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The thermochemistry of some polycyclic compounds
J. Chem. Thermodynamics 1998, 30, 1455]1464 Article No. ct980413
The thermochemistry of some polycyclic compounds V. P. Kolesov, a S. M. Pimenova, V. A. Lukyanova, T. S. Kuznetsova, and M. P. Kozina Chemistry Department, Moscow State Uni¨ ersity, Moscow 119899, Russia
The standard massic energies of combustion of eight polycyclic compounds were measured at T s 298.15 K by static-bomb combustion calorimetry. The standard enthalpies of vaporization and sublimation were measured in a Calvet microcalorimeter, or adiabatic vaporization calorimeter, or derived from ebulliometric measurements of the vapour pressure as a function of temperature. The standard molar enthalpies of formation in the condensed and gaseous states were obtained from these data. The peculiarities in the strain energies of polycyclic compounds are discussed. q 1998 Academic Press KEYWORDS: thermochemistry; calorimetry; enthalpies of combustion; enthalpies of formation; polycyclic compounds
I. Introduction Polycyclic compounds have been always considered very interesting substances for thermochemical investigations for several reasons. First, polycyclic compounds constitute the most promising substances for studying correlations between energetic parameters and molecular geometry. Cyclization of hydrocarbons, for example, is usually accompanied by considerable changes in their geometry, which lead to the appearance of strain in the molecules and to a corresponding change in their enthalpies of formation. Second, cyclic compounds are the most important structural units in organic chemistry, forming sequences of molecules of similar structures gradually increasing in complexity, which are natural to study by thermochemical methods. The strain in molecules of cyclopropane and cyclobutane, as well as in polycyclic compounds containing three- and four-membered rings is especially significant due to the distortion of valence angles and internuclear distances. The strain energy causes unusually high massic energies of combustion and opens the prospect for using some of these compounds in designing effective fuels. Thermochemical investigations of cyclic and polycyclic compounds with threemembered rings have been carried out in the laboratory of thermochemistry of a
To whom correspondence should be addressed ŽE-mail: [email protected]..
0021]9614r98r121455 q 10 $30.00r0
q 1998 Academic Press
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V. P. Kolesov et al.
FIGURE 1. The structural formulae of the studied polycyclocompounds: I, cis-bicycloŽ2.1.0.pentane; II, spirocyclopropane-1.6-tricycloŽ3.2.1.0 2,4 .4octane; III, dispiroŽ3.0.2.1.octane; IV, ethyl ether of cis-2phenylcyclopropanecarboxylate; V, ethyl ether of trans-2-phenylcyclopropanecarboxylate; VI, 1cyclopropylcyclobutene; VII, cis-7-methylenebicyclo Ž4.1.0.heptene-2; VIII, bicyclopropylidene.
Moscow State University for many years.Ž1,2. Recently, the enthalpies of formation of highly strained polycyclic compound ‘‘triangulanes’’ were determined.Ž3. The triangulanes can be defined as a class of compound whose skeleton is constructed of spiro-attached three-membered rings. In such structures, the addition of each cyclopropane fragment results in formation of the new spiro-centre which is associated with extra strain and with accumulation of the strain energy in the triangulane molecule. In this work, the enthalpies of formation have been measured for some other compounds containing cyclopropane or triangulane fragments.
2. Experimental All polycyclic compounds Žsee figure 1. were prepared according to the procedures described previously: cis-bicycloŽ2.1.0.pentane ŽI.;Ž4,5. spirocyclopropane-1.6tricycloŽ3.2.1.0 2,4 .4 octane ŽII.;Ž6. dispiroŽ3.0.2.1.octane ŽIII.;Ž7. two isomeric ethyl ethers of cis- ŽIV. and trans- ŽV. 2-phenylcyclopropanecarboxylate;Ž8. 1cyclopropylcyclobutene ŽVI.;Ž9. cis-7-methylenebicyclo Ž4.1.0.heptene-2 ŽVII.;Ž10. and bicyclopropylidene ŽVIII..Ž11. All these substances were purified by preparative g.c. Ž15 mass per cent of SE 30 on Chromaton W-AW-DMCS, columns 6 m long, 5 mm i.d.. followed by drying over sodium, or 0.4 nm molecular sieves. The purity of all samples was checked by elementary analysis and g.l.c. Žfor compound V the cryometric method was employed.. The physical properties and purities of the compounds are given in table 1. The molar masses of all compounds were calculated from the table of relative atomic masses recommended by IUPAC.Ž12. All substances were available in very limited quantity. In view of the little knowledge, and the great importance, of thermochemical quantities of these compounds, we have measured the energy of combustion for all eight compounds, in spite of the fact that compounds I, III, and
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Thermochemistry of polycyclic compounds TABLE 1. Physical properties and purities of the compounds Compound I II III IV V VI VII VIII
a
Tb
b
r420
K
g . cmy3
319 447 359 360 f 404 g 383 410 374
0.7910 0.9567 0.8270 1.055 1.120 h 0.8304 0.8304 0.8544
n20 D
xc
1.4219 1.4920 1.4522 1.5159
0.990 d 0.999 0.989 e 0.999 0.999 i 0.997 0.993 j 0.999
1.4650 1.5059 1.4520
a I, cis-bicycloŽ2.1.0.pentane; II, spirocyclopropane-1.6-tricycloŽ3.2.1.0 2,4 .4octane; III, dispiroŽ3.0.2.1.octane; IV, ethyl ether of cis-2-phenylcyclopropanecarboxylate; V, ethyl ether of trans-2phenyl-cyclopropanecarboxylate; VI, 1-cyclopropylcyclobutene; VII, cis-7-methylenebicyclo Ž4.1.0.heptene-2; VIII, bicyclopropylidene. b Accuracy of T b measurements was about 0.5 K. c Mol fraction of substance. d Mol fraction f 0.010 of methylcyclobutane impurity. e Mol fraction f 0.011 of 1-methylenspiroŽ2,3.hexane impurity. f At p s 300 Pa. g At p s 1300 Pa. h Density of the compound V was determined by floating some crystals in a salt solution in Žethanol q water. of known density. i Checked by depression of the triple point Ž331 K.. j Mol fraction 0.007 of isomeric impurity.
