reactions are homogeneous, unimolecular, and follow a first-order rate law. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for 3chloro-1-phenylpropane,log kl (s-') = (1 3.99 0.26) - (238.4 3.5) kJ-mol-' (2.303RT)-'; for 4-chloro-l-phenylbutane, log k , (s-I) = (13.07 & 0.43) - (220.5 5.8) kJ.mol-I (2.303RT)-'; and for 5-chloro-l-phenylpentane,log k , (s-l) = (13.75 f 0.36) - (231.2 4.9) kJ.mol-I (2.303RT)-I. When the rates of HC1 elimination among each of the phenylalkyl chlorides are compared with those for the corresponding unsubstituted alkyl chloride, participation of the C6H5group at the 3 position is more favored, while that at the 5 and 6 positions is rather weak. Participation of C6H5group at the 4 position is apparently absent.
*
*
*
Introduction Phenyl participation has been reported in the gas-phase pyrolysis of unsubstituted and ring-substituted phenylethyl chlorides.lJ The aromatic substituent was found to cause an order of magnitude increase in the pyrolysis rate relative that of ethyl chloride, attributable to the formation of a very discrete phenonium ion in the transition state (1). The presence of a methyl group at the
A -----_-& , a 1
three isomeric positions of the benzene ring2 gave similar k values when compared to unsubstituted 2-phenylethyl chloride. These results insinuated that the electron donation of the CH3 substituent is rather ineffective in strengthening the phenyl assistance for rate enhancement. However, the presence of CH30at the para position of the benzene ring in 2-phenylethyl chloridel causes a significant increase in the rates by participation of the p-anisyl when compared with ethyl chloride and 2-phenylethyl chloride. Since phenyl assistance is apparently responsible for rate augmentation in the gas-phase dehydrochlorination of these types of compounds, it seemed interesting to examine the participation by more remote phenyl rings in w-phenylalkyl chlorides. Consequently, the present work addressed the study of the pyrolysis kinetics of 2-chloro- 1-phenylpropane, 4-chloro- 1-phenylbutane, and 5-chloro-1-phenylpentane.
Experimental Section The reagent 3-chloro- 1-phenylpropane was bought from K & K Laboratories, while 4-chloro-1-phenylbutane and 5-chloro-lphenylpentane were prepared by treating the corresponding phenylbutanol and phenylpentanol in dimethylaniline with thionyl chloride as previously de~cribed.~4-Chloro-1-phenylbutane had a boiling point of 114 OC at 6 torr (lit. bp 122-123 OC at 17 torr4). 5-Chloro-1-phenylpentanehad a boiling point of 117 OC at 4 torr (lit. bp 134 OC at 18 torr4). These halide substrates were distilled several times and the fraction with over 99.1% purity (gas-liquid chromatography) was used (FFAP 7%-Chromosorb G AW DMCS 80-100 mesh). 3-Phenyl- 1-propene was acquired from K & K Laboratories, whereas 4-phenyl-1-butene and l-phenyl2-butene were from Pfaltz and Bauer. The above FFAP column (1) Hernandez, A,, J. A.; Chuchani, G.-Int.J. Chem. Kinet. 1978, IO, 923. ( 2 ) Chuchani, G.;Medina, J. D.; Martin, I.; Hernandez A., J. A. J. Phys. Chem. 1981, 85, 3900. (3) Bergmann, E.; Weizmann, A. J. Org. Chem. 1939, 4, 266. (4) von Braun, J. Ber. 1910, 43, 2837.
TABLE I: Stoichiometry of the Reaction 3-Chloro-1-phenylpropaneat 469.8 5 7 time, min 3 26.5 39.8 50.2 reaction, % press. HC1, % titration 26.4 38.5 49.3
4-Chloro-1-phenylbutaneat 43 1 .O 20 5 10 time, min reaction, % press. 18.3 31.4 51.8 HC1, % titration 17.7 29.5 50.9
OC 10 79.7
78.6 OC 30 75.5 74.8
5-Chloro-1-phenylpentaneat 440.1 "C time, min 2.5 5 10 15 10.1 20.0 32.7 47.6 reaction, % press. HC1, %titration 9.7 18.8 31.1 46.9
20 57.4
57.0
was also used for quantitative analyses of the olefinic aromatic hydrocarbon products. The identities of the substrates and products were additionally verified by mass spectrometry and by infrared and nuclear magnetic resonance spectroscopy. The chloride reagents were pyrolyzed in a static system5 and in the presence of the chain inhibitor propene. The reaction vessel was seasoned with the decomposition product of allyl bromide5 and the rate coefficients were determined manometrically. The temperature was maintained within *0.2 OC with a calibrated platinum-platinum-1 3% rhodium thermocouple. No temperature gradient was found in the reaction vessel.
