3068
of competition reactions. These experiments are briefly summarized as follows. (1) Addcd bromide, hydrogen bromide, butene-1 or isobutene cause no change in the observed rate of isomerization though the over-all rate of decay of the spectrum is increased slightly. ( 2 ) Added t-butyl bromide and t-butyl bromide-dl cause an increase in the rate of isomerization. For instance, the rate is doubled by the addition of about 6% of t-butyl bromide. However, in both cases a tenline spectrum is produced, whereas the (CH3)&H2D radical gives a nine-line spectrum. (3) Irradiation of a 1:l mixture of isobutyl bromide and t-butyl bromidedl gives a mixture of fiveline and nine-line spectra initially; on standing, the ten-line spectrum of the t-butyl radical appears, but there is no increase in the size of the nine-line spectrum. (4) The removaI of traces of f-butyl bromide by repeated gas chromatographic separation (i.e., to less than 0.1%) caused the rate of the observed reaction to decrease by a factor of 50 to 100. (.5) The rate of isomerization was shown to be insensitive to changes in surface area by adding powdered quartz before irradiation. (6) Continuous irradiation with visible light has no effect on the rate. These results are to be compared with observations on y-irradiated isobutene and isobutane which gave mixtures of five- and ten-line spectra (attributed to isobutyl and t-butyl radicals) but, provide no evidence for isomerization.
KOTES
if the observed reaction is simply intramolecular isomerization, it is difficult to account for the extraordinary sensitivity toward traces of t-butyl bromide. The effect is certainly not chemical, so it must result from modification of the trapping sites of the isobutyl radicals. Without much further study one can only speculate about the nature of such a modification. It may be a simple physical distortion resulting in greater mohility for the trapped radical, or it may be a change in the local crystal fields which reduces the activation energy of the isomerization step. Perhaps more likely is the production of metastable domains on rapid quenching of the doped samples: the increased mobility of the radicals during the consequent slow phase change, together with the release of energy during this process, may allow isomerization to occur in these systems but not in a very pure sample. Some earlier observations made in this laboratory indicate a relationship between phase changes in solid chloroform and carbon tetrachloride and slow, reversible changes in the e.s.r. spectra following y-irradiation.6 Acknowledgment. We wish to acknowledge generous financial support from D.S.I.R. in the form of maintenance and capital grants. We are also grateful to Professor F. S. Dainton, F.R.S., for permitting the use of radiation facilities presented to him by the Rockefeller Foundation. (6) A. P. McCann, P1i.D. Thesis, University of Leeds, England, 1961.
Discussion
Thermal Reactions of Organic Nitrogen
One of the special problems encountered TT hen applying e.s.r. methods to the study of reactions in solid matrices is associated with the necessarily low activation energy of processes occurring in solids a t low temperatures. Because of this, the rate of, for instance, recombination of trapped species is likely to be greatly influenced by variations in the local environment which affect the rate of diffusion through the matrix. Reactions occurring in the trapping site, e.g., intramolecular isomerization, should not be SO sensitive to such environmental factors. The evidence presented in this paper appears t o contradict this view if this is indeed the observed reaction. Apart from recombmation, the three most likely reactions of isobutyl radicals iri this system are (1) isomerization, ( 2 ) I.)romine abstraction, and ( 3 ) addition to olefin. The experiments using monodcutcrated t-butyl bromide show that bromine abstraction is negligible, and addition to olefin is eliminated by the negligible effect of addcd butetie-1 and isobutene On the other hand,
Compounds.
T h e Journal of Physical Chemistru
111. 1-1sopropylpyrrole
by I. A. Jacobson, Jr., and H. B. Jensen Laramie Petroleum Research Center, Bureau of M i n e s , U . S. Department of the Interior, Laramie, W y o m i n g (Receiaed .Vouember 4, 1965)
This is the third and final publication of a series dealing with the thermal reactions of alkylpyrrolcs. Only a limited amount of the literature on tlierinal reactions of organic nitrogen compounds deals with pyrrolic cornpound~,~-7 arid the majority of tho perti(1) G. Ciarnician and P. Maghaghi, Wer., 18, 1828 (1885). (2) G. Ciarnician and 1'. Silber, ibid., 2 0 , 698 (1887). (3) P. Crespieux and A. Pictet, ibid., 28, 1904 (1895). (4) A. Pictet, ibid., 37, 2792 (1904): 38, 1946 (1905). ( 5 ) A. G. Oosterhuis and J. P. Wibaut, Rec. tran. chim., 5 5 , 348 (1936). (6) W.Reppe. et al., Ann., 596, 80 (1955).
