Jan., 1961
NOTES
185
packed into a 220 cm. column of Pyrex glass tubing of 0.45 cm. internal diameter which was wound into a helix 4.0 cm. in diameter and 20 om. high. This form of the column was convenient for high temperature treatment and for immersion in suitable cryostatic baths used in the retention time measurements. The OSSAR sample weighed 16.5 g. and its specific surface area waa 100 m.*/g. In the most definitive measurements of the present research, the column was dried overnight in a stream of dry helium at 480”. This treatment presumably removed substantially all the adsorbed water without destruction of the hydroxyapatite structure. The nitrogen pulses effluent from the column were observed by means of a Gow-Mac katharometer and were traced on a standard type of recorder. The flow rate employed was 35 cm.a per minute. The input pressure of the helium stream was maintained constant by a simple device described by James and Martin.6 The volume of the nitrogen pulses contained between two four-way stopcocks was 2.3 0111.3. The pulses used were either pure nitrogen or a mixture of 1% nitrogen with 99% helium. The observed retention time measured at the peak of the recorded effluent nitrogen pulse was corrected to allow for the time that mould have been required for pawage through a non-adsorbing column. The magnitude of this correction at each temperature employed was determined by substituting hydrogen for nitrogen in the pulses, making the reasonable assumption that hydrogen waa not being appreciably adsorbed on the column at these temperatures. The corrected retention times for the pure nitrogen pulses on the 480” dried column measured at 0, -31 and -78” were 17.1, 54.0 and 783 seconds, respectively. Similar measurements with 1% nitrogen to 99% helium in the pulses at the above three temperatures resulted in corrected retention times of 34.6, 109 and 1820 seconds, respectively. Following the method of Habgood and Hanlans we have calculated heats of adsorption from the chromatographic data. The valum obtained for the pure nitrogen and the one per cent nitrogen pulses are 5.6 and 5.8 kcal./mole, respectively.
It is noteworthy that the retention time is greatly increased by dilution of the nitrogen pulse passing through the OSSAR column. In fact, it was approximately doubled by a hundred-fold dilution of the pulse a t each of the temperatures cited above. We believe this effect is due to a slight non-linearity in the adsorption isotherm over the range of partial pressures of nitrogen covered in the present experiments. However, regardless of the cause, it is apparent that this change in retention time with partial pressure must be guarded against in the operation of a gas-solid chromatographic column for gas analysis. Acknowledgment.-The authors are grateful to Dr. Frank S. Stone for helpful discussion and especially for his pertinent suggestions in interpreting the chromatographic data, and to Dr. James M. Holmes who made the calorimetric measurements cited here. One of us (R.A.B.) is also grateful to W. A. Van Hook for his expert assistance and advice in reducing the time required to put the chromatographic system into operation.
The calorimetrically determined differential heats of adsorption for successive increments of nitrogen on the bone mineral a t -195” fall off more or less linearly from 5.4 kcal. for an initial increment covering 3% of the monolayer to approximately 2.2 kcal. for the monolayer. In view of the low coverage obtained in the chromatographic work it seems reasonable to compare the results of the present work with the calorimetric data at the lowest coverage. More exact agreement than that obtained is probably not to be expected because of small differences in the state of the OSSAR surface as well as the difference in the temperature of measurement by the two methods. I n some preliminary experiments conducted under less carefully controlled conditions we have determined retention times for nitrogen on OSSAR which was partially dried by treating it at room temperature with a stream of helium as taken directly from the tank. This drying procedure presumably left a monolayer or more of adsorbed water on the sample. For four separate nitrogen pulse runs in the temperature range -78 to -145” we obtained retention times yielding heat of adsorption values in the range 2.5 to 3.0 kcal. in qualitative agreement with calorimetric heats ranging from 3.3 kcal. for 8% coverage down to 2.0 kcal. for a monolayer. It is probable that the surface was less completely dehydrated in the chromatographic work than in the calorimetry. Thus we might expect a lower binding energy for nitrogen a t low coverage as determined in the chromatographic experiments.
The earliest evidence for extra “stabilization energy” in certain conjugated polyenes was noted in the thermod,ynamic properties of those compounds. Their heats of combustion and heats of hydrogenation differed appreciably from the values they may have been expected to have from an examination of similar non-stabilized models. The term “stabilization energy” in fact implies reference to a model, as pointed out by Kistiakowsky and 0thers.l It has also been pointed out by Turner2 that the stabilization energies of compounds include electron delocalization energies, terms for bond length changes, bond hybridization changes, steric effects, and other factors. Dewar and Schmeising* have attempted to show that simple conjugated dienes, such as butadiene, have no bond delocalization energy, and all of the so-called “resonance energy” of such molecules can be explained by taking into account the type of bond hybridization. In support of this hypot,hesis they have calculated the single bond energies for various types of C hybridization using molecular orbital methods and have shown that the single bonds between Csp~Cspt, C 8 p ~ C e pCsp~Cspz, ~, CsprH, and Cspl-H are indeed quite different. Any stabilization energies based on heats of hydrogenation must take into account the corresponding changes in hybridization. There remains an area of Dewar’s hypothesis
(6) D H. Jamas and A. J. P. Martin, Biochsm. J., 60, 682 (1862).