Results and Discussion The gas-phase pyrolysis of w-phenylalkyl chlorides in vessels seasoned with the decomposition products of allyl bromide yields the corresponding olefinic aromatic hydrocarbons and hydrogen chloride: C6H5(CH,),CH,CH2Cl C6H5(CH,),CH=CH2 f HCl n = 1, 2, 3 (1) The stoichiometry based on eq 1 required that, for long reaction times, the final pressure Pr should be twice the initial pressure Po. The average experimental results of Pr/Povalues at four different temperatures and ten half-lives were 2.08 for 3-chloro1-phenylpropane, 2.23 for 4-chloro-l-phenylbutane,and 2.26 for 5-chloro-1-phenylpentane. The departure from PI = 2P0 is due to a slight decomposition of the olefinic products. The above stoichiometry, up to 6 0 4 0 % reaction, was satisfactorily verified by comparing the percentage decomposition of the substrate calculated from pressure measurements with that obtained from
-
( 5 ) Maccoll, A. Chem. Reu. 1969, 69, 33. ( 6 ) Evans, P. J.; Ichimura, T.; Tschiukow-Roux, E. Znt. J . Chem. Kinet. 1978, IO, 855.
(8) Hartman, H.; Bosch, H. G.;Heydtmann, H. Z . Phys. Chem. (Frankfurt am M u i n ) . 1964, 42 329. (9) Grant, R. C . S.; Swinbourne, E. S.J . Chem. SOC.1965, 4423.
0022-3654/85/2089-0551~01.50/00 1985 American Chemical Society
552
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985
Chuchani et al.
TABLE II: Variation of the Rate Coefficients with Temperature 3-Chloro-1 -phenylpropane
temp, OC 104k,, s - ~
419.4 1.05
429.2 1.90
temp, "C 104kl, s-I
398.8 0.88
temp, OC 104k,, s-1
410.1 1.19
440.2 3.61
450.0 5.83
460.0 10.58
469.8 16.95
480.6 31.12
409.0 1.41
4-Chloro-1-phenylbutane 420.1 431.0 2.89 5.15
440.3 8.72
449.8 15.21
460.8 24.32
420.1 2.15
5-Chloro-1-phenylpentane 430.1 435.2 3.86 4.85
440.1 6.42
450.1 10.72
460.1
TABLE III: Comparative Kinetic Parameters of w-Phenylalkyl Chlorides at 440 OC compound 1044, s-I Ea, kJ/mol CH3CH2CI 1.35 241.8 f 4.2 220.9 i 4.6 C6HSCH2CHzCI 7.73 i 0.10
CH,CH2CH,C1 4.47 C~HJCH~CH~CH~CI 3.36 f 0.16 CH,CH2CH$H2CI 5.37 C,H&H2CH2CH,CH,CI 8.27 i 0.39 CH,CH2CH$H2CH&I 5.44 f 0.12 C ~ H J C H ~ C H ~ C H ~ C H ~ C H ~ C 6.51 I
229.2 i 5.4 238.4 f 3.5 230.7 f 3.1 220.5 i 5.8 244.0 f 6.2 231.2 f 4.9
E,'
285.9 222.6 225.9 228.9 224.8 222.2 224.7 223.7
19.8 1
ref
log A , s-l
13.84 i 0.20 13.07 i 0.35 13.44 i 0.28 13.99 f 0.26 13.63 i 0.24 13.07 f 0.43 14.61 i 0.46 13.75 i 0.36
6b 1 l b
this work 8 this work
9 this work
"Scaled activation energy. bThesedata from the shock tube method have the advantage that a much larger temperature can be covered and that, since the heating is homogeneous and the reaction times are short, heterogeneous effects are not present. hydrogen chloride titration with 0.05 N sodium hydroxide solution (Table I). 3-Chloro- 1-phenylpropane yielded, up to 80% decomposition, largely 3-phenyl-l-propene, while 4-chloro- 1-phenylbutane gave, up to 80% reaction, mostly 4-phenyl- 1-butene, small amounts of cis- and tram-l-phenyl-2-butene, and traces of 1-phenyl-1-butene. For 5-phenyl-l-chloropentane,the products are, up to 60% decomposition, mainly 5-phenyl-l -pentene and traces of l phenyl- 1-pentene and cis- and trans-5-phenyl-2-pentene. The homogeneity of these reactions was examined in a vessel with a surface-to-volume ratio of 6.0 times greater than that of a normal vessel which is equal to one. The rates were unaffected in the packed and unpacked seasoned vessels, but a significant heterogeneous effect was obtained in the packed and unpacked clean vessels. The halides were always pyrolyzed in seasoned vessels and in the presence of at least a threefold excess of the free chain radical inhibitor propene. The first-order rate coefficients of these chlorides estimated from k , = (2.303/t) log P0/(2P0 - P,) were invariable of their initial pressures. A plot of log (2P0 - P,) against time t gave a straight line up to 6 0 4 0 % decomposition. The temperature dependence of the rate coefficients is listed in Table 11. The results of Table I1 are expressed, when using the leastsquares procedure and 80% confidence limits, by the following Arrhenius equations 3-chloro- 1-phenylpropane