VII contained almost 1 per cent of impurities Žsee table 1.. It was important that the estimated massic energies of combustion of the main impurities in I, III, and VII Žprecursor in synthesis and isomers. were close to the massic energies of combustion of the compounds under study and could not seriously affect the results. Only three runs could be made with III because of the small quantity of the substance. The energies of combustion were determined by using two static-bomb isoperibolic macrocalorimeters.Ž13. The temperature rise was measured with a copper resistance thermometer and a bridge circuit Ž14. with a sensitivity of f 4 . 10y5 K. The energy equivalent « of the calorimeters was determined by combustion of thermochemical standard benzoic acid with a certified massic energy of combustion of yŽ26434.0 " 2.2. J . gy1 under the test bomb conditions at T s 298.15 K. It was equal to Ž54107.5 " 6.0. J . Vy1 for calorimeter number 1 and Ž54263.1 " 5.8. J . Vy1 for calorimeter number 2, from eight calibration experiments in both cases Žthroughout this paper the results are given as "t . s, where s is the standard deviation of the mean value, and t is a Student’s coefficient at the 0.05 significance level.. The samples of the compounds I, IV, and VII were placed in a platinum crucible in sealed glass ampoules; Terylene film bags were used in combustion experiments with II, III, VI, and VIII. Benzoic acid was used as auxiliary material in all runs with VIII to ensure complete combustion and to produce a suitable temperature
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V. P. Kolesov et al.
rise. The ethyl ether of trans-2-phenylcyclopropanecarboxylic acid ŽV. was burnt in pellet form. The bombs with 1 cm3 of added water were charged with purified oxygen, usually to a pressure of 3.04 MPa. In calorimetric experiments with I and VII, the initial pressure was reduced to p s 2.5 MPa and p s Ž2.0 to 3.0. MPa to avoid explosion-like combustion. Ignition was provided using a platinum wire Žcompounds II, III, VI, and VIII., or an iron wire Žcompounds I, IV, V, and VII. heated by the discharge of a capacitor. After each run, the combustion products were analysed for CO 2 by the Rossini method.Ž15. Qualitative tests for COŽg. with indicator tubes were negative within the limits of their sensitivity x ŽCO. - 1 . 10y6 4 . The bomb was examined carefully for soot after each combustion, and the experiment was rejected if any was found. The molar enthalpy of vaporization D vap HT m of IV, and the enthalpy of sublimation D sub HT m of V were determined in a Calvet microcalorimeter at T s 298.15 K by the standard procedure.Ž16. The D vap HT m value of II was measured in an adiabatic vaporization calorimeter LKB 8721-3 ŽSweden. at T s 298.15 K.Ž17. The enthalpies of vaporization of I and III were calculated from the temperature dependence of the equilibrium vapour pressure p measured in a differential ebulliometer of the Swietosławski type.Ž18.
3. Results The results of typical combustion experiments are reported in table 2: m denotes sample mass; « 9, the energy equivalent corrected for heat capacity of the bomb contents; D R c , the increase of the thermometer resistance corrected for heat exchange; q Žf., q Žaux., and q ŽFe., the combustion energies of Terylene film, of benzoic acid, and of iron wire; q ŽHNO3 ., correction for the energy of formation of aqueous nitric acid from nitrogen, oxygen, and water; q Žs., the correction to standard states; D c u8, the standard massic energy of combustion; A, ratio of mass of CO 2 recovered after the experiment to the mass of CO 2 expected on the basis of the mass of the sample; n, number of experiments; and ² D c u8:, the average value of the standard massic energy of combustion. In experiments with II, IV, and V, the results of analyses of combustion products for CO 2 were in accordance with expectation on the basis of the mass of the samples; it proved the completeness of combustion and the purity of the compounds. In experiments with the other five compounds, a noticeable deficiency of CO 2 , about Ž0.2 to 0.3. per cent, was detected in the products of combustion, that exceeded the limits of experimental error. In these runs, the mass of the compound burnt in the experiment was calculated from the results of the CO 2 analysis of the combustion products. It was assumed in this approach that some samples could contain traces of moisture because the drying procedure was not effective enough for small quantities of substances. The standard massic energy of combustion of Terylene film, D c u8 s yŽ22927.9 " 6.3. J . gy1 , and the resulting ratio of carbon dioxide to that of the film, Ž2.2897 " 0.0006., were measured previously.Ž19. The standard massic energy of combustion of benzoic acid, yŽ26413.7 " 2.2. J . gy1 , was derived from the
1.4 4.0 y45074 1.0002 5 45074
0.21432 9938.6 273.0
II
III
1.3 4.0 y46776 0.9988 3 46812
0.24318 a 11618.7 238.4 37.5 2.7 7.6 y33731 0.9999 7 33731
0.38106 12901.5
IV
29.6 1.8 9.0 y33537 0.9998 8 33535
0.44806 15067.0
V
34.2 4.5 4.9 y46100 0.9972 6 46089
0.25694 a 12199.7 311.1
VI
33.0 6.8 5.9 y45016 0.9995 8 45022
0.29764 a 13444.4
VII
0.3 2.5 y47209 0.9963 5 47250
0.12614 a 11506.4 346.9 5201.7
VIII
b
The mass of the compound burnt in the experiment was determined from the results of the CO 2 analysis of the combustion products. The uncertainties of the average results given as "t . s Žwhere s is the standard deviation, and t is a Student’s coefficient at the 0.05 significance level. were as follows: I, 19 J . gy1 ; II, 12 J . gy1 ; III, 50 J . gy1 ; IV, 6 J . gy1 ; V, 6 J . gy1 ; VI, 9 J . gy1 ; VII, 10 J . gy1 ; and VIII, 31 J . gy1 .
a
mrg 0.24759 a « 9 . D R crJ 11796.7 q Žf.rJ q Žaux .rJ q ŽFe .rJ 32.3 q ŽHNO 3 .rJ 2.6 q Žs .rJ 2.9 D c u8r ŽJ . gy1 . y47493 A 0.9990 n 6 ²yD c u8:r ŽJ . gy1 . b 47515
I
TABLE 2. Results of typical combustion experiments at T s 293.15 K for VI, cis-bicycloŽ2.1.0.pentane; II, spirocyclopropane-1.6-tricycloŽ3.2.1.0 2,4 .4octane; III, dispiroŽ3.0.2.1.-octane; IV, ethyl ether of cis-2-phenylcyclopropanecarboxylate; V, ethyl ether of trans-2-phenylcyclo-propanecarboxylate; VI, 1-cyclopropylcyclobutene; VII, cis-7-methylenebicyclo Ž4.1.0.heptene-2; and VIII, bicyclopropylidene
Thermochemistry of polycyclic compounds
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V. P. Kolesov et al. TABLE 3. Results and derived quantities at T s 298.15 K
Compounda IŽl. IIŽl. IIIŽl. IVŽl. VŽcr. VIŽl. VIIŽl. VIIIŽl.
