NoTEs
1231
In view of the fact that the splitting betuecm the lines is not due to hyperfine interaction, the spectrum may he analyzed using a simple form of the spin Hatniltoriian
x = pzigl!!
(1)
whew S = 1 1 2 , p is the Bohr magneton, and g is a tensor. The resonance condition is given by
hv = gpH (2, By pulling thc specirrien in the form of thread, there is molecular orientation along the draw axis. If each ion has axial synimetry, then gz = gv = g;l g L = 911; and g2 = g,,2 (vs2e g12 siri2e,where e is the azimuthal angle. I The symmetrical variation in intensity of the spectrum due to the peroxy radical in stretched polypropylene with rcspwt to angular orientation suggests that corresponding to thch 0 orientation, it has the highest field arid lowest crnter of gravity, gl. This strongly suggests that thc 0 0 bonds of thc pcroxy group are perpendicular to the draw axis. The highest g is expected to he nearly parallel to the 0-0 bond.
+
O
Acknowledgment. The author is thankful to Prof. R. Mecke for permission to use the e.p.r. instrurnent.
Decarboxylation.
I.
Kinetic Study of the
Vapor Phase Thermal Decarboxylation of 3-Rutenoic Acid
by Grant, Gill Smith and Sullivan E. H a u l D e p r t m d of Chimistry, Ctah State University, Logan, C'tah (Recticrd Septemhcr 28, 1.968)
Decarboxylation is an important rcAaction in many branches of organic chemistry, e.g., synthetic organic, mechanism studies, arid many life processes in both plants and aninials. JZost studies have been in the condensed phase under catalytic conditions. Several rcviews have appeared, some very recently, dcalirig with the decarboxylat,ion under acidic and basic conditions.2 I ~ s is s known about how structural changes influence vapor phase thcrnial decarboxylations. In condensed phase systems where solvcnt, catalysts, and other tLnvironmcnta1 conditions niarkedly influence the rate of the reaction it is difficult to evaluate the individual influences. A better understanding of the details of decarboxylation can be obtained through a
fundamental approach on a carefully defined system. Homogeneous vapor phasr reactions provide this system. This is the first of a swies of kinetic studies on the effect of structure on t h r ease of drcarboxylation of organic acids in the vapor phase.
Experimental Reagents. 3-Butenox Acid. 8-htenoic acid was prepared by hydrolyzing 67 g. (1.0 mole) of allyl cyanido (Natheson Coleman and Bell, practical grade) with 100 nil. (1.2 tiiol(2s) of concentrated hydrochloric acid according to the niethod of Reitz.3 The purified acid was collected at 88-90' (34 n m . ) , Literature values are 69-70' (12 mm.) 7 ~ ~ 1.4242. ~ 1 ) and n T 5 u1.4237.4 Cyclohezene. Cyclohexcme (Matheson Coleman and M l ) was stored 3 days over sodium and distilled from sodium through a 20-in. Stedrnan column. After discarding a IOCr, forecut, cyclohexcnc was collected a t 77.5' (645 mm.), n * ' ~1.4411. Reported values are 81 (725 mm.)arid n2%1.4441.5 Kinetzcs. 3-Rutenoic acid was injected with a microsyringe arid needle into a stainless steel reaction chamber as a 1: 6 mole ratio solution in cyclohexene. The increase in pressure as the reaction progressed was plotted continuously on a 2-niv. strip chart recorder. The apparatus is described in detail elsewhcre.6 The temperature was constant during a run to < + O 2", being nieasured with two chromel-alumel thermocouples in sc2ric.s mouritchd in the aluminurn therniostat on either side of the reaction vessel; these thermocouples were calibrated to +0.1 O against a platinum resistance thermometer. Homogeneity. The homogeririty of the reaction was checked by packing thc reaction chamber with a stainless sted sponge (Gottschalk Yo. 723) which iricrcasrd the surface area more than tcn times while leaving the volume essentially unchanged. Pyodurt Analysis. The products of the reaction were collected in a trap cooled in liquid nitrogcri as they were rern0vc.d froni the reaction vessel after a run. O
(1) This study is abstracted from a thesis presented t.o the Graduate S(8hool. 1Ttah State I'niversity, by S. E. Blnu in p:irt,i:il fulfillment of the requirements for the degree of M.S..Sept.. 1903. (2) (a) R. H. Brown. Quart. R (London). 5 , 131 (1952); (b) E. M , Kosower, "Molecwl:~r Bioeher try." .\I(Gr:iw-liill Rook (-0..New York, N. Y., 1962, p. 71; (I:) I,. L. Ingraham. "Riocheniic:rl Met-hanisms." John Wiley Br Sons, Inc.. New l-ork, N. Y.,1902, p. 58. ( 3 ) E. Ileitz. Org. S y n . , 24, 00 (1944). (4) I. 11. HeiLbron, "Dirtionary of OrRaniv ('ompourids," Vol, 4, Oxford Irniversity Press, New York, N. Y..1953, p. (165.
