Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 0537 609

olefin (67a), saturated amine (67b) or even saturated carbocycle (67c), for enthalpies of formation of any of these more conventional species have yet to be determined.

H. Aromatic Aldimines

There are but two members of this class of compounds that have been studied with explicit interest as to their enthalpies of formation158. The first is benzaldimine (68a), studied by gas-phase ion molecule reactions141 for which the alternative possibility of troponimine (68b) was considered plausible. It is unequivocal that the enthalpy of formation of PhCHX is significantly more negative than that of cyclo-(CHDCH)3CX for both the archival X D O and the newly generated value of X D CH2, cf Reference 8. X D NH is expected, as before, to interpolate the ketone and olefinic cases and so benzaldimine is most assuredly more stable than troponimine. However, this species was ‘synthesized’ by deprotonation of the corresponding cation and protonated troponimine is probably more stable than protonated benzaldimine159. If it is the former ion that was seen in Reference 141, simple deprotonation is not expected to yield benzaldimine.

H

N NH

Ph

Ph N

The second species is N-phenylbenzaldimine (69), very commonly known as benzalaniline. Our archive cites an earlier study that resulted in an enthalpy of formation of 253.6 š 7.4 kJ mol 1, while more recently160 a value of 278.7 š 2.2 kJ mol 1 was reported. This result may be compared with earlier data. We earlier cited the enthalpy of formation of propenaldimine of ca 125 kJ mol 1. We can employ the standard86 ca 30 kJ mol 1 increase accompanying the transformation of ViX to PhX and so conclude the enthalpy of formation of benzaldimine is ca 155 kJ mol 1. Or alternatively acknowledging unresolved difficulties with unsaturated and conjugating groups X87, we may assume the same increase from CH2DCHCHDNH to PhCHDNH as from CH2DCHCHDCH2 to PhCHDCH2. This ‘fine-tuning’ will be assumed but needn’t be made. This results in a value of ca 163 kJ mol 1. Either way, as a result of N-phenylating benzaldimine, let us assume that the same increase of enthalpy of formation accompanies this process as from ethyl amine to EtNHPh, or ca 113 kJ mol 1. We thus predict an enthalpy of formation of benzalaniline of 276 kJ mol 1, essentially identical to the recently measured enthalpy of formation. The results and reasoning are both found to be credible and that is what we endeavoured to show.

I. Polyimines

The key question for such species is whether two imine groups will provide intramolecular stabilization or destabilization. There are three examples of these species for which there are relevant thermochemical data: 70a, the conjugated glyoxal bisimine, t BuNDCHCHDNBut (see Reference 153); 70b, the cyclic and likewise conjugated

non-conjugated ethylenediamine bis-anil PhCHDN(CH2)2NDCHPh from our archive. The first two species were investigated using hydrogenation calorimetry, the last by combustion calorimetry. We wish to put all three species on a common footing with each other and preferably with the mono-imines earlier discussed. Enthalpies of hydrogenation would appear to be such an approach: after all, any stabilization energy

0 š 10 kJ mol 1, suggestive of comparatively little stabilization. To investigate the last species it is necessary to estimate the enthalpy of formation of the saturated diamine N,N0- dibenzylethylenediamine, PhCH2NH(CH2)2NHCH2Ph. This may be facilely estimated by twice adding the υ(sec/prim, prim) methylene/amine increment of 73 kJ mol 1 from Reference 143 to the enthalpy of formation of Ph(CH2)6Ph and deriving this by adding the enthalpy of formation of four strainless methylene groups to that of 1,2- diphenylethane. The enthalpy of formation of the saturated diamine is ca 205 kJ mol 1. This corresponds, to an enthalpy of hydrogenation of the bis-imine of ca 150 kJ mol 1 or ca 30 kJ mol 1 stabilization. That the enthalpy of hydrogenation of the non-conjugated bis-imine is less than either of the conjugated species seems most unreasonable. However, it is then remembered that the same authors who investigated this bis-imine had erred by ca 30 kJ mol 1 for both benzalaniline and the alicyclic iPrCHDNBun. Assuming the same error results in the non-conjugated bis-imine lacking either stabilization or destabilization. We admit the numbers are not likely to be as precise as we have written them. Too many estimates have been made. However, the conclusion remains valid there is but a small enthalpic consequence of the interaction of imines with other imines.