T yD c Um Ž l or cr . . kJ moly1
yD c HT m Ž l or cr . kJ . moly1
D f HT m kJ . moly1
D vap or sub HT m kJ . moly1
D f HT m Žg. kJ . moly1
3236.6 " 1.3 6049.8 " 1.6 5064.3 " 5.0 6417.0 " 1.1 6379.7 " 1.1 4339.6 " 0.9 4779.8 " 1.1 3786.1 " 2.5
3241.6 " 1.3 6058.5 " 1.6 5071.8 " 5.0 6423.1 " 1.1 6385.8 " 1.1 4345.8 " 0.9 4786.0 " 1.1 3791.1 " 2.5
130.7 " 1.5 122.6 " 1.8 208.8 " 5.0 y299.9 " 1.9 y337.2 " 1.9 162.1 " 1.3 208.7 " 1.5 286.6 " 2.6
28.0 " 0.5 b 47.8 " 0.1c 35.6 " 0.5 b 70.7 " 0.6 d 96.9 " 0.4 d 39.2 e 43.1e 37.7 e
158.7 " 1.6 170.4 " 1.8 244.4 " 5.0 y229.2 " 2.0 y240.3 " 1.9 201.3 251.8 324.3
a I, cis-bicycloŽ2.1.0.pentane; II, spirocyclopropane-1.6-tricycloŽ3.2.1.0 2,4 .4octane; III, dispiroŽ3.0.2.1.octane; IV, ethyl ether of cis-2-phenylcyclopropanecarboxylate; V, ethyl ether of trans-2phenylcyclopropanecarboxylate; VI, 1-cyclopropylcyclobutene; VII, cis-7-methylenebicyclo- Ž4.1.0 . heptene-2; and VIII, bicyclopropylidene. b Calculated from Ž p,T . behaviour. c Measured in the LKB 8721-3 calorimeter. d Measured by using a Calvet microcalorimeter. e Estimated according to the Klages formula.Ž20.
above-mentioned certificate value. The energy of combustion of iron to form Fe 2 O 3 , D c u8 s y7500 J . gy1 , was taken from the reference 15. The uncertainties of the average D c u8 values were calculated in the same way as for « . The overall random uncertainties listed in table 2 also included the uncertainty of calibration. Other contributions such as uncertainties of the combustion energies of Terylene film and of iron wire were found to be negligible. The results of our measurements and derived quantities are summarized in table 3. The standard molar enthalpies of formation of CO 2 Žg. and H 2 OŽl., recommended by CODATA,Ž21. yŽ393.51 " 0.13. kJ . moly1 and yŽ285.830 " 0.040. kJ . moly1 , were employed in the calculation of D f HT m values.
4. Discussion As a result of the experimental work of thermochemists, many D f HT m values of polycyclic compounds have been accumulated.Ž22,23. The great variety of polycyclic compounds makes it difficult to describe these data on the basis of one general scheme. For that reason, special contributions are usually introduced for formation of various cyclic Žthree-, four-membered., or polycyclic fragments Žnorbornane, spiropentane..Ž22,23. In principle, according to this approach, each parent cyclic compound forms its own group of derivatives, within which conventional additivity schemes can be used. Nevertheless, with the growing complexity of polycyclic compounds, the D f HT m values usually became unpredictable. So, taking into account the variety of polycyclic compounds, it is often necessary to discuss experimental data in terms of interactions between diverse molecular fragments, or to introduce additional contributions. Even within the framework of one group of polycyclic compounds, the enthalpies of formation often do not obey additivity rules.
Thermochemistry of polycyclic compounds
1461
Thus, D f HT values and strain energies of 1- and 2-methylderivatives of m bicycloŽ2.2.1.heptane Žnorbornane. differ by f 10 kJ . moly1 .Ž1. The specific features of the thermochemistry of polycyclic compounds are displayed, in particular, by compounds with cage structures. One can expect that additional strain energy should arise with the growing complexity of polycyclic compounds. But experimental data often disagree with this assumption. On the contrary, it was shown in a review Ž1. that the strain energy UŽstrain. in polycyclic hydrocarbons with a cage structure is very often less than the sum of the molar strain energies of the independent component rings. A slight decrease of UŽstrain. in comparison with the sum of the strain energies of the component rings was also found recently in pentacycloŽ5.4.0.0 2,6 0 3,10 0 5,9 .undecane, a compound of rather complicated cage structure.Ž24. On the other hand, for some other hydrocarbons with a cage structure, cubane, for example, a substantial increase of the strain energy was detected. This increase was usually connected with structural factors, for example, deformation of the valence and dihedral angles, or repulsion of closely opposed hydrogen atoms.Ž1. The compounds under study belong to different classes and their molecules consist of diverse fragments, including oxygen atoms and double bonds. It would be interesting to know if there are some peculiarities in their strain energies due to this structure. As before,Ž1,2. Benson’s additivity scheme Ž22. was used to compare experimental and calculated D f HT m values. During previous years, other schemes have been developed on the basis of a group-additivity approach;Ž23,25,26. the latest of themŽ25,26. represented modifications of the original Benson scheme.Ž22. Regarding hydrocarbons, the schemes Ž25,26. introduced only minor changes in the group contribution values. This is not surprising, because the database for these compounds was not changed significantly. For this reason, and for the sake of uniformity with previous papers,Ž1,2. we chose the scheme Ž22. in this work. The experimental values of enthalpies of formation for the gaseous state, as well as the strain energies are presented in table 4. In this table, UŽstrain. denotes the strain energy of a compound; SUŽstrain, cycl., the sum of strain energies of component rings; and DUŽstrain., the difference UŽstrain. y UŽstrain, cycl.. The data for some relevant compounds Žfigure 2. are given in the same table. One can see that for bicycloŽ2.1.0.pentane ŽI. the excess strain energy DUŽstrain. s 11.6 kJ . moly1 , for bicycloŽ1.1.0.butane ŽIa. it increases to 43.5 kJ . moly1 , whereas for bicycloŽ3.1.0.hexane ŽIb. and bicycloŽ4.1.0.heptane ŽIc. it is rather small and comparable to experimental errors. It is obvious, when considering a series of cyclopropanated bicyclo compounds, that the excess strain energy decreases with increasing size of the ring to which the cyclopropane ring is attached. We can assume, therefore, that for large cyclopropanated rings, such as the norbornane fragment in the compound ŽII., the extra strain energy can be neglected. In that case, DUŽstrain. can probably be applied to the spiro junction; in fact Žsee table 4., it is very small, only 5.3 kJ . moly1 . About the same DUŽstrain., 6.2 kJ . moly1 for a spiro atom, is detected in compound IIa. Taking error limits into account, we can conclude that there is no essential extra strain energy associated with the spiro junction between cyclopropane and norbornane rings.
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V. P. Kolesov et al.
