Sept., 1963
PHOTOLYSIS O F
2,3-DIAZABICYCLO [2.2.112-HEPTEKE
1779
PHOTOLYSIS OF BICYCLIC AZO COMPOUNDS.
I. 2,3-DIAZABICYCLO[k?.k?.I]k?-HEPTENE BY COLINSTEEL^ Chemistry Laboratory, Itek Corporation, Lexington, Massachusetts Received January I?', 1,963 The photolysis of 2,3-diazabicyclo [2.2.1]2-heptene (I) has been studied both in solution and in the gas phase. The photolysis in solution yields the same products, namely, nitrogen and bicyclo [2.1.O]pentane (11), as does the pyrolysis. However, in the gas phase cyclopentene (111) is also formed. The ratio of 1II:II formed is pressure sensitive indicating the initial formation of excited I1 and excited 111.
Introduction As part of our work on the reactions of bicyclic azo compounds2 we have recently studied the photolysis of 2,3-diazabicyclo [2.2.1]2-heptene (I). The results are significantly different from those obtained for the thermal decomposition of I, but have close parallelisms to those obtained in the photolysis of ketenes.s Experimental The preparation of I and the identification of the products of pyrolysis, N2 and bicyclo[2.1.0]pentane (II), has already been The structures of the photolysis products, Nz, 11, and cyclopentene (111) were confirmed by their n.m.r. and infrared spectra and by their retention times on various gas chromatographic columns. The added gases NZ (99.996%), 02 (99.6Y,), and KO (99.0%) were all from the Matheson Co. The isooctane was Fisher Spectrogade. Three experimental methods were employed. For the runs a t high pressure, 1.0 to 7.5 cm., a given volume of I a t a known pressure was frozen into a 0.5-cc. cylindrical thin-walled reaction ampoule which was then sealed under high vacuum. In the runs with added gm the latter was added after the azo compound, the amount being determined manometrically. The contenta of the ampoule were photolyzed with light from a 100-watt Osram high-pressure mercury arc which had been pasaed through Corning filters 7 5 4 and 0-53. This combination allows pmsnge of light in the 280 to 400 mp range. The azo compound itself absorbs in the range 300 to 350 mp
.
After photolysis the total contents of the ampoule were analyzed by gas chromatography using an ampoule crusher a t the injection port and a 4-ft. silver nitratcethylene glycol analysis column.6 The relative amounts of 11, 111, and Nl were determined from their peak areas. Pyrolysis of I under similar conditions to the photolysis (except temperature of course) provided a convenient calibration source since our earlier work2 had established that 1 mole of I when pyrolyzed yields 1 mole of NZand 1 mole of 11. Calibration with pure samples of I1 and I11 showed that both materials had the same sensitivity with respect to the thermal conductivity detector. For the runs a t intermediate pressure, 0.07 to 2.0 cm., I was frozen into a 15-cc. vessel which was then sealed. The vessel waa equipped with a small (0.5 oc.) thin-walled side arm and with a break-seal. After the photolysis the condensables were frozen with liquid nitrogen into the side arm which was then drawn off, the contents of the resultant ampoule being analyzed ae before. The noncondensable NZwaa pumped off via the break-seal to a gas buret and measured there.' The presence of any impurity (NO or 0 2 ) was determined by gas chromatographic analysis using a Linde Molecular Sieve column. For the runs a t the lowest pressure, 0.001 cm., the procedure was similar except that a 1000-cc. reaction vessel was employed. All photolyses were carried out in a well-stirred air oven which could be therrnostated to f0.1". In most runs the extent of reaction was 60% or greater; however runs were carried out in which the percentage conversion (1) Chemistry Department, Brandeis University, Waltham, Mass. ( 2 ) S.G. Cohen, R. Zand, and C. Steel, J . Am. Cfwm. SOC.,83, 2895 (1961). (3) H.M. Prey and G. 13. Kstiakowsky, ihid., 79, 6373 (1957). (4) G.B. Xistakowsky and B. H. Mahan, ibid., 74, 2412 (1957). (5) R. Criegee and A. Rimmelin. Chem. Ber.. 90, 414 (1957). (6) R. Smith and R. Ohlson, Acto Chem. Scand., 13, 1253 (1959). (7) C. Steel and M. Srwarc, J . Chem. I'hys., 83, 1677 (1960).