( 5 ) A. J. Streiff. *J. C . Zimmerrrian, L. 1:. Soule, .\I. 'r. B u t t , V, A. Sedlnk. C. B. Willingham, and 1 . D. Rossirii, .J. Rcu. .Val(. Rlcr. Std., 41, 357 (19.18). (6) (a) C:. G . Smith and 1:. D. Ragley. Rrr. Sci. Instr., 32, 703 (1961); (b) C . 0 . Sniith and D. A . K . Jones, J . Orll. f'hrm.. 28, 3496 (1903).
NOTEB
1232
Several 0.3 to 0.4-nil. samples of vapor were withdrawn froni the trap by means of a hypodermic syringe and an 11-in. needle as the trap slowly warmed up from liquid nitrogctn to Dry Ice temperature ; the products were soparated on a 2-m. lorig silica gel gas chromatograph column maintained at 241 '. Identification of the products was niado by comparing their retention tirnes with CO, from Dry Ice arid propene obtained from the phosphoric acid arihydridc dehydration of isopropyl alcohol.
Results Kinetics o j the Decarboxylation o j 3-Rutenoic Acid. Determinations of thc rates of decarboxylation of 3buterioic acid werc made at temperatures ranging from 334.6 f 0.2' to 378.4 f 0.2". The rate constants for the reaction were obtained hy plotting log (Zfm - I$() against time, where I!: is equal to the output of the pressure transducw as plotted on the strip chart recorder and is proportional to the pressure inside the rcaction chamber. Even though product analysis by vapor phase chromatography showed only two products (CO, and propene) there was apparently a very slow secondary reaction (ca. '/bo the rate of the decarboxylation) that caused the prcssure to increase beyond that expected froni the decarboxylation. For this reason E a t 8 half-lives' (98.7%) was taken as Em. Where this value of E , was used in determining the rate, straight lines werc obtained in first-order plots through 75-90y0 of the reaction. The data for these detcrniinations are shown in Tablc I. The rate of the reaction at a given temperature was found to be iridcpcndcnt of thc initial pressure. This confirmed tho order of the reaction indicated by the straight lines obtained in the first-ordcr plots. The data in Table I1 demonstrate that the reaction was essentially homogeneous. The surface : volunie ratio was incrcased in cxccss of ten times while the ratio of the rate in thc packed reaction vessel to that of the unpacked vessel was 1.20, which is considered satisfactory.8 The Arrhcnius plot obtained by treating the data by the method of least, squares gave E , = 39.3 + 1.6 kcal./riiolc and A S * = - 10.2 f 2.5 C.U. a t 650'K. I'rodiict Analysis. Three poaks were obtained when the products \vcro separated in the gas chromatograph. The first pcak was nitrogen which was used to flush out tho reaction chamber. The retention tinics of the other two peaks corresponded to thc rctcntion tinics of known satnplcs of CO, arid proponc. The retention times for thc reactioii products and the known conipounds arc shown in Table 111. The .JOltrnd of I'husical Chemistry
Table I : Kinetic Data for t,he Decarboxylation of 3-13utenoic Acid 1OSk.
'Ielll~l.,
Run
"C.
122 123 124 125
IIP/"
-log k
8 w - I
369.6 3tj9, 6 369 . 6 389. 4
1.556 1 556 1.556 1 556
2.03 2.06 2.05 2.03
9.31 8.78 8.89 0.33
128 129 130
358 8 358.8 358.6
1 5x2 1.582 1 . 683
2.23 2.24 2.23
5.8.5 5.74 5 !IO
132 133 134 135
348 6 34x, 3 347.8 348.0
1.609 1.609 1.till 1.610
2.45 2.48 2.46 2.46
3.53 3.32 3.46 X46
138 139 I40
334.8 334.7 :33 4 . 2
1.645 1 ,645 1.646
2.77 2 77 2.77
1 71 1.73 1.69
146 147 148 149
378.6 378.5 378.5 378.2
1.534 I . 535 1 ,535 1.536
1.80 1.81 1.81 1.81
16.0 15.4 15.5 15.5
Av. IO' k
dev.
9.08
0.28
5 83
0.08
3.44
0.09
1.71
0.02
15.6
0.28
Std.
Table I1 : Homogeneity of Decomposition Temp.,
.-lOZk,
-....
aec,
Run
O C .