TABLE 4. Comparison of thermochemical data and strain energies UŽstrain. of some polycyclic compounds; SUŽstrain, cycl. is the sum of strain energies of component rings ŽT s 298.15 K. Compounda I Ia Ib Ic II IIa III IV V VI VII VIII IX
D f HT m Žg. kJ . moly1
kJ . moly1
SU Ž strain, cycl . kJ . moly1
DU Ž strain . kJ . moly1
References
158.7 " 1.6 217.2 " 0.8 38.6 " 2.1 1.3 " 2.7 170.4 " 1.8 384.3 " 4.2 244.4 " 5.0 y229.2 " 2.0 y240.3 " 1.9 201.3 251.8 324.3 200.5 " 1.8
236.7 274.5 137.3 120.7 303.7 568.0 364.5 105.5 94.4 212.9 162.5 320.6 172.5
225.1 231.0 141.8 115.5 298.4 555.7 367.8 115.5 115.5 240.2 121.3 231.0 115.5
11.6 43.5 y4.5 5.2 5.3 12.3 y3.3 y10.0 y21.1 y27.3 41.2 89.6 57.0
this work 1, 27 1, 28 1, 29 this work 30 this work this work this work this work this work this work 1, 27
U Ž strain .
a I, cis-bicycloŽ2.1.0.pentane; Ia, bicycloŽ1.1.0.butane; Ib, bicycloŽ3.1.0.hexane; Ic, bicycloŽ4.1.0.heptane; II, spirocyclopropane-1.6-tricycloŽ3.2.1.0 2,4 .4octane; IIa, dispirocyclopropane-6,1’pentacycloŽ6.3.1.0 2,7 0 3,5 0 9,10 .dodecane-12,10-cyclopropane 4; III, dispiro-Ž3.0.2.1.octane; IV, ethyl ether of cis-2-phenylcyclopropanecarboxylate; V, ethyl ether of trans-2-phenylcyclopropanecarboxylate; VI, 1-cyclopropylcyclobutene; VII, cis-7-methylenebicyclo- Ž4.1.0.heptene-2; VIII, bicyclopropylidene; and IX, methylenecyclopropane.
It is interesting to compare these results with the data for other spirocompounds. It was shown in reference 3 that the extra strain in spirocondensed cyclopropanes is approximately proportional to the number of spiro-centres, which means that the Ž . spiro atom C s3,3 has an additional contribution of 27.2 kJ . moly1 to D f HT m g as compared with the ordinary C-ŽC.4 contribution Žbeing calculated in terms of Benson’s scheme.. Beckhaus et al.Ž31. later confirmed the proportionality of the extra strain energy in spirocondensed cyclopropanes to the number of spiro-centres, with slightly greater additional contribution of C s3,3 atoms. On the other hand, no excess strain energy was reported in reference 31 for spirocyclopropanated cyclobutanes. Considering dispiroŽ3.0.2.1.octane ŽIII. Žtable 4., we can see that if
FIGURE 2. The structural formulae of some relevant compounds: Ia, bicycloŽ1.1.0.butane; Ib, bicyclo Ž3.1.0..hexane; Ic, bicyclo Ž4.1.0..heptane; IIa, dispiro cyclopropane-6,19- pentacyclo Ž6.3.1.0 2,7 0 3,5 0 9,11 .dodecane-12,10-cyclopropane 4; IX, methylenecyclopropane.
Thermochemistry of polycyclic compounds
1463
the additional contribution of C s3,3 atom is accepted to be equal to 27.2 kJ . moly1 ,Ž3. the extra strain energy which can be applied to the C s3,4 spiro atom is very small, in accordance with reference 31. Obviously, only for spirocondensed cyclopropanes is the extra strain energy essential in a series of spirocyclopropanated compounds. A decrease of UŽstrain. in comparison with the cyclopropane ring, which is found in two isomers of the ethyl ether of 2-phenylcyclopropanecarboxylic acid ŽIV and V., can probably be associated with the influence of the phenyl group. The slight lessening of UŽstrain . in phenylderivatives of cyclopropane was also deduced for phenylcyclopropane Ž DUŽstrain. s y13.1 kJ . moly1 . and for the trans isomer of 2-phenylcyclopropanecarboxylic acid Ž DUŽstrain. s y13.1 kJ . moly1 ., on the Ž1,32,33. basis of D f HT m values obtained previously. The strain energy of bicyclic compounds with rings separated by a s-bond is usually close to SUŽstrain, cycl.:Ž1. e. g. for bicyclopropyl, UŽstrain. s 228 kJ . moly1 , about twice as much as UŽstrain. of cyclopropane itself. Meanwhile, there is a visible lessening of the strain energy in cyclopropylcyclobutene ŽVI. Žsee table 4., which indicates that the effect of conjugation may be noticeable in this compound. The introduction of a double bond in alkenyl derivatives of cyclopropane greatly increases the strain energy of the three-membered ring Žtable 4.. Considering the increase of UŽstrain. in methylenecyclopropane ŽIX., DUŽstrain . s 57.0 kJ . moly1 , the authors Ž2. paid attention to the fact that DUŽstrain . for this compound is about equal to the estimated strain energy for each trigonal C atom in cyclopropene. This value Ž54.6 kJ . moly1 . was taken as half the difference between the strain energies of cyclopropene and cyclopropane.Ž2. For compounds VII and VIII, DUŽstrain. is slightly less than in IX: 41.2 kJ . moly1 in VII, and 44.8 kJ . moly1 for each individual cyclopropylidene fragment in VIII, but it is very likely of the same nature. Obviously, the introduction of an exocyclic or endocyclic double bond increases the strain energy of three-membered rings about equally. This fact can be used for the further development of the additivity scheme for estimating D f HT m values of cyclopropane derivatives.
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12. Atomic Weights of the Elements 1993. IUPAC Commission on Atomic Weights and Isotopic Abundances. J. Phys. Chem. Ref. Data 1995, 24, 1561]1576. 13. Kozina M. P.; Timofeeva L. P.; Lukyanova V. A.; Pimenova S. M.; Kasyan, L. I. Zh. Fiz. Khim. 1988, 62, 1203]1208. Ž Russ. J. Phys. Chem. 1988, 62, 609]612.. 14. Skuratov, S. M.; Goroshko, N. N. Izmer. Tekh. 1964, 2, 6]8. 15. Experimental Thermochemistry. Rossini, F. D.: editor. Interscience: New York. 1956, p. 56. 16. Papina, T. S.; Kolesov, V. P.; Vorobieva, V. P.; Golovkov, V. F. J. Chem. Thermodynamics 1996, 28, 307]311. 17. Varushchenko, R. M.; Druzhinina, A. I.; Pashchenko, L. L. Zh. Fiz. Khim. 1996, 70, 228]231. Ž Russ. J. Phys. Chem. 1996, 70, 212]216.. 18. Varuchchenko, R. M.; Druzhinina, A. I. J. Chem. Thermodynamics 1995, 27, 355]368. 19. Papina, T. S.; Kolesov, V. P. Zh. Fiz. Khim. 1985, 59, 2169]2172. Ž Russ. J. Phys. Chem. 1985, 59, 1289]1291.. 20. Klages, F. Chem. Ber. 1949, 82, 358]375. 21. Cox, J. D.; Wagman, D. D.; Medvedev, V. A. CODATA Key Values for Thermodynamics. Hemisphere: New York. 1989. 22. Benson, S. W.; Cruickshank, F. R.; Golden, D. M.; Haugen, G. R.; O’Neal, H. E.; Rodgers, A. S.; Shau, R.; Walsh, R. Chem. Re¨ . 1969, 69, 279]324. 23. Domalski, E. S.; Hearing, E. D. J. Phys. Chem. Ref. Data 1993, 22, 805]1159. 24. Kabo, G. J.; Kozyro, A. A.; Diky, V. V.; Simirsky, V. V.; Ivashkevich, L. S.; Krasulin, A. P.; Sevruk, V. M.; Marchand, A. P.; Frenkel, M. J. Chem. Thermodynamics 1995, 27, 707]720. 25. Cohen, N.; Benson, S. W. Chem. Re¨ . 1993, 93, 2419]2438. 26. Cohen, N. J. Phys. Chem. Ref. Data. 1996, 25, 1411]1481. 27. Wiberg, K. B.; Fenoglio, R. A. J. Am. Chem. Soc. 1968, 90, 3395]3397. 28. Chang, S.; McNally, D.; Shary-Fehrany S.; Hickey, M. D.; Boyd, R. H. J. Am. Chem. Soc. 1970, 92, 3109]3118. 29. Timofeeva, L. P.; Kozina, M. P.; Skuratov, S. M.; Nefedov, O. M.; Novitskaya, N. N. Vestnik Mosko¨ . Gos. Uni¨ . Ser. Khim. 1970, 3, 300]304. 30. Lukyanova, V. A.; Timofeeva, L. P.; Kozina, M. P.; Kazimirchik, I. V.; Kotelnikova, T. A. Zh. Fiz. Khim. 1991, 65, 823]827. Ž Russ. J. Phys. Chem. 1991, 65, 436]439.. 31. Beckhaus, H.-D.; Ruchardt, C.; Kozhushkov, S. I.; Belov, V. N.; Verevkin, S. P.; de Meijere, A. J. Am. Chem. Soc. 1995, 117, 11 854]11 860. 32. Kozina M. P.; Kropivnitskaya, A. G.; Gembitskii, G. A.; Loim, N. M. Vestnik Mosko¨ . Gos. Uni¨ . Ser. Khim. 1969, 6, 85]89. 33. Pimenova, S. M.; Masyutina, L. V.; Kozina, M. P.; Skuratov, S. M.; U, Ii Sein; Bolesov, I. G.; Levina, R. Ya. Zh. Obshch. Khim. 1970, 60, 2117]2120.