log k , (s-1) = (13.99 f 0.26) - (238.4 f 3.5) kJmol-l (2.303RT)-1 4-chloro- 1-phenylbutane log k , (s-I) = (13.07 f 0.43)
- (220.5
f 5.8) kJ.mo1-I (2.303RT)-'
5-chloro- 1-phenylpentane log k , (s-1) = (13.75 i 0.36) - (231.2 f 4.9) kJ-mol-] (2.303RT'" If the rates for HC1 elimination in w-phenylalkyl chlorides are compared with those for the corresponding unsubstituted alkyl chlorides (Table 111),one can see that participation of the C6HS group at the 3 position is more favored, while participation of the C6HSgroup at the 5 and 6 positions can be rationalized as weak. On the other hand, participation of the C6H5 group at the 6 position is of no importance. This interpretation arises from the fact that, when the positions of the phenyl substituents of these primary halides (Table IV) are projected on the recently reported Taft correlation of the log krel,of substituted ethyl chlorides, i.e.,
TABLE I V Effect of Phenyl Substituent in ZCH2CH2CI Pyrolyses at 440 OC Z 104k1,S-' log kz/k, u*' CH3 4.47 0.000 0.000 C6HS C6HSCH2
C~HSCH~CH~ C6HSCH2CH2CHI
7.73 3.36 8.27 6.51
0.238 -0.124 0.268 0.164
0.60 0.215 0.08
0.02
"Hansch, C.; Leo, A. J. "Substituent Constant for Correlation Analysis in Chemistry and Biology"; Wiley: New York, 1979. ZCH2CH2Cl,against u* values," the plot of the C ~ H S C position H~ falls on the slope of the line for alkyl halides pyrolyses. However, the plot of the C6H5position is far above the slope of the line, while that of C6H5CH2CH2and C6H5CH2CH2CH2 are slightly above and significant. In addition to the above argument, the participation of the phenyl group may well be reflected in the activation energies. If the preferred log A is not much different from the transition state estimate of 13.3 for ethyl chloride and 13.2 for all other ZCH2CH2CI" then the activation energies can be scaled with the rate coefficient shown in Table 111. The comparison of the scaled activation energy between the phenyl-substituted alkyl chloride with that of the corresponding unsubstituted parent compound (column 3, Table 111) suggests that only the C6H5group at the 3 position assists in the rate of elimination of phenylethyl chloride. The relative sequence of phenyl participation obtained in the present work appears to be somewhat similar to the relative sequence found by Heck and Winstein12 for the rates of acetolysis and formolysis of unsubstituted and ring methoxy substituted w-phenylalkyl p-bromobenzenesulfonates. In the reactions participation of the C6H5group at the 4 position is unimportant, participation of the C6H5group at the 3 position occurs most readily, and participation of the C6HS group at the 5 and 6 positions is rather weak. Acknowledgment. We are thankful to the Consejo Nacional de Investigaciones Cientificas y TecnolBgicas (CONICIT) for their support through Project No. 51.78.31, S1-1072. Registry No. 3-Chloro-l-phenylpropane, 104-52-9; 4-chloro-lphenylbutane, 4830-93-7; 5-chloro-l-phenylpentane,15733-63-8; phenylbutanol, 3360-41-6; phenylpentanol, 10521-91-2. (10) Chuchani, G . ; Martin, I.; Rotinov, A,; Hernlndez, A,, J. A,; Reikonnen, N. J . Phys. Chem. 1984, 88, 1563. (1 1) Benson, S. W.; ONeal, H. E. Natl. Stand Re$ Data Ser. ( U S . Natl. Bur. Stand 1970, NSRDS-NBS No. 21. (12) Heck, R.; Winstein, S . J . A m . Chem. SOC.1957, 79, 3144.
