J. Phys. Chem. 1989, 93, 203-205
203
Assessment of the Cyano Substituent Effect in the Inhibited Pyrolysis Kinetics of o-Bromonitriles in the Gas Phase Gabriel Chuchani* and Rosa M. Domhguez Centro de Qulmica, Instituto Venezolano de Investigaciones Cient$cas (I.V.I.C.), Apartado 21 827, Caracas 1020-A, Venezuela (Received: December 10, 1987)
The kinetics of the gas-phase pyrolysis of several bromonitriles were determined in a static system over the temperature range of 389.7-470.1 OC and the pressure range of 38.5-139 Torr. The reactions in seasoned vessels, and in the presence of at least a 4-fold excess of the free-radical inhibitor propene and/or toluene, are homogeneous and unimolecular and follow a first-order rate law. The rate coefficients are given by the following Arrhenius equations: for 3-bromopropionitrile, log k l (8)= (13.56 f 0.44) - (231.9 f 6.0) kJ mol-' (2.303RT)-I; for 4-bromobutyronitrile, log kl (s-l) = (14.12 f 0.33) - (233.5 f 4.5) kJ mol-' (2.303RT)-'; for 5-bromovaleronitrile, log k l (8)= (13.79 0.19) - (225.7 f 2.6) kJ mol-' (2.303RT)-'. The electron-withdrawing effect of the CN group along the carbon chain deactivates the HBr elimination during pyrolysis. A plot of the log krclof the cyano substituents in w-bromonitriles against u* values gives a good straight line with p* = -0.24, correlation coefficient r = 0.988, and intercept = -0.054 at 400 "C. This slope inflects at u*(CH3) = 0.00 with another good straight line derived by plotting the log k,l vs u* values of alkyl @-substitutedethyl bromides (p* = -1.87, r = 0.991, and intercept = 0.056 at 400 "C). The present result is explained in terms of slight alteration in the polarity of the transition state due to changes of electronic transmission at the carbon reaction center.
*
Introduction
TABLE I: Stoichiometry of the Reaction"
Because of the high electronegativity or electron-withdrawing effect of the C N group at the 2-position of ethyl chloride, the C 4 1 bond polarization in the transition state is reduced, thus causing destabilization for the dehydrochlorination process in the gas phase. In this respect, the study of the pyrolysis of 3-chloropropionitrile in a static system was found to be very complex and a detailed kinetic determination and mechanistic interpretation proved to be very difficult.' However, the use of the very low pressure pyrolysis (VLPP) technique with this compound gave primary thermal processes in unimolecular elimination without complications of secondary reactiom2 Since the cyano group has shown to slow the rate of HCl elimination of 3-chloropropionitrile with respect to the corresponding unsubstituted ethyl c h l ~ r i d e ,the ~ , ~present work was addressed to study the effect of the electron-withdrawing C N substituent in the pyrolysis kinetics of a series of w-bromonitriles.
Experimental Section 3-Bromopropionitrile, 4-bromobutyronitrile, and 5-bromovaleronitrile were from Aldrich. These bromonitriles were distilled several times, and the fraction of better than 98.4% purity (gas-liquid chromatography) was used. The products acrylonitrile and allyl cyanide were from Aldrich. The column used for quantitative analysis was the 10% Dow Corning 200/ 1OO-Chromosorb W A W DMCS (80-100 mesh). The identities of substrates and products were verified with a mass spectrometer and by infrared and nuclear magnetic resonance spectroscopy. The pyrolysis reactions were performed in a static system, seasoned with allyl bromide, and in the presence of at least a 4-fold excess of the free-radical inhibitor propene and/or toluene. The rate coefficients were determined manometrically, and the temperature was kept to better than f0.2 OC with a calibrated iron-constantan thermocouple. N o temperature gradient was found in the reaction vessel, and the substrates were injected directly into the reaction vessel with a syringe through a silicon rubber septum. For the kinetic determination of 3-bromopropionitrile, the reaction vessel was seasoned before each run.