(Recei¨ ed 5 March 1998; in final form 26 June 1998)
SPŽ1.-01
The thermochemistry of some polycyclic compounds V. P. Kolesov, a S. M. Pimenova, V. A. Lukyanova, T. S. Kuznetsova, and M. P. Kozina Chemistry Department, Moscow State Uni¨ ersity, Moscow 119899, Russia
The standard massic energies of combustion of eight polycyclic compounds were measured at T s 298.15 K by static-bomb combustion calorimetry. The standard enthalpies of vaporization and sublimation were measured in a Calvet microcalorimeter, or adiabatic vaporization calorimeter, or derived from ebulliometric measurements of the vapour pressure as a function of temperature. The standard molar enthalpies of formation in the condensed and gaseous states were obtained from these data. The peculiarities in the strain energies of polycyclic compounds are discussed. q 1998 Academic Press KEYWORDS: thermochemistry; calorimetry; enthalpies of combustion; enthalpies of formation; polycyclic compounds
I. Introduction Polycyclic compounds have been always considered very interesting substances for thermochemical investigations for several reasons. First, polycyclic compounds constitute the most promising substances for studying correlations between energetic parameters and molecular geometry. Cyclization of hydrocarbons, for example, is usually accompanied by considerable changes in their geometry, which lead to the appearance of strain in the molecules and to a corresponding change in their enthalpies of formation. Second, cyclic compounds are the most important structural units in organic chemistry, forming sequences of molecules of similar structures gradually increasing in complexity, which are natural to study by thermochemical methods. The strain in molecules of cyclopropane and cyclobutane, as well as in polycyclic compounds containing three- and four-membered rings is especially significant due to the distortion of valence angles and internuclear distances. The strain energy causes unusually high massic energies of combustion and opens the prospect for using some of these compounds in designing effective fuels. Thermochemical investigations of cyclic and polycyclic compounds with threemembered rings have been carried out in the laboratory of thermochemistry of a
To whom correspondence should be addressed ŽE-mail: [email protected]..
0021]9614r98r121455 q 10 $30.00r0
q 1998 Academic Press
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V. P. Kolesov et al.
FIGURE 1. The structural formulae of the studied polycyclocompounds: I, cis-bicycloŽ2.1.0.pentane; II, spirocyclopropane-1.6-tricycloŽ3.2.1.0 2,4 .4octane; III, dispiroŽ3.0.2.1.octane; IV, ethyl ether of cis-2phenylcyclopropanecarboxylate; V, ethyl ether of trans-2-phenylcyclopropanecarboxylate; VI, 1cyclopropylcyclobutene; VII, cis-7-methylenebicyclo Ž4.1.0.heptene-2; VIII, bicyclopropylidene.
Moscow State University for many years.Ž1,2. Recently, the enthalpies of formation of highly strained polycyclic compound ‘‘triangulanes’’ were determined.Ž3. The triangulanes can be defined as a class of compound whose skeleton is constructed of spiro-attached three-membered rings. In such structures, the addition of each cyclopropane fragment results in formation of the new spiro-centre which is associated with extra strain and with accumulation of the strain energy in the triangulane molecule. In this work, the enthalpies of formation have been measured for some other compounds containing cyclopropane or triangulane fragments.
2. Experimental All polycyclic compounds Žsee figure 1. were prepared according to the procedures described previously: cis-bicycloŽ2.1.0.pentane ŽI.;Ž4,5. spirocyclopropane-1.6tricycloŽ3.2.1.0 2,4 .4 octane ŽII.;Ž6. dispiroŽ3.0.2.1.octane ŽIII.;Ž7. two isomeric ethyl ethers of cis- ŽIV. and trans- ŽV. 2-phenylcyclopropanecarboxylate;Ž8. 1cyclopropylcyclobutene ŽVI.;Ž9. cis-7-methylenebicyclo Ž4.1.0.heptene-2 ŽVII.;Ž10. and bicyclopropylidene ŽVIII..Ž11. All these substances were purified by preparative g.c. Ž15 mass per cent of SE 30 on Chromaton W-AW-DMCS, columns 6 m long, 5 mm i.d.. followed by drying over sodium, or 0.4 nm molecular sieves. The purity of all samples was checked by elementary analysis and g.l.c. Žfor compound V the cryometric method was employed.. The physical properties and purities of the compounds are given in table 1. The molar masses of all compounds were calculated from the table of relative atomic masses recommended by IUPAC.Ž12. All substances were available in very limited quantity. In view of the little knowledge, and the great importance, of thermochemical quantities of these compounds, we have measured the energy of combustion for all eight compounds, in spite of the fact that compounds I, III, and
1457
Thermochemistry of polycyclic compounds TABLE 1. Physical properties and purities of the compounds Compound I II III IV V VI VII VIII
a
Tb
b
r420
K
g . cmy3
319 447 359 360 f 404 g 383 410 374
0.7910 0.9567 0.8270 1.055 1.120 h 0.8304 0.8304 0.8544
n20 D
xc
1.4219 1.4920 1.4522 1.5159
0.990 d 0.999 0.989 e 0.999 0.999 i 0.997 0.993 j 0.999
1.4650 1.5059 1.4520
a I, cis-bicycloŽ2.1.0.pentane; II, spirocyclopropane-1.6-tricycloŽ3.2.1.0 2,4 .4octane; III, dispiroŽ3.0.2.1.octane; IV, ethyl ether of cis-2-phenylcyclopropanecarboxylate; V, ethyl ether of trans-2phenyl-cyclopropanecarboxylate; VI, 1-cyclopropylcyclobutene; VII, cis-7-methylenebicyclo Ž4.1.0.heptene-2; VIII, bicyclopropylidene. b Accuracy of T b measurements was about 0.5 K. c Mol fraction of substance. d Mol fraction f 0.010 of methylcyclobutane impurity. e Mol fraction f 0.011 of 1-methylenspiroŽ2,3.hexane impurity. f At p s 300 Pa. g At p s 1300 Pa. h Density of the compound V was determined by floating some crystals in a salt solution in Žethanol q water. of known density. i Checked by depression of the triple point Ž331 K.. j Mol fraction 0.007 of isomeric impurity.