was varied from 5% to greater than 90Cy. I n all cases the ratio of hydrocarbon to nitrogen remained invariant a t 1.0. Monochromatic light for the quantum yield measurements in isooctane solution was obtained with the aid of a Bausch and Lomb grating monochromator. The light intensity incident upon the solution was measured by ferrioxalate while the fraction of the light absorbed by the solution was determined froin the transmission curve of I as recorded by a Beckman DK-1 recording spectrophotometer. The solution of I was contained in a 5-cm. quartz optical cell. After a given time of photolysis this was removed from the light and the extent of reaction determined by the decrease in the absorbance.
Results In agreement with earlier work26it was found that when I was pyrolyzed below 200' the only product besides nitrogen was bicyclo [2.1.0]pentane (11). Above 200', I1 isomerizes thermally to cyclopentene (111). When I, in the gas phase, was photolyzed a t 110' and at a pressure of 0.08 cm., Nz, 11, and cyclopentene (111) were formed in the ratio 1.0:0.55:0.45 (Fig. 1).
I
I1
111
IV
As the pressure of I was increased to 5 cm., the ratio III/II steadily decreased, the analysis at the latter pressure being 1.0:0.9:0.1 When I, at 0.08 cm., was photolyzed in the presence of 60 cm. of Nzthe ratio once again decreased, but as before for every mole of photolytic nitrogen produced 1mole of hydrocarbon (I11 11) was formed: the analysis was 1.0; 0.95; 0.05. In this case, since the photolytic nitrogen could not be determined directly, known amounts of I were photolyzed to completion. Oxygen had an effect similar to that of nitrogen. On the other hand, the addition of 1 cm. of nitric oxide completely suppressed the formation of I11 and 11, while gas chromatographic analysis showed that under these conditions complete photolysis of 1 mole of I still yielded 1 mole of nitrogen. Although due to the low volatility of I we could not vary its pressure over the same range a t 30' we were able to repeat the other experiments and to obtain the same results a t the lower temperature, viz., the quenching effect of O2 and K2 and the suppresion of products by S O . Photolysis of I in methanolic solution a t this temperature gave III/II = 0.014. Photolysis of the pure solid also gave the same value.
+
Discussion The above data are con8istent with the rnechaiiism ( 8 ) C. G. IIatchard and C. A . Parker, I'rar. Roy. SOC.(London), AX36, 518 (1956).
C O I SIEEL ~
1’780 loa !
-4
-3
I
I
cw/k.i).
-2
-1
0
0
-1
. \
-3
-2
-1 log l’azo
(Clil
)
Fig. 1.-Variation in ratio of cyclopentene and tkycIo[2.1.0]pentane formed as a function of sribstrute azo pressure: 0, no added gas; 8, 1.5 mi., 02,0, 60 cin. 01,0 , 60 cm. X2. The full cwve refers to the upper abscissa and is the theoretical curve. For definition of w and k - I see test.