Unpacked
216 217 218
369.6 369.6 369.6
1.01 1.00 0.99
222 223 224
369.6 369.6 369,6
-I
Packed
-
1.20 1.18 1.22
~~
Table 111: Retention Times of the P r o d u c t s of the Reaction ---------.Retention Compound
Nz CO, Propene
Standards
0 71 1 28 2 68
times, inin.----Reaction products
0 71 1 27 2 68
Discussion Thermal decarboxylation of p, y-unsaturated acids in the vapor phase procccds siiioothly to olcfiri arid (7) The half-life used was obtained by a method of approximation. The first approximatio7,of the half-life was t l l Z ' , at A E / 2 . The serond approximation was t ~ , / ~= E / 2 where E was taken at 8 X t ~ / ~ 'The . third approximation to E , w w set equal t o E a t 8 X 11/~''. Further approximation did not significantly change the value of E - . (8) A. Maccoll and I? J. Thomas, J . Chem. SOC.,979 (1955).
NOTES
1233
carbon dioxide. It has been suggestedg that this deconiposition reaction takes place via a transition state of type I. I t is significant to note, however, that this conclusion is only valid theoretically if the mechanism can bc, proved to he hoth homogeneous and uniniolecular, and have a negative entropy of activation.
iHz!%
CIIZ
H
CHI-CH=CH~
+ COZ
0 CHa
COOH
\
C-c
/
I/
-
c--0-
(:€I2
CsHaN
\
/
/
\
C-CTI
(1)
C-0
\/ 0
/ CH3
/
I This study clearly demonstrates that the thermal decomposition of 8-butenoic acid follows first-order kinetics and that the rate const.ants are independent of the initial prcssure and are essentially independent of the volunie to surface ratio. The products of the reaction, as nicasured by gas phase chromatography, are propcnc arid carbon dioxide, and the rate of their formation shows no inductive periods. These facts are adequatc to exclude radical or radical-chain mechanism and to establish that the reaction is a true homogent~us,unimolecular reaction which proceeds with a negative entropy of activation (-10.2 e.u.). These data provide the strongest. support for the cyclic transition state mechanism (I) for this decarboxylation suggested by Arnold, et UZ.,~who have suggested that a,p-unsaturated acids first rearrange to p, r-unsaturated acids before decarboxylating. A number of reactions have been reported which give prodPreliminary" ucts consistent with this mechanism. studies show that the isomer of 3-buknoic acid, crotonic acid, pyrolyzes a t a much slower rate than 3-butenoic acid. A determination of the rate of decarboxylation of crotonic acid will be, therefore, a measure of the rearrangement of crotonic acid to 3-butenoic acid. A similar mechanism has been proposed for the condensed phase decarboxylation of a number of alkylidene malonic acid derivatives in pyridine." It was proposed that the +unsaturated acids tautomerize to the P , r isomer which subsequently loses carbon dioxide as the anion (eq. 2 ) . The rates of decarboxylation of the & y acids are found to be faster than their C U , ~ isomers. The thermal decarboxylation of 3-hexendioic acid (ey. 3 ) has been recently report,ed,12and, though no kinetic studies were made, the mechanism
COO13
C"3
CHa
\
C=C H--C 00E t
/ CH, suggested by the investigators is a two-step process equivalent to the one-step process proposed by Arnold, et a1.9
HOOC--CHZ--CTI 1
K
.C€IZ
1 H
I
c-0 \/ :o:
heat
+
C €I +
HOOC-C"2--CH2
/\
C'Hz
+ ().-r(:Ly()
HOO(' --VI12 --Ct12
(3)
c rr2-=cr i Acknowldgmwzt. Acknowlcdgmcnt is kindly given to the donors of the I'ctrolcum Research Fund, ad(9) R . T.Arnold. 0. (:. ICIiner, rriid I?. XI. 1)odson. .I. A m . C h e m Sor., 72, 4359 (1950). (10) (a) I?. T. Arnold and X I , .J. Dniizip, ibid., 79, 892 (1957); (11) E. IC. Rlaise aiid A . ('ourtot, C'ompt. r r n d . , 139, 292 (1904): (c) 0. Wallwh. Ann., 353, 304 (1907): (d) 0. Wnll:ich, ibid., 360, fiX (1908); ( e ) W. ,J. le Noble and 1'. .J. Crean, .I. O r y j . Cham.. 27, 3875 (1962). (11) E. J. Corer, .I. ilm. C'hem. Soc., 74, 5x97 (1952): 75, 1 I63 (1953). (12) F. Rennington and R . D. >forin, .I. Org. Chem., 26, 5210 ( I B ( i 1 ) .