VII contained almost 1 per cent of impurities Žsee table 1.. It was important that the estimated massic energies of combustion of the main impurities in I, III, and VII Žprecursor in synthesis and isomers. were close to the massic energies of combustion of the compounds under study and could not seriously affect the results. Only three runs could be made with III because of the small quantity of the substance. The energies of combustion were determined by using two static-bomb isoperibolic macrocalorimeters.Ž13. The temperature rise was measured with a copper resistance thermometer and a bridge circuit Ž14. with a sensitivity of f 4 . 10y5 K. The energy equivalent « of the calorimeters was determined by combustion of thermochemical standard benzoic acid with a certified massic energy of combustion of yŽ26434.0 " 2.2. J . gy1 under the test bomb conditions at T s 298.15 K. It was equal to Ž54107.5 " 6.0. J . Vy1 for calorimeter number 1 and Ž54263.1 " 5.8. J . Vy1 for calorimeter number 2, from eight calibration experiments in both cases Žthroughout this paper the results are given as "t . s, where s is the standard deviation of the mean value, and t is a Student’s coefficient at the 0.05 significance level.. The samples of the compounds I, IV, and VII were placed in a platinum crucible in sealed glass ampoules; Terylene film bags were used in combustion experiments with II, III, VI, and VIII. Benzoic acid was used as auxiliary material in all runs with VIII to ensure complete combustion and to produce a suitable temperature
1458
V. P. Kolesov et al.
rise. The ethyl ether of trans-2-phenylcyclopropanecarboxylic acid ŽV. was burnt in pellet form. The bombs with 1 cm3 of added water were charged with purified oxygen, usually to a pressure of 3.04 MPa. In calorimetric experiments with I and VII, the initial pressure was reduced to p s 2.5 MPa and p s Ž2.0 to 3.0. MPa to avoid explosion-like combustion. Ignition was provided using a platinum wire Žcompounds II, III, VI, and VIII., or an iron wire Žcompounds I, IV, V, and VII. heated by the discharge of a capacitor. After each run, the combustion products were analysed for CO 2 by the Rossini method.Ž15. Qualitative tests for COŽg. with indicator tubes were negative within the limits of their sensitivity x ŽCO. - 1 . 10y6 4 . The bomb was examined carefully for soot after each combustion, and the experiment was rejected if any was found. The molar enthalpy of vaporization D vap HT m of IV, and the enthalpy of sublimation D sub HT m of V were determined in a Calvet microcalorimeter at T s 298.15 K by the standard procedure.Ž16. The D vap HT m value of II was measured in an adiabatic vaporization calorimeter LKB 8721-3 ŽSweden. at T s 298.15 K.Ž17. The enthalpies of vaporization of I and III were calculated from the temperature dependence of the equilibrium vapour pressure p measured in a differential ebulliometer of the Swietosławski type.Ž18.
3. Results The results of typical combustion experiments are reported in table 2: m denotes sample mass; « 9, the energy equivalent corrected for heat capacity of the bomb contents; D R c , the increase of the thermometer resistance corrected for heat exchange; q Žf., q Žaux., and q ŽFe., the combustion energies of Terylene film, of benzoic acid, and of iron wire; q ŽHNO3 ., correction for the energy of formation of aqueous nitric acid from nitrogen, oxygen, and water; q Žs., the correction to standard states; D c u8, the standard massic energy of combustion; A, ratio of mass of CO 2 recovered after the experiment to the mass of CO 2 expected on the basis of the mass of the sample; n, number of experiments; and ² D c u8:, the average value of the standard massic energy of combustion. In experiments with II, IV, and V, the results of analyses of combustion products for CO 2 were in accordance with expectation on the basis of the mass of the samples; it proved the completeness of combustion and the purity of the compounds. In experiments with the other five compounds, a noticeable deficiency of CO 2 , about Ž0.2 to 0.3. per cent, was detected in the products of combustion, that exceeded the limits of experimental error. In these runs, the mass of the compound burnt in the experiment was calculated from the results of the CO 2 analysis of the combustion products. It was assumed in this approach that some samples could contain traces of moisture because the drying procedure was not effective enough for small quantities of substances. The standard massic energy of combustion of Terylene film, D c u8 s yŽ22927.9 " 6.3. J . gy1 , and the resulting ratio of carbon dioxide to that of the film, Ž2.2897 " 0.0006., were measured previously.Ž19. The standard massic energy of combustion of benzoic acid, yŽ26413.7 " 2.2. J . gy1 , was derived from the
1.4 4.0 y45074 1.0002 5 45074
0.21432 9938.6 273.0
II
III
1.3 4.0 y46776 0.9988 3 46812
0.24318 a 11618.7 238.4 37.5 2.7 7.6 y33731 0.9999 7 33731
0.38106 12901.5
IV
29.6 1.8 9.0 y33537 0.9998 8 33535
0.44806 15067.0
V
34.2 4.5 4.9 y46100 0.9972 6 46089
0.25694 a 12199.7 311.1
VI
33.0 6.8 5.9 y45016 0.9995 8 45022
0.29764 a 13444.4
VII
0.3 2.5 y47209 0.9963 5 47250
0.12614 a 11506.4 346.9 5201.7
VIII
b
The mass of the compound burnt in the experiment was determined from the results of the CO 2 analysis of the combustion products. The uncertainties of the average results given as "t . s Žwhere s is the standard deviation, and t is a Student’s coefficient at the 0.05 significance level. were as follows: I, 19 J . gy1 ; II, 12 J . gy1 ; III, 50 J . gy1 ; IV, 6 J . gy1 ; V, 6 J . gy1 ; VI, 9 J . gy1 ; VII, 10 J . gy1 ; and VIII, 31 J . gy1 .