I (2) III* t ,IF’* I-11* (1)
111
I1
We visualize the first step in the photolysis as the production of tlic cyclopciitadiyl biradical (IT’) with excitation energy e* over the ground state; this will rapidly form either 11* or III* by iiiteriial combination and disproportioiiation, respectively. If these species do not suffer a deactivating collision witli the wall or with a gas molecule ( S )then we have tlic rewrse reaction, where 1 ’1;-1 aiid 1 ’k-* arc the lifctiiiics of 11* and HI*, respectively. I n the limiting case of high pressures, wliicli is best approximated i i i solution, tlir effective collision frequency w will bc i n i i r l i greater than 116- and 1 ’12- so that the ratio of 111 ’I1 formed should simply depend upon kz/Icll every III* and I1* molecule formed bring deactivated hefore it has a chance to undergo a reverse reaction, The fact that photolysis in metliaiiolic solution yields predominantly I1 (ie., k1 >> k,) is consistent with the fact that Zf2,the energy of activation for reaction IV + 111, is greater than El, the activation energy for I V + 11. These activation energies and related topics are discussed in dctail in a forthcoming paper on the reactions of small ring compounds.g At this stage we may observe that the activation energy difference is consistent with the fact that pyrolysis of I, which will proceed by the lowest potential path other factors being equal, yields I1 ratlier than 111. I’roiii our knowledge of the heats of formation of I1 and 111 we know that I1 is less stable than I11 t)jr about 28 kcal. niolc-l.10 we By Itice-Ramsperger-Iiasscl (R.R.K.) can show that k-1 >> k2for all reasonable values of e*. This means, if we consider the fate of a IV* species, initially there will be a high probability of finding it in the 11*form but with time there is an increasing probability of finding it in the III* form. Thus, a t low (9) C. Steel, I ’. Hurwit7, R. Zand, s n d S. G. Colien, “The Thermal Isomerization of Bicyrlo[2.1.O]pentari~and l> li2 tlic photolysis can be regarded as a method for producing a “hot” cyclopropane derivative (II*) which, unless quenclied, isomerizes to the corresponding olefin (HI*): in this respect there is a close similarity between our results and those of Frey and l i i s t i a k o ~ s k y who , ~ showed that ‘ % ~ tcyclopro” pane formed by the addition of methylene to ethylene isomerizes to propylene unless quenched. Tliis simplified inechanism suggests the following semiquantitative argument with the assumption that every collision which an excited II* suffers is deactivating. At pressure P all 11* molecules whose lifetime is less than 1 / w p will, on the average, react to form III* and subsequently 111. If their lifetime is greater than l / w P then they will, on the average, be deactivated to form 11. Thus the experimental curve in Fig. 1 can be interpreted as follows: twenty per cent of the molesec. (which is the time cules have lifetimes > between collisions a t cm.). Forty per cent have lifetimes > sec.; 60% have lifetimes > IOp6sec.; arid so on. This would imply that the 11* iriolccules are formed with a more or less uniform spectrum of energies, having lifetimes in the range lo-* to lo-* sec. Kow sirice we knobvn the activation energy for reaction I1 + I11 is 46 kcal. inole-’ l 2 and the number of effective oscillators which the molecule possesses is 18,lZ it can readily be shown by the R.R.K. theory that the energy of 11* must lie in the range 62 to 82 kcal. mole.-’ Since the strength of the C-C bridgclicad bond in I1 is known to be about 30 kcal. mole-’ the exce~s energy associated with IV* must lie in the range 32 to (12) M. L. IIalberstadt a n d J. P. Cliesick, J . Am. Chem. SOC., 84, 2688 (1962).
Sept., 1963
EXCHANGE SORPTION IN
THE
SYSTEMS Na-Cs-Ba MONTMORILLONITE
52 kcal. mole-’. This range is a little larger than might be expected because the energy difference between 1 einstein of 350 mp radiation and 1 einstein of 300 mp radiation is 13 kcal. mole-I. (iii) From their studies on the photolysis of methyl ketene, Kistiakowsky and Mahan4 showed that only “hot” ethylidene biradicals undergo internal disproportionation to form ethylene. The equivalent situation exists in our system in that I11 is formed less readily than I1 from IV. Of course, the detailed mechanisms of hydrogen migration and concbmitant double bond formation must differ somewhat in the two reactions. (iv) Because of the undoubtedly low value of‘ E,, the activation energy of reaction 1, for any reasonable value of e* the lifetime of IV* will not be much greater than a vibration frequency. This picture of a shortlived biradical may be an oversimplification. If the carbons associated with the lone electrons are sp3 hybridized then the free electrons are in orbitals pointing away from each other and there may well be a small energy barrier to the formation of the strained C-C bridgehead bond in 11. Thus, a t very low temperatures one might be able to detect IV. However, if the carbons are sp2 hybridized, as in the case of a methyl radical, then the electrons would be in p-orbitals perpendicular to the ring and we should expect interaction under all conditions. The use of the term biradical would then be incorrect in the strict sense. The reaction might better be regarded as molecular with the concerted elimination of nitrogen and formation of the