STABILIZATION ENERGIES I N NONAROMATIC CONJUGATED POLYENES BY WM. F. YATES Research Department, Monsanto Chemical Company, Tczaa City Tczaa Received July 16, 1960
(1) (a) G . B. Kistiakownky, J. R. Ruhoff, H. A . Smith and W. E. Vaughan, J . Am. Chem. Sac., 68, 146 (1936); (b) J. B. Conant and G. B. Kistiakowsky, Chem. Reus., 20, 181 (1937). (2) R. B. Turner, Prooeedings of the Eighth Rice Institute Research Conferenae, April 24, 1969. (3) M. J. 8. Dewar and H. N. Schmeising, Tetrahedron. 6 , 166 (1969).
NOTES
186
which must face a test of old fashioned thermodynamic consistency: that all of the stabilization energy of butadiene can be accounted for in the Csp2-Csp~ hybridization in the central single bond. Mulliken4has shown that even in very simple molecules, such as ethylene, the C bonds are not pure sp2 hybrids, and one cannot expect to calculate the bond energies in a classical butadiene molecule with sufficient accuracy to make the necessary hybridization corrections to account for the heat of hydrogenation. In this paper we shall attempt t o find the average bond energies by the usual methods using the standard heats of formation of the compounds involved. The purpose of the work will be to see whether changes in conformation between C atoms with sp2hybridizations have any effect on the bond energies, thereby throwing some light on the problem of bond delocalization. The C-C Bond Energy in Conjugated PolyenesFrom the hydrogenolysis of butadiene CHF=CH-CH=-CH~
+ Hz +2CzH4
cyclooctatetraene of 67.90 was used and was calculated from the heat of hydrogenation (determined by Turner's and co-workers), and the heat of formation of cyclooctane.8 Turner’s heat of hydrogenation was determined in solution, so its use to determine a gas phase heat of formation may introduce an error. In the equation &C’
f EHH- 2E’CH (1) where EccIBis the central bond energy in butadiene, E” is the bond energy in the Hz molecule, and E’CHis the bond energy for the CH bond in ethylene. The value of E ’ C H can be computed by the usual procedure from the standard heat of formation of ethylene, g:raphite and atomic hydrogen E‘CCB
(&HI +2C (graphite) 4 = U ’ C H - g Lc
+ 4H
in which Q is the heat of reaction shown and LC is the latent heat of vaporization of graphite. E‘CHis then found to have the value of 106.20 kcal./mole. The value of E’ccB in equation 1is then 106.47 kcal./mole. It should be noted here that if Dewar’s value af 100.9 kcal. for E’CHis used the value of E’ccBis 95.87 against the value of 100.4which he calculates by the molecular orbital treatment. The difference probably arises largely from the fact that the C bonds involved are not pure sp2-sp2 bonds, and there may be extra delocalization energy (the very point we are examining). Cyc1ooctat)etraeneprovides an interesting model for examination because here again all of the single overlap, but the bonds are formed from Csp~-Cspl double bonds have been rotated through 90°,6 thus greatly diminishing the possibility of bond delocalization between adjacent double bonds. There still exists, however, the possibility for some trans-annulay r-bond interaction, and, as Mulliken4 points out, secondary hyperconjugation can be introduced. By the previous method used with butadiene, the complete hydrogenolysis of cyclooctatetraene to ethylene has a heat of reaction, QCI, of -17.90 kcal./mole. In this calculation a value of AHfOfor (4) R. S. Mulliken, T e t r o h ~ d + ~6n, ,68 (1959). (5) “Selected Values of the Properties of Hydrooarbons,” A P I Project 44, a t the Carnegie Institute of Teohnology, April 30, 1859. (6) (a) W. B. F’erson, G. C. Pimentel and K. 8.Pitzer, J . A m . Chem S O C 74, ~ , 3487 (1962); (b) I. L. Karie, J. Chsm. P h y s . , 10, 66 (1P62).
a’ccc8 f
- 8E’c~.