NOTES
nent papers appeared more than 50 years ago. The first paper8 of the Bureau's study of pyrrolic shale oil compounds described the thermal reactions of 1methylpyrrole and the secondY dealt with l-n-butylpyrrolc. To obtain information on the thermal reactions of other types of alkyl-substituted pyrroles, 1-isopropylpyrrole mas selected as a representative branched-chain alkyl member. Experimental
Materials. 1-Isopropylpyrrole was synthesized by the thermal decomposition of the diisopropylamine salt of mucic acidlo and was purified by distillation, refluxing with calcium hydride, and redistillation. The final product was more than 98 mole % pure as determined by a freezing-point method. 2-Isopropylpyrrole and 3-isopropylpyrrole were prepared for use in some flow studies and for g.1.c. calibrations. A silver nitrate solution (0.25 mole in 50 ml. of water) was stirred into an equimolar mixture of pyrrole (0.25 mole), 2-iodopropaneJ and sodium bicarbonate in 50 ml. of toluene. The heterogeneous mixture was heated and stirred a t 65" for 6 hr." The 2- and 3-isopropylpyrrole were separated from the reaction mixture and distilled. G.1.c. studies on selected fractions indicated a purity of about 97% for the 3-isopropylpyrrole, and a purity of about 72% for the 2-isopropylpyrrole. The principal contaminant in each case was the other isomer. Apparatus. The experimental work was performed using both a flow system and a static system. The equipment has been previously described. The flow system's collection train was modified so that a representative sample of the liquid and of the gaseous products could be collected after conditions had stabilized. Procedure. Flow studies were performed using the general procedure previously described. Flow runs were made on 1-isopropylpyrrole a t selected temperatures between 465 and 575"; several residence times were used a t each temperature studied. The flow runs were made using combinations of the following conditions: empty reaction tube, packed reaction tube, presence of a diluent gas (purified nitrogen), absence of a diluent gas, and presence of nitric oxide. For all runs, the products were vented until steady-state conditions werc obtained. This was assumed to be after 1 nil. of sample had been introduced into the reaction system. The product flow then was directed through a liquid nitrogen trap. The material that did not condense was collected in an evacuated gas bulb. After the completion of a run, the material that had condensed in the liquid nitrogen trap was warmed to
3069
room temperature and the gases that evolved were collected in the same gas bulb. Flow runs on 2- and 3-isopropylpyrrole were made in the same manner. These compounds were studied a t selected temperatures in the range of 500 to 550 ". Static thermal runs were made on l-isopropylpyrrole in the manner previously d e ~ c r i b e d . ~These runs were made between 340 and 400 " with several residence times used a t each temperature. Analysis. Relative concentrations of the various compounds in the liquid products were determined by g.l.c., and the composition of the gaseous products was determined by mass spectral analysis. Results and Discussion It was found that 1-isopropylpyrrole isomerized irreversibly to 2-isopropylpyrrole which in turn isomerized reversibly to 3-isopropylpyrrole. These isomerization reactions were not affected by changes in the area-to-volume ratio, by presence or absence of an inert diluent gas, or by addition of nitric oxide. These data indicate that these isomerization reactions are homogeneous, nonchain, and unimolecular. There were decomposition reactions in conjunction with the various isomerization reactions. The decomposition products from the 1-isopropylpyrrole work consisted of 3-methylpyridine, pyrrole, 2-methylpyrrole, 3-methylpyrrole, 2-ethylpyrroleJ 3-ethylpyrrole, and hydrocarbons. The absence of 1-substituted pyrroles in thc decomposition products demonstrated that the alkyl group had isomerized to the 2-position of the pyrrole ring before any decomposition took place. The relative concentrations of the deconiposition products were affected by changes in the areato-volume ratio, by absence or presence of any inert diluent gas, and by addition of nitric oxide. These data indicate that these decomposition reactions were heterogeneous, free-radical reactions. Samples of 2-isopropylpyrrole and 3-isopropylpyrrole were also thermally studied. When either of these compounds was the starting material, the reaction products consisted of a mixture of 2-isopropylpyrrole and 3-isopropylpyrrole and of decomposition materials. No 1-isopropylpyrrole was found in the reaction products, thus demonstrating the irreversibility of the 1~
~~
(7) J. M. Patterson and P. Drenchko, J . O ~ Q .Chem., 2 7 , 1650 (1962). (8) I. A. Jacobson, Jr., H . H. Heady, and G. U. Dinneen, J . P h y s . Chem., 6 2 , 1863 (1958). (9) I. A. Jacobson, Jr., and H. B. Jensen, i b i d . , 6 6 , 1245 (1962).