1234
NOTES
Isomet Corp., Palisades Park, N. J., with a stated NI6 content of 95.5 and 97.4%, which was confirmed by us. Normal nitrogen as well as oxygen and hydrogen were AirCo research grade reagents. All gases were passed through traps immersed in suitable freezing mixtures to remove any condensable impurities eventually present. Isotope Exchange in Gaseous Nitrogen under Dosimetry. The nitrous oxide system was used X-Ray and Cobalt-60 y-Irradiation for the determination of the absorbed dose. A value of 12 for G--N~owas adopted for the dose calculations3; the proportions between Nz:O2:YO2 formed were by M. Anbar and P. Perlstein taken as 1:0.14:0.48. The pressure of products, The Weizmann Institute of Science and the Soreq Research noncondensable in liquid nitrogen, was measured after Establishment, Rehovoth, Israel (Received October 3, 1963) irradiating N20in the same vessels. Procedure. The preparation of gas mixtures as well as the filling of the irradiation vessels were perThe radiolytically induced isotopic exchange reacformed on a vacuum line a t lop5 mm. The filling tion between S214,14 and X216,15 was investigated by was carried out using a manifold on which up to eight Klein and Hod, who irradiated solid N214,14-11'215,16 irradiation vessels could be simultaneously filled by mixtures a t 4.2 and 20'K. with 15-20 kev. electrons. means of a Toepler pump and then sealed off. The An isotopic exchange was observed with a low G filling pressures were determined by means of a mercury value (Gx914,15 = 0.1-0.3). On the other hand, nitrogen manometer. A special cold trap was attached to the atoms (active nitrogen) produced in a condensed filling manifold and held in a C02-acetone bath (- 78') discharge were found not to undergo any isotopic to remove mercury vapor originating from the Toepler exchange with nitrogen molecules in the gas phase.2 pump and from the manometer. We have found that nitrogen undergoes isotopic One of the mixtures was prepared in the presence exchange in the gas phase under radiolytic conditions. of a sodium mirror condensed on the walls of the The present work was undertaken to study this isotopic vessel to assure the elimination of any trace of oxygen. exchange reaction and possibly to elucidate its mechaThe vessel was first flushed with pure nitrogen to renism. move any hydrogen which might have been formed Experimental from the reaction between sodium and traces of water Radiation Sources. X-Ray irradiations were perabsorbed on the walls. formed with a Picker Vanguard X-ray unit, with a The composition of the mixtures was in most experiMachlett EG-302 tube. The tube was operated a t ments approximately 1: 1 1\7214714/N215,15. The initial 280 KVP with a current of 20 ma. The y-irradiations amount of "scrambled" nitrogen, N214,15, was between were performed with a 3.5-kc. cobalt-60 source (in a 2.3 and 4.2%. Gammacell 200 produced by The Atomic Energy of I n the X-ray irradiations, the dose rates applied Canada Ltd.) with a dose rate of 5.75 X lo5 rads/hr. were 0.64 x 106 rads/hr., 2.5 X lo6 rads/hr., and Irradiation Vessels. All the vessels were made of most runs were'carried out a t 6.23 X lo6 rads/hr. Pyrex glass with break-off tips. The vessels were The total doses varied between 1.16 X lo7 rads and washed with hot concentrated nitric acid and then 1.25 X lo8rads. with triple-distilled water. Then they were outbaked I n the y-irradiations, the dose rate measured was in a vacuum oven a t 140'. Finally they were outgas5.75 X 105 rads/hr. and the total doses varied from sed on a vacuum line a t 500'. Silica wool-filled 1.84 X l o 7 rads to 1.06 X lo8rads. irradiation vessels were treated alike and the outgassing The temperature in the Coo0 y-irradiations was 33' was rechecked by successive pressure determinations, while in the X-ray irradiations it was 25' initially, until no gas could be desorbed from the silica. but it rose to about 60' for the high dose rate experiThe volume of the vessels was approximately 12 cc. ; ments. the walls were of 1.5-2 mm. thickness. Spherical Analysis. Analyses were performed with a CEC irradiation vessels were used in the X-ray irradiations and vessels of cylindrical shape were irradiated in the (1) R. Klein and E. M. Horl, J . Chem. Phys., 32, 307 (1960). C060 y-experiments. (2) R. A. Back and J. Y . P. Mui, J . Phys. Chem., 66, 1362 (1962). Materials. The nitrogen-15 was obtained from (3) P. Harteck and S.'Dondes, Nucleonics, 14, 66 (1966). ministered by the American Chemical Society, and to the Utah State University Division of University Research for partial support of this research.