E’ccCs is found to be 103.73 kcal./mole, or 2.74
f
0.3 kcal. less than the value in butadiene. An alternate value for the heat of formation of cyclooctatetraene vapor (71.0 kcal.) is available from combustion data7b and the heat of vapori~ation.~~ If this alternate value is used E’ccC8is found to be 103.0 kcal./mole. Cyclooctatriene provides still another model for examination in which the conjugated double bonds are rotated through approximately 90”. Here the problem is not quite so simple, however, in that two of the bonds broken in hydrogenolysis are formed by sp2-sp3overlap. In the reaction Cyclooctatriene
the heat of reaction, QB, from standard heats of formation5 is - 1.75 kcal./mole. &B
Vol. 65
+ 4Hz -+ CzH6 + 3C2H4 &c” = -25.04
Here again Turner’s6 heat of hydrogenation was used to calculate AHfO = 42.30 for cyclooctatriene and the same possible error is included. We then have &C”
= 2E”CCC8
+ 2E’CCC8+
- 6 E ’ c ~- 2EcH (2)
in which E”ccC8is the C,p*Cspsbond energy in cyclooctatriene and ECH is the CH bond energy in ethane. ECHis found to be 98.54 kcal./mole from the reactions C2H6 -+ 2C (diamond)
+ 6H
E”ccC* was calculated from the hydrogenolysis of
+
cis-cyclooctene to 3CzHs C2H4 and the value of EccC8, the Csps-Cspr bond in the hydrogenolysis of cyclooctene was determined from the hydrogenolysis of cyclooctane. In this way compensation for bond weakening due to strain characteristic of the Cs saturated ring is taken into account. We have for these values EccC8(C,,a - C,,a) in cyclooctane, 80.17 kcal./mole E”ccC8,(Cap) - G P s ) in cis-cyclooctene, 92.72 kcal./mole
Using these values then in equation 2 we get 103.54 kcal./mole for E’ccCS, the bond energy for CsplC,,, single bonds in cyclooctatriene. This is in excellent agreement with the value of 103.73 in cyclooctatetraene. For still another model for study we have chosen cycloheptatriene. In this compound the three conjugated double bonds have been assumed to be near planargand the value of E‘ccC’ should lie somewhere between the 106.47 value in butadiene and the 103.6 value in the 8-member ring polyenes. (7) (a) R. B. Turner, W. R. Rleador, W. von E. Doering, J. R ’ Rlayer and D. W. Wiley, J. A n . Cham. Soc., 79, 4127 (1957); (b) E. J . Prosen, W. H. Johnson and F. D. Rossini, $bad., 72, 626 (1950); ( 0 ) D.W. Scott, .If. E. Gross, G. D. Oliver and H. 15. Huffman, ibid , 71, 1634 (1949). ( 8 ) H. L. Finke, D. W. Scott, & E. I. Gross, J. F. Messerly and G. Waddington. %bid.. 78, 5469 (1956). (9) W. von E. Doering, G. Labor, R. Vonderwahl, N. F. Chamberlain and R,€3. Williams, ibid.,78,5448 (1856).
Jan., 1961
NOTES
187
Using methods identical in all respects 00 the cases unexpected spin-spin coupling of the protons in the above a value of 105.87 for E’ccC7is found. The ar-positions of the two heterocyclic rings proved to calculations necessitate the use of 99.50 kcal./mole be the key to the interpretation of the spectra; alfor ECHin methane, 80.88 kcal./mole for E ’ ’ c c C 7 though these protons are separated by six bonds, in cycloheptatriene and cis-cycloheptene, and the the coupling constant amounts to about 1 C.P.S. standard heats of formation reported in the litera- On the basis of the spectra of representative comture or computed from heats of h y d r o g e n a t i ~ n . ~ J ~ pounds !~l in the series, we wish to report here the interpretation of the spectrum of N-benzylthienoConciusions [3,2-b]pyrrole (I), the synthesis of whichis reported To summarize then, we have these bond energies e1sewhe1-e.~ The relative magnitudes of the crossfor Cspz-CSp2 single bonds ring couplings in this compound can be explained in terms of a a-electron mechanism. in butadiene 106.47kcal./mole The a-proton of a 5-membered aromatic heteroin cycloheptatriene 105.87 kcal./mole cycle generally absorbs at a lower magnetic field in cyclooctatetraene 103.73 kcal./mole than a corresponding P-proton.KsBI n the spectra in cyclooctatriene 103.54kcal./mole of thieno [3,2-b]pyrroles the peak assigned to the It is still impossible to attribute all of the difference 5-proton occurs between -3.19 and -2.18 p.p.m. in energy of ca. 2.8 kcal./mole between the single (relative to water) and that to the 6-proton between bond in butadiene and the C8 cyclopolyenes t o - 1.84 and - 1.31 p.p.m., all spectra showing good resonance or an;y other single phenomenon. The peak separation a t both 40 and 60 Me.’ I n the difference in bond energy is, however, in good agree- spectra of several 2,3-disubstituted pyrroles (11) ment with the irotational barrier in butadiene as and 2,3-disubstituted (111) and 2,3,4-trisubstituted calculated by Millliken and Parr12 and later refined (IV) thienopyrroles the value of the coupling conby M ~ l l i k e n . ~Turner has derived a value of 2.4 stant3r5 JaB(Js6 in the thienopyrroles) is in the kcal./mole for the stabilizat,ion energy of cycloocta- range 3.0 f 0.5 C.P.S. (In the NH series, there is t,etraene as referred to a cyclohexene-cyclooctatriene further splitting of the a and P protons by the NH model.’& If this 2.4 kcal./mole is due mostly to proton, and, since J45 % J.IB J 5 6 , a pair of triplets trans-annu’lar a-bond interaction, then half of it is observed3.) The J 5 6 values of several 2,3,4-trimust be added t o the 2.8 kcal. difference in bond substituted thienopyrroles are included in Table I. energy between t,he coplanar and 90” rotated model to get 4.0 A 0.3 kcal./mole for the rotational barrier TABLE I in butadiene, in excellent agreement with Mulliken. PROTON-PROTON COUPLING CONSTANTS IN C.P.S. OBSERVED The conclusioin we then draw is that there is THIENO [3,2-b]significant st,abilization energy in butadiene which I N THE N.M.R. SPECTRA O F SUBSTITUTED PYRROLES (Iv) cannot be attributed to a pure CSp~-Csp~ bond Coupling alone. Even if the use of average bond energies ,.Substituents constantsa Rz Ra Itr RB J66 Ja does not subsequently prove to be exact (ie., if OH COzEt H 3.7 .. E’CHin cyclooctatetraene is different from E‘CHin COzEt OH Et H 2.7 .. ethylene), we would see an error in the E’cc cal- COzEt OCHi COzEt H 3.2 .. culated in the models used but probably very little COzEt OCHI Et H 3.0 .. error reflected in. the differences between the E’CC C02Et COzEt OAC COzEt H 3.5 .. values.
-
(10) R. B. Turner and W. R. Meador, ibid.. 79, 4133 (1957). (11) J. B. Conn, G. 13. Kistiakowsky and E. A. Smith, ihid.. 61, 1868 (1939). (12) R. S. Mullikm and R. G. Parr, J . Chem. Phys., 19, 1271 (1951).
H OAC COCH, H COCH, OAC COCH, H H OAC COCH, CHzCsHs The values given are the average splitting8 60 Mc. with moderate resolution, uncorrected order effects.
3.5 3.5
1.2
.. 1.3 observed at for second-
..
NUCLEAR MAGNETIC RESONANCE SPECTRUM OF N-BENZYLTHIENO [3,2-b1PYRROLE‘
With the knowledge of the approximate line positions of the 5 and 6 protons and of the value for J56, the spectrum of 3-acetoxy-4-acetylthienoBY R. J. TVITE,HI. R. SNYDER,A. L. PORTE A N D H. S. [3,2-b]pyrrole (V) can be explained. I n addition to GUTOWSKY the expected coupling of the 5-proton a t -2.36 N o y e s Chemical Laboratory, University of Illinois, L’rbana, Illinois pap.m.with the 6-proton a t -1.70 p.p.m. (JSSS Received July $6,1960 3.5 c.P.s.), there is a further splitting of the -2.36 We have found that the position of nuclear sub- p.p.m. peak by the 2-proton, which appears a t 1.2 c.P.s.). The latter stitution in thieno [3,2-b]pyrroles2p3usually can be -2.00 p.p.m. ( J 2 5 deduced directly from n.m.r. spectral data. An cross-ring splitting is also observed in the spectrum of VI (J25 1.3c.p.s.).4 (1) This investigation was supported in part by the Officeof Naval
Research and in part by a grant IC3969(Cl)Bio] from the National Cancer Institute, Public Health Service. Also, grateful acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of part of the work. (2) D. S. Matteson and H. R. Snyder, J . Am. Chem. Soc., 79, 3610 (1957). (3) W.R. Carpenter and H, R. Snyder, ibid.. 81,2582 (1960).
=
(4) 9. D. Josey, R. J. Tuite and H. R. Snyder, ibid., 83,1597 (1960). ( 5 ) R. Abraham and H. Bernstein, Canad. J . Chem., 37, 1056
(1959). (6) E. J. Corey, G . Slomp, S. Dev, S. Tobinaga and E. R. Glazier, J . A m . Chem. Soc., 80, 1204 (1958). (7) The spectra were determined with a Varian Associates V-4300-B high resolution n.m.r. spectrophotometer on 20% solutions in deuterioohlorofwm with methylene chloride aa an external stsndsrd.