a
mrg 0.24759 a « 9 . D R crJ 11796.7 q Žf.rJ q Žaux .rJ q ŽFe .rJ 32.3 q ŽHNO 3 .rJ 2.6 q Žs .rJ 2.9 D c u8r ŽJ . gy1 . y47493 A 0.9990 n 6 ²yD c u8:r ŽJ . gy1 . b 47515
I
TABLE 2. Results of typical combustion experiments at T s 293.15 K for VI, cis-bicycloŽ2.1.0.pentane; II, spirocyclopropane-1.6-tricycloŽ3.2.1.0 2,4 .4octane; III, dispiroŽ3.0.2.1.-octane; IV, ethyl ether of cis-2-phenylcyclopropanecarboxylate; V, ethyl ether of trans-2-phenylcyclo-propanecarboxylate; VI, 1-cyclopropylcyclobutene; VII, cis-7-methylenebicyclo Ž4.1.0.heptene-2; and VIII, bicyclopropylidene
Thermochemistry of polycyclic compounds
1459
1460
V. P. Kolesov et al. TABLE 3. Results and derived quantities at T s 298.15 K
Compounda IŽl. IIŽl. IIIŽl. IVŽl. VŽcr. VIŽl. VIIŽl. VIIIŽl.
T yD c Um Ž l or cr . . kJ moly1
yD c HT m Ž l or cr . kJ . moly1
D f HT m kJ . moly1
D vap or sub HT m kJ . moly1
D f HT m Žg. kJ . moly1
3236.6 " 1.3 6049.8 " 1.6 5064.3 " 5.0 6417.0 " 1.1 6379.7 " 1.1 4339.6 " 0.9 4779.8 " 1.1 3786.1 " 2.5
3241.6 " 1.3 6058.5 " 1.6 5071.8 " 5.0 6423.1 " 1.1 6385.8 " 1.1 4345.8 " 0.9 4786.0 " 1.1 3791.1 " 2.5
130.7 " 1.5 122.6 " 1.8 208.8 " 5.0 y299.9 " 1.9 y337.2 " 1.9 162.1 " 1.3 208.7 " 1.5 286.6 " 2.6
28.0 " 0.5 b 47.8 " 0.1c 35.6 " 0.5 b 70.7 " 0.6 d 96.9 " 0.4 d 39.2 e 43.1e 37.7 e
158.7 " 1.6 170.4 " 1.8 244.4 " 5.0 y229.2 " 2.0 y240.3 " 1.9 201.3 251.8 324.3
a I, cis-bicycloŽ2.1.0.pentane; II, spirocyclopropane-1.6-tricycloŽ3.2.1.0 2,4 .4octane; III, dispiroŽ3.0.2.1.octane; IV, ethyl ether of cis-2-phenylcyclopropanecarboxylate; V, ethyl ether of trans-2phenylcyclopropanecarboxylate; VI, 1-cyclopropylcyclobutene; VII, cis-7-methylenebicyclo- Ž4.1.0 . heptene-2; and VIII, bicyclopropylidene. b Calculated from Ž p,T . behaviour. c Measured in the LKB 8721-3 calorimeter. d Measured by using a Calvet microcalorimeter. e Estimated according to the Klages formula.Ž20.
above-mentioned certificate value. The energy of combustion of iron to form Fe 2 O 3 , D c u8 s y7500 J . gy1 , was taken from the reference 15. The uncertainties of the average D c u8 values were calculated in the same way as for « . The overall random uncertainties listed in table 2 also included the uncertainty of calibration. Other contributions such as uncertainties of the combustion energies of Terylene film and of iron wire were found to be negligible. The results of our measurements and derived quantities are summarized in table 3. The standard molar enthalpies of formation of CO 2 Žg. and H 2 OŽl., recommended by CODATA,Ž21. yŽ393.51 " 0.13. kJ . moly1 and yŽ285.830 " 0.040. kJ . moly1 , were employed in the calculation of D f HT m values.
4. Discussion As a result of the experimental work of thermochemists, many D f HT m values of polycyclic compounds have been accumulated.Ž22,23. The great variety of polycyclic compounds makes it difficult to describe these data on the basis of one general scheme. For that reason, special contributions are usually introduced for formation of various cyclic Žthree-, four-membered., or polycyclic fragments Žnorbornane, spiropentane..Ž22,23. In principle, according to this approach, each parent cyclic compound forms its own group of derivatives, within which conventional additivity schemes can be used. Nevertheless, with the growing complexity of polycyclic compounds, the D f HT m values usually became unpredictable. So, taking into account the variety of polycyclic compounds, it is often necessary to discuss experimental data in terms of interactions between diverse molecular fragments, or to introduce additional contributions. Even within the framework of one group of polycyclic compounds, the enthalpies of formation often do not obey additivity rules.
Thermochemistry of polycyclic compounds
1461
Thus, D f HT values and strain energies of 1- and 2-methylderivatives of m bicycloŽ2.2.1.heptane Žnorbornane. differ by f 10 kJ . moly1 .Ž1. The specific features of the thermochemistry of polycyclic compounds are displayed, in particular, by compounds with cage structures. One can expect that additional strain energy should arise with the growing complexity of polycyclic compounds. But experimental data often disagree with this assumption. On the contrary, it was shown in a review Ž1. that the strain energy UŽstrain. in polycyclic hydrocarbons with a cage structure is very often less than the sum of the molar strain energies of the independent component rings. A slight decrease of UŽstrain. in comparison with the sum of the strain energies of the component rings was also found recently in pentacycloŽ5.4.0.0 2,6 0 3,10 0 5,9 .undecane, a compound of rather complicated cage structure.Ž24. On the other hand, for some other hydrocarbons with a cage structure, cubane, for example, a substantial increase of the strain energy was detected. This increase was usually connected with structural factors, for example, deformation of the valence and dihedral angles, or repulsion of closely opposed hydrogen atoms.Ž1. The compounds under study belong to different classes and their molecules consist of diverse fragments, including oxygen atoms and double bonds. It would be interesting to know if there are some peculiarities in their strain energies due to this structure. As before,Ž1,2. Benson’s additivity scheme Ž22. was used to compare experimental and calculated D f HT m values. During previous years, other schemes have been developed on the basis of a group-additivity approach;Ž23,25,26. the latest of themŽ25,26. represented modifications of the original Benson scheme.Ž22. Regarding hydrocarbons, the schemes Ž25,26. introduced only minor changes in the group contribution values. This is not surprising, because the database for these compounds was not changed significantly. For this reason, and for the sake of uniformity with previous papers,Ž1,2. we chose the scheme Ž22. in this work. The experimental values of enthalpies of formation for the gaseous state, as well as the strain energies are presented in table 4. In this table, UŽstrain. denotes the strain energy of a compound; SUŽstrain, cycl., the sum of strain energies of component rings; and DUŽstrain., the difference UŽstrain. y UŽstrain, cycl.. The data for some relevant compounds Žfigure 2. are given in the same table. One can see that for bicycloŽ2.1.0.pentane ŽI. the excess strain energy DUŽstrain. s 11.6 kJ . moly1 , for bicycloŽ1.1.0.butane ŽIa. it increases to 43.5 kJ . moly1 , whereas for bicycloŽ3.1.0.hexane ŽIb. and bicycloŽ4.1.0.heptane ŽIc. it is rather small and comparable to experimental errors. It is obvious, when considering a series of cyclopropanated bicyclo compounds, that the excess strain energy decreases with increasing size of the ring to which the cyclopropane ring is attached. We can assume, therefore, that for large cyclopropanated rings, such as the norbornane fragment in the compound ŽII., the extra strain energy can be neglected. In that case, DUŽstrain. can probably be applied to the spiro junction; in fact Žsee table 4., it is very small, only 5.3 kJ . moly1 . About the same DUŽstrain., 6.2 kJ . moly1 for a spiro atom, is detected in compound IIa. Taking error limits into account, we can conclude that there is no essential extra strain energy associated with the spiro junction between cyclopropane and norbornane rings.