Results
3-Bromopropionitrileat 449.8 OC time, min 7 12 17 22 decomp, % (press.) 24.8 41.5 52.6 61.7 HBr, % (titration) 23.9 43.9 48.2 61.3 4-Bromobutyronitrileat 419.5 OC time, min 7 13 20 30 decomp, % (press.) 14.1 21.9 32.3 43.5 HBr, % (titration) 14.9 21.0 33.1 44.0
45 59.6 60.9
60 70.2 69.5
5-Bromovaleronitrileat 430.0 OC time, min 3 6 10 16 decomp, % (press.) 19.8 30.9 44.7 61.8 HBr, % (titration) 23.6 29.2 44.1 62.6
20 69.2 67.6
25 76.4 74.7
"The percentage of error is less than 5%. TABLE II: Ratio of Final to Initial Pressure" temp, OC Po, Torr Pr, Torr Pr/Pn
430.0 440.5 459.8 470.1
a
109 98 76 84
430.0 439.8 450.6 460.3
55.5 74 46 61
420.0 430.0 439.6 450.0
60.5 109.5
av
3-Bromopropionitrile 201.5 1.85 f 0.03 186.5 1.90 f 0.03 147 1.93 f 0.02 158 1.88 f 0.03
1.89 f 0.03
4-Bromobutyronitrile 101.5 1.83 f 0.04 145.5 1.97 f 0.03 92 2.00 f 0.02 126.5 2.07 f 0.02
1.97 f 0.10
5-Bromovaleronitrile 123.5 2.04 f 0.03 219.5 2.00 f 0.03 75 146.5 1.95 f 0.03 61.5 114 1.85 f 0.03
1.96 f 0.08
Under maximum inhibition of propene and/or toluene.
toluene, was examined by comparing the amount of decomposition of the substrate calculated from the pressure increase with that obtained by direct titration of the hydrogen bromide produced (Table I).
--
The stoichiometry for reactions 1-3, in a static system seasoned with allyl bromide and maximally inhibited with propene and/or
BrCH2CH2CH2CN
Barroeta, N.; Rincbn, M. Acta Cient. Venez. 1976, 27, 247. (2) King, K. D.J . Chqm. SOC.,Faraday Trans. 1 1978, 74, 912. (3) Chuchani, G.;Martin, I.; Rotinov, A,; Hernindez, A. J. A,; Reikonnen, N. J . Phys. Chem. 1984,88, 1563.
BrCH2CHzCH2CHzCN
BrCH2CH2CN
+
CH2=CHCN CH2=CHCH2CN
HBr f
(1)
HBr
(2)
(1)
0022-365418912093-0203$01.50/0
CH&H=CHCN
-
tcis.tran.9
CH2=CHCH2CH2CN
0 1989 American Chemical Society
-I- HBr
(3)
204
The Journal of Physical Chemistry, Vol. 93, No. 1, 1989
TABLE III: Effect of the Inhibitor on Rates4 PS? 104kI, s-1 Torr Pi,Torr PJP, (pressure) 3-Bromopropionitrile at 440.5 OC 98 16.08 f 0.63 117 306 2.62 5.56 f 0.28 68 244.5 3.60 3.99 f 0.18 76.5 359 4.69 3.97 f 0.20 40 316 7.90 3.84 f 0.15
8.40 f 0.40 6.40 f 0.30 3.94 f 0.13 3.84 f 0.05 3.66 f 0.15
121.5 128.5 100 117 76.5 48.5
4-Bromobutyronitrile at 419.5 OC 6.66 f 0.24 148 1.15 6.87 f 0.32 247 2.47 3.37 f 0.16 357.5 3.06 3.10 f 0.15 324.5 4.24 3.23 f 0.15 278.5 5.74 3.40 f 0.17
2.57 f 0.13 3.02 f 0.15 3.46 f 0.16 3.22 f 0.08 3.16 f 0.09 3.34 f 0.10
56 120.5 101.5 100 62 75
5-Bromovaleronitrile at 439.6 O C 10.37 f 0.80 10.55 f 0.50 17.89 f 0.25 86.5 0.72 24.63 f 1.06 17.19 f 0.30 166 1.64 22.43 f 1.10 17.24 f 0.20 232.5 2.33 22.31 f 0.92 17.46 f 0.21 196 3.16 19.04 f 1.08 17.54 f 0.17 338 4.51 17.48 f 0.28
104kl,s-I (titration)
P, = pressure of the substrate, Pi = pressure of the inhibitor. As an additional check on the stoichiometry of reactions 1-3, the pyrolyses were carried out until no further increase in pressure was observed. The average ratio of the final pressure, Pf, to the initial pressure, Po, which should be equal to 2, was found at four temperatures and 10 half-lives to be a little less than 2 (Table 11). The small departure from the theoretical Pf = 2P0 results from the very small polymerization of the corresponding olefinic nitrile products (eq 1-3). The primary products from these pyrolysis reactions, within the range of rate determination, were the following: (a) 3Bromopropionitrile, up to 65% reaction, gave acrylonitrile and HBr gas. (b) 4-Bromobutyronitrile, up to 70% reaction, gave around 57% allyl cyanide, 11% cis-crotonitrile, 32% trans-crotonitrile, and HBr gas. The formation of cis- and trans-crotonotriles arises from the rapid isomerization and equilibrium of the primary product allyl cyanide in the presence of HBr gas. (c) 5-Bromovaleronitrile, up to 80% reaction, gave mostly 4-pentenonitrile, traces of unsaturated isomers (