J. Unusual Tautomers

Numerous well-known nitrogenous heterocycles have unconventional tautomers that are formally imines: e.g. pyrrole and 2H-pyrrole (71a and 71b), indole and 3H-indole (72a and 72b), imidazole and 4H-imidazole (73a and 73b), purine, 6H- and 8H-purine (74a, 74b and 74c). Of all of the diverse species we can posit, the only tautomeric imine we will discuss is 3H-indole. This is the last imine, indeed the last compound, that we will discuss in the current chapter. Generated161 by photochemical reactions of both N-phenacyl-indolenine (75a) and of the parent heterocycle (75b, with added ketone), its aqueous-phase Gibbs energy of isomerization to the more conventional (1H)-indole is 33š1 kJ mol 1. If one presumes to equate Gibbs energies and enthalpies, and aqueous and gaseous media, then one may derive an enthalpy of formation of gaseous 3H-indole given the enthalpy of formation of the conventional 1H-indole tautomer. While both of these assumptions are unequivocally invalid, nonetheless they are simple and useful and a value of ca 190 kJ mol 1 was suggested. Additionally, this value was shown to be numerically

plausible by reproducing it within 15 kJ mol 1 upon assuming that thermoneutrality for the following reactions;

These equations are appealing, but suffer from the debits that both pyridine and benzalaniline have the imine group flanked by unsaturated carbon on both sides while 3H-indole is better described as a cyclic N-alkylbenzaldimine with unsaturated carbon only on one side of the imine functionality. In that the following reaction is thermoneutral to 2 kJ mol 1:

we are optimistic that the following reaction is likewise essentially thermoneutral as well162:

This results in a predicted enthalpy of formation of 3H-indole of 193 kJ mol 1. That this value is so close to that suggested in the literature gives credence to the enthalpies of

formation of both 3H-indole and the earlier one cited above for 1-pyrroline. We are encouraged163.

XIII. REFERENCES AND COMMENTARY

1. J. F. Liebman and A. Greenberg, in The Chemistry of the Cyclopropyl Group, Part 2 (Ed.

Z. Rappoport), Wiley, Chichester, 1987.

2.J. F. Liebman and R. M. Pollack, in The Chemistry of Enones, Part 1 (Eds. S. Patai and

Z.Rappoport), Wiley, Chichester, 1989.

3.J. F. Liebman, K. S. K. Crawford and S. W. Slayden, in Supplement S: The Chemistry of Sulphur-containing Functional Groups (Eds. S. Patai and Z. Rappoport), Wiley, Chichester, 1993.

4.S. W. Slayden and J. F. Liebman, in Supplement E2: The Chemistry of Hydroxyl, Ether and Peroxide Groups (Ed. S. Patai), Wiley, Chichester, 1993.

5.J. F. Liebman and H. M. Perks, in The Chemistry of Enamines, Part 1 (Ed. Z. Rappoport), Wiley, Chichester, 1994.

6.S. W. Slayden, J. F. Liebman and W. G. Mallard, in Supplement D2: The Chemistry of Organic Halides, Pseudohalides and Azides (Eds. S. Patai and Z. Rappoport), Wiley, Chichester, 1995.

7.J. F. Liebman, in The Chemistry of the Cyclopropyl Group, Vol. 2 (Ed. Z. Rappoport), Wiley, Chichester, 1995.

8.J. F. Liebman, in The Chemistry of Dienes and Polyenes (Ed. Z. Rappoport), Wiley, Chichester, in press.

9.D. D. Wagman, W. H. Evans, V. B. Parker, R. H. Schumm, I. Halow, S. M. Bailey, K. L. Churney and R. L. Nuttall, ‘The NBS tables of chemical thermodynamic properties: Selected values for

inorganic and C1 and C2 organic substances in SI units’, J. Phys. Chem. Ref. Data, 11, Supplement 2 (1982).

10.J. S. Chickos, A. S. Hyman, L. H. Ladon and J. F. Liebman, J. Org. Chem., 46, 4294 (1981).

11.J. P. Guthrie, in The Chemistry of Enols (Ed. Z. Rappoport), Wiley, Chichester, 1990.

12.G. Pilcher, in Supplement B: The Chemistry of Acid Derivatives, Vol. 2 (Ed. S. Patai), Wiley, Chichester, 1992.

13.K. Pihlaja, in The Chemistry of Amidines and Imidates, Vol. 2 (Eds. S. Patai and Z. Rappoport), Wiley, Chichester, 1991.

14.R. Shaw, in The Chemistry of Cyanates and Their Thio Derivatives (Ed. S. Patai), Wiley, Chichester, 1977.