1462
V. P. Kolesov et al.
TABLE 4. Comparison of thermochemical data and strain energies UŽstrain. of some polycyclic compounds; SUŽstrain, cycl. is the sum of strain energies of component rings ŽT s 298.15 K. Compounda I Ia Ib Ic II IIa III IV V VI VII VIII IX
D f HT m Žg. kJ . moly1
kJ . moly1
SU Ž strain, cycl . kJ . moly1
DU Ž strain . kJ . moly1
References
158.7 " 1.6 217.2 " 0.8 38.6 " 2.1 1.3 " 2.7 170.4 " 1.8 384.3 " 4.2 244.4 " 5.0 y229.2 " 2.0 y240.3 " 1.9 201.3 251.8 324.3 200.5 " 1.8
236.7 274.5 137.3 120.7 303.7 568.0 364.5 105.5 94.4 212.9 162.5 320.6 172.5
225.1 231.0 141.8 115.5 298.4 555.7 367.8 115.5 115.5 240.2 121.3 231.0 115.5
11.6 43.5 y4.5 5.2 5.3 12.3 y3.3 y10.0 y21.1 y27.3 41.2 89.6 57.0
this work 1, 27 1, 28 1, 29 this work 30 this work this work this work this work this work this work 1, 27
U Ž strain .
a I, cis-bicycloŽ2.1.0.pentane; Ia, bicycloŽ1.1.0.butane; Ib, bicycloŽ3.1.0.hexane; Ic, bicycloŽ4.1.0.heptane; II, spirocyclopropane-1.6-tricycloŽ3.2.1.0 2,4 .4octane; IIa, dispirocyclopropane-6,1’pentacycloŽ6.3.1.0 2,7 0 3,5 0 9,10 .dodecane-12,10-cyclopropane 4; III, dispiro-Ž3.0.2.1.octane; IV, ethyl ether of cis-2-phenylcyclopropanecarboxylate; V, ethyl ether of trans-2-phenylcyclopropanecarboxylate; VI, 1-cyclopropylcyclobutene; VII, cis-7-methylenebicyclo- Ž4.1.0.heptene-2; VIII, bicyclopropylidene; and IX, methylenecyclopropane.
It is interesting to compare these results with the data for other spirocompounds. It was shown in reference 3 that the extra strain in spirocondensed cyclopropanes is approximately proportional to the number of spiro-centres, which means that the Ž . spiro atom C s3,3 has an additional contribution of 27.2 kJ . moly1 to D f HT m g as compared with the ordinary C-ŽC.4 contribution Žbeing calculated in terms of Benson’s scheme.. Beckhaus et al.Ž31. later confirmed the proportionality of the extra strain energy in spirocondensed cyclopropanes to the number of spiro-centres, with slightly greater additional contribution of C s3,3 atoms. On the other hand, no excess strain energy was reported in reference 31 for spirocyclopropanated cyclobutanes. Considering dispiroŽ3.0.2.1.octane ŽIII. Žtable 4., we can see that if
FIGURE 2. The structural formulae of some relevant compounds: Ia, bicycloŽ1.1.0.butane; Ib, bicyclo Ž3.1.0..hexane; Ic, bicyclo Ž4.1.0..heptane; IIa, dispiro cyclopropane-6,19- pentacyclo Ž6.3.1.0 2,7 0 3,5 0 9,11 .dodecane-12,10-cyclopropane 4; IX, methylenecyclopropane.
Thermochemistry of polycyclic compounds
1463
the additional contribution of C s3,3 atom is accepted to be equal to 27.2 kJ . moly1 ,Ž3. the extra strain energy which can be applied to the C s3,4 spiro atom is very small, in accordance with reference 31. Obviously, only for spirocondensed cyclopropanes is the extra strain energy essential in a series of spirocyclopropanated compounds. A decrease of UŽstrain. in comparison with the cyclopropane ring, which is found in two isomers of the ethyl ether of 2-phenylcyclopropanecarboxylic acid ŽIV and V., can probably be associated with the influence of the phenyl group. The slight lessening of UŽstrain . in phenylderivatives of cyclopropane was also deduced for phenylcyclopropane Ž DUŽstrain. s y13.1 kJ . moly1 . and for the trans isomer of 2-phenylcyclopropanecarboxylic acid Ž DUŽstrain. s y13.1 kJ . moly1 ., on the Ž1,32,33. basis of D f HT m values obtained previously. The strain energy of bicyclic compounds with rings separated by a s-bond is usually close to SUŽstrain, cycl.:Ž1. e. g. for bicyclopropyl, UŽstrain. s 228 kJ . moly1 , about twice as much as UŽstrain. of cyclopropane itself. Meanwhile, there is a visible lessening of the strain energy in cyclopropylcyclobutene ŽVI. Žsee table 4., which indicates that the effect of conjugation may be noticeable in this compound. The introduction of a double bond in alkenyl derivatives of cyclopropane greatly increases the strain energy of the three-membered ring Žtable 4.. Considering the increase of UŽstrain. in methylenecyclopropane ŽIX., DUŽstrain . s 57.0 kJ . moly1 , the authors Ž2. paid attention to the fact that DUŽstrain . for this compound is about equal to the estimated strain energy for each trigonal C atom in cyclopropene. This value Ž54.6 kJ . moly1 . was taken as half the difference between the strain energies of cyclopropene and cyclopropane.Ž2. For compounds VII and VIII, DUŽstrain. is slightly less than in IX: 41.2 kJ . moly1 in VII, and 44.8 kJ . moly1 for each individual cyclopropylidene fragment in VIII, but it is very likely of the same nature. Obviously, the introduction of an exocyclic or endocyclic double bond increases the strain energy of three-membered rings about equally. This fact can be used for the further development of the additivity scheme for estimating D f HT m values of cyclopropane derivatives.
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(Recei¨ ed 5 March 1998; in final form 26 June 1998)
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