(10) L. C. Craig and 12. M. Hixon, J . Am. Chem. SOC.,5 3 , 187 (1931). (11) The authors thank Dr. C. A. VanderWerf of the University of Kansas for suggesting this synthesis method.
Volume 68, Number 10 October, 1965
SOTES
3070
to 2- isornerization and the reversibility of the 2- to 3isoinerization. The preceding data demonstrated that the only route by which 1-isopropylpyrrolc disappeared was by an irreversible isonierization to 2-isopropylpyrrole. First-order ratc equations were used for the kinetic calculation of the isomerization of 1-isopropylpyrrole to 2-isopropylpyrrole based on the disappearance of the I- isomer. The specific reaction rate constant was calculated for each temperature studied for both the static and the flow work. Froni these specific reaction rate constants, activation energies were calculated for the flow work and for the static work. Thesc two activation energies showed no significant difference ; therefore, the data from both the flow work and from the static work were used to evaluate the Arrhenius equation for the isoinerization of 1-isopropylpyrrole to the 2-isopropylpyrrole isomer. I n the temperature range studied, 340 to 575", this equation was found to be
kI2 = 1.10
x
10]3~-(55.500
+ ZSOO/rZU
sec.-l
The two previous papers of this seriesB9have reported the isoriierization reactions of 1-methylpyrrole and 1-n-butylpyrrole. From these results and those reported for 1-isopropylpyrrole in this paper, it was found that in each case the alkyl group isomerized by the route 1-alkylpyrrole
-+
2-alkylpyrrole S 3-alkylpyrrole
A H * , kcel./moleb
km krz
-1.3 - 29 - 10
56.8 f 1 . 6 3 4 . 5 rt 3 . 9 48.1 f 2 . 7
n-Bu tylpyrrole kl% k2I krz
-0.7 - 15 -8.9
hz
Isopropylpyrrole -2.7
k20
kaz
j 6 . 4 rt 0 . 2 44.0 f 1 . 5 4 8 . 3 i. 1 . 7
- 14 - 12
54.0 f 0 . 6 44.7 rt 3 . 7 4 5 . 2 =k 3 . 8
kiz
For the reaction: 1 alkylpyrrole slkylpyrrole. de v.iat,ion.
--f
k23
2-alkylpyrrole
kaz
3-
The plus or minus values are for one standard
T h e Journal of Physical Chemistry
Resistance-Pressure Relation of the Reaction Product of Pyromellitic Anhydride
by $J. H. Lupinski, C. 11. Huggins, and €1. G. Pfeiffer
Electrically conducting powders exhibiting a strong pressure dependence have long been known as, for exaniple, in the carbon microphone. Thesc materials have not been overly successful as pressure-measuring devices because of drift and hysteresis in the pressureconductivity relationship. In several recent publications,l-3 a new family of pressure-sensitive compounds has been reported. Some theoretical arguments were presented which correlated the pressure-conductivity relation with their intrinsic molecular properties rather than with the area and quality of contact between the
h'tethylpyrrole k2z
(12) J. E. Leffler, J . Org. Chem., 20, 1202 (1955).
General Electric Research Laboratory, Schenectady, h'ew York (Received J a n u a r y IO, 1964)
Table I : Entropies and Heats of Activation for the Isomerization Reactions of Alkylpyrroles a t 500" AS*, e.u.
Acknowledgnaent. Thanks are extended to AI'I Research Project 52 for purification and purity deterinination of 1-isopropylpyrrole. The work reported in this note was done under a cooperative agreeinent between the University of Wyoming and the Bureau of JIines, U. S. Departinent of the Interior.
and Pyrene
Therefore, a total of nine isomerization reactions have been studied-three for each compound.
Reactiona
Entropies and heats of activation were calculated for all nine of the pyrrole isomerizations studied. These calculated values are presented in Table I. When the heat of activation was plotted against the entropy of activation, a straight line resulted. The slope of this line, as deterinined by a least-squares niethod, weighing each point in proportion to its variance, gave thc isoltinetic teniperature. This temperature was found to Le 860°K. and had a correlation index of 0.92. The existence of this linear relationship indicates that either the reaction mechanism or the transition state is siinilar for all nine isomerization reactions. l 2 The studies were all carried out below the isokinetic teniperature; therefore, the isomerization reactions were enthalpy controlled.
(1) J. J. Brophy and J. W. Buttrey, "Organic Semiconductors," The SIacinillan Company, New York, N.Y., 1962, p. 143. (2) 11. A. Pohl and E. H. Engelhardt, J . P h y s . Chem., 6 6 , 2085 (1982). (3) H. A. Pohl, A. Rembaum, and 4.Henry, J . Am. Chem. Soc., 84, 2699 (1962).