15.S. Sunner and M. Mansson˚ (Eds.), Experimental Chemical Thermodynamics, Vol. 1: Combustion Calorimetry, Pergamon, Oxford, 1979.

16.J. B. Pedley, R. D. Naylor and S. P. Kirby, Thermochemical Data of Organic Compounds (2nd ed.), Chapman & Hall, New York, 1986. This is the ‘chosen’ archival source of enthalpy-of- formation data, and so an unreferenced datum may be assumed to arise from this source. Where cited explicitly in the text, this volume will usually be referred to as ‘Pedley’.

17.We will also occasionally take data from

(a) E. S. Domalski, J. Phys. Chem. Ref. Data, 1, 221 (1972).

(b) D. R. Stull, E. F. Westrum and G. C. Sinke, The Chemical Thermodynamics of Organic Compounds, Wiley, New York, 1969.

(c) M. S. Kharasch, Bur. Stand. J. Res., 2, 359 (1929).

These last three sources will usually be referred to as ‘Domalski’, ‘Stull, Westrum and Sinke’ and ‘Kharasch’, respectively.

18.Admittedly unreferenced phase-change enthalpies are taken from any of the following sources:

(a) J. S. Chickos, in Molecular Structure and Energetics, Vol. 2 (Physical Measurements) (Eds.

J.F. Liebman and A. Greenberg), VCH Publishers, Deerfield Beach, 1987.

(b)W. E. Acree, Thermochim. Acta, 189, 37 (1991); 219, 97 (1993).

(c)E. S. Domalski and E. D. Hearing, J. Phys. Chem. Ref. Data, 25, 1 (1996). (This study subsumes the earlier ones by these authors: E. S. Domalski, W. H. Evans and E. D. Hearing, “Heat Capacities and Entropies of Organic Compounds in the Condensed Phase”, J. Phys. Chem. Ref. Data, 13, Supplement 1 (1984) and E. S. Domalski and E. D. Hearing, J. Phys. Chem. Ref. Data, 19, 881 (1990).

19. R. Shaw, in Supplement A: The Chemistry of Double-bonded Functional Groups, Part 1 (Ed.

S. Patai), Wiley, London, 1977.

20.J. D. Cox and G. Pilcher, Thermochemistry of Organic and Organometallic Compounds, Academic Press, New York, 1970.

21.W. V. Steele and R. D. Chirico, J. Phys. Chem. Ref. Data, 22, 377, (1993).

22.J. L. Jensen, ‘Heats of Hydrogenation: A Brief Summary’, in Progress in Physical Organic Chemistry, Vol. 12 (Ed. R. W. Taft), Wiley, New York, 1976.

23.D. W. Rogers, O. A. Dagdagan and N. L. Allinger, J. Am. Chem. Soc., 101, 671 (1979).

24.D. W. Rogers and S. Skanupong, J. Phys. Chem., 78, 2569 (1974); D. W. Rogers, J. Phys. Chem., 83, 2430 (1979).

25.D. W. Rogers and E. L. Crooks, J. Chem. Thermodyn., 15, 1087 (1983).

26.D. W. Rogers, E. Crooks and K. Dejroongruang, J. Chem. Thermodyn., 19, 1209 (1987).

27.D. W. Rogers and K. Dejroongruang, J. Chem. Thermodyn., 20, 675 (1988).

28.D. W. Rogers and K. Dejroongruang, J. Chem. Thermodyn., 21, 1115 (1989).

29.D. W. Rogers, K. Dejroongruang, S. D. Samuel, W. Fang and Y. Zhao, J. Chem. Thermodyn., 24, 561 (1992).

30.K. B. Wiberg, D. J. Wasserman and E. Martin, J. Phys. Chem., 88, 3684 (1984).

31.K. B. Wiberg and D. J. Wasserman, J. Am. Chem. Soc., 103, 6563 (1981).

32.K. B. Wiberg and S. Hao, J. Org. Chem., 56, 5108 (1991).

33.Two examples illustrate this point. The enthalpy of formation of gaseous 1-heptene was deter-

mined from the enthalpy of hydrogenation to be 62.6 š 1.6 kJ mol 1. The enthalpies of combustion and of vapourization (35.6 š 0.2 kJ mol 1) of the liquid were later measured, from which the enthalpy of formation of the gas was determined to be 62.3 š 1.0 kJ mol 1. Yet Pedley did not incorporate the former measurement in the selected value although it is listed among the literature cited. This choice of experimental values is in contrast to a selected composite enthalpy of formation for gaseous 1-pentene which includes an enthalpy of formation derived from the enthalpies of combustion and vapourization of the liquid ( 21.5 š 0.8 kJ mol 1) and an enthalpy of formation derived from the equilibrium isomerization of 1-pentene and trans-2- pentene ( 21.0 š 1.4 kJ mol 1).