1781
C-C bridgehead bond to fok-m 11, or migration of hydrogen to form 111. Because of the nitrogen these transition states will differ somewhat from that for the reaction I1 & 111. (v) The absence of reaction with oxygen other than as an inert gas indicates the absence of a triplet biradical. It is known for example that 0 2 reacts rapidly with triplet methylene13 but not with singlet methylene . (vi) The reaction with nitric oxide is often regarded as a test for free radicals. However, in this system nitric oxide may be reacting with 11* and III* directly rather than with the biradical intermediate IV*. We hope that experiments with mixed added gases and metallic mirrors will answer some of these problems. (vii) Quantum yield measurements, in terms of consumption of I, in isooctane solution gave the value 1.0 f 0.10 (for both 313 and 334 mp radiation). Thus, the primary step is unexceptional with no complications: from internal or external quenching or from fluorescence.l5 Acknowledgment.-It is a pleasure to acknowledge helpful discussions with Dr. G. B. Kistiakowsky (Harvard University) and Dr. M. Rosenblum (Brandeis University). The azo compound was prepared by Dr. R. Zand in work done under N.S.F. Grant G-14049, Department of Chemistry, Brandeis University. (13) H. M. Frey, J . Am. Chem Soc., 82, 5947 (1960). (14) A. W. Strachan a n d W.A. Noyes, Jr., ibid., 76,3258 (1954). (15) The absence of fluorescence was confirmed b y Mr. K. Norland of Brandeis who kindly ran the fluorescence spectrum,
ADSORPTION STUDIES ON CLAY MINERALS. 17111. A COXSISTENCY TEST OF EXCHANGE SORPTION IN THE SYSTEMS SODIUM-CESIUM-BARIUM MONTMORILLONITE1 BY RUSSELLJ. LEWISAND HENRYC. THOMAS Department of Chemistry, University of North Carolina, Chapel Hill,N . C. Received January 25, 1563
-
Standard free energies of exchange from solutions of the chlorides a t 0.04 N on a purified montmorillonite clay Na, -2152 cal./mole; Ba 2Na, -496 cal./mole; have been determined for the three reactions: Cs 2Cs -+ Ba, -3677 cal./mole. These satisfy the requirement of additivity. Activity coefficients as functions of surface composition are given.
Studies of the ion-exchange behavior of siliceous minerals have in rnost cases been confined to single pairs of ions so that tests of the additivity of the standard free energies of the reactions have not often been made. Such additivity is a minimum requirement of consistency in this work. It has, for example, been demonstrated conclusively for Li-Na-K exchanges in sodalite.2 In the only case involving clay minerals of which the writers are aware3 the result was not entirely convincing. Such a consistency test is particularly desirable in work with the clays, where the definition of the extent of the surface region under study, as determined by the nature of the experiment, can give different results with different ions. Thus the ion-exchange (1) We are indebted t o the International Atomic Energy Agenoy for a grant which made this work possible. 12) R. M. Barrer a n d J. D. Falconer, Proc. Rou. SOC.(London), A236, 234 (1956). (3) C.N. Merriam, Jr., a n d H. C. Thomas, J . Chem. Phys., 24, 995 (1956).
-+
“capacity” of a clay mineral as determined by a chromatographic elution depends directly on the method used for the determination of the free volume of the column. Results differing by as much as 5% in the capacity are obtained when comparing a free volume determined by weight with that determined by isotopic anion elution in the case of montmorillonite in contact with sodium chloride solution. In this case the anion is definitely repelled from the clay surface. Such effects if undetected might produce serious discrepancies in thermodynamic calculations. The effect is very small or absent in the presence of kesium and barium. We report here examinations of the equilibria of the three pairs of Na+, Cs+, Ba++ on a purified wontmorillonite which satisfy the consistency test. The clay used in this work mas the Reference Clay Mineral KO. 23 of the American Petroleum Institute, from Chambers, Arizona. I n unpurified form this material