34.Discussions of the linear relationship between enthalpy of formation and carbon number are found in E. J. Prosen, W. H. Johnson and F. D. Rossini, J. Res. Natl. Bur. Stand., 37, 51 (1946) and in References 3 and 20.

35.The uncertainty interval for the quantity being determined is the square root of the sum of the

squares of the individual enthalpies used in the calculation.

36. L. Flitcroft, H. A. Skinner and M. C. Whiting, Trans. Faraday Soc., 53, 784 (1957);

H. A. Skinner and A. Snelson, Trans. Faraday Soc., 55, 404 (1959).

37.H. F. Bartolo, Ph.D. Thesis. Carnegie Institute of Technology (1956).

38.H. F. Bartolo and F. D. Rossini, J. Phys. Chem., 64, 1685 (1960).

39.M. C. Loeffler and F. D. Rossini, J. Phys. Chem., 64, 1530 (1960).

40.J. D. Rockenfeller and F. D. Rossini, J. Phys. Chem., 65, 267 (1961).

41.K. Yates and R. S. McDonald, J. Org. Chem., 38, 2465 (1973).

42.We might have written equation 7 in the form Hf(alkene, g) D mž Hf(alkane, g) ‘ Hhyd’ so that Hf(alkene), which is more likely to be unknown, is calculated directly from the other two enthalpies. However, this form has a statistical disadvantage. The extrapolation to the y- axis [ Hf(alkene)] to generate ‘ Hhyd’ (the intercept) is much longer in this form than in the preferred equation and so the uncertainty in the estimated ‘ Hhyd’ is approximately doubled.

43.The enthalpies of vapourization of interest found in Reference 16 and Reference 21 vary no more than 0.1 kJ mol 1, except for the values for cis-2-hexene which differ by 0.7 kJ mol 1. We chose Hv D 32.2 kJ mol 1 from the latter source.

46.W. D. Good and N. K. Smith, J. Chem. Thermodyn., 11, 111 (1979).

47.Admittedly, the butene data can be shown to be outliers if different enthalpies of formation are chosen for the analysis. For example, cis-2-butene is slightly stabilized compared to the C5 C7 points if only data from References 30 and 31 are plotted according to equation 4a. Trans-2- butene is slightly stabilized if the ‘best’ straight line in a plot of equation 4a is drawn with a slightly different slope.

48.We have often wondered if there is a ‘large enough’ nc such that equation 4 exhibits a break or curvature and the bond additivity changes within a homologous series to become essentially the

same as for the n-alkanes. If such a change were to occur in the 2- and 3-n-alkenes at nc ca 7 9, it might avoid the current conundrum of reversed stability order.

49.The enthalpy data for the 2-alkenes come from References 16, 21, 26 and 28. Where more than one enthalpy of formation for a compound is available a simple average is taken. The enthalpy data for the 1-alkenes are those specifically cited in the previous section.

50.The enthalpy of formation of 2,4-dimethyl-2-pentene from Reference 21 is less negative than that of 3,4-dimethyl-2-pentene and would be the only one of the three 1,1-dimethyl-2-alkyl ethenes to be less stable than its 1,2-dimethyl-2-alkyl isomer.

51.D. W. Rogers, H. von Voithenberg and N. L. Allinger, J. Org. Chem., 43, 360 (1978).

52.See, for example, Reference 7.

53.See, for example, A. Greenberg and J. F. Liebman, Strained Organic Molecules, Academic Press, New York, 1978.

54.W. R. Roth, O. Adamczak, R. Breuckmann, H.-W. Lennartz and R. Boese, Chem. Ber., 124, 2499 (1991).

55.We know of no experimental measurements for the enthalpy-of-formation difference of (Z)- and (E)-cycloheptene or cyclohexene. Admitting for now the 1-phenyl derivative, the (Z)-/(E)- differences are 56, 121 and 197 kJ mol 1 for the substituted cyclooctene, cycloheptene and cyclohexene. [These results derived from photoacoustic calorimetric measurements were reported by A. D. Strickland and R. A. Caldwell, J. Phys. Chem., 57, 13394 (1993).]

56.J. F. Liebman and A. Greenberg, Chem. Rev., 89, 1225 (1989).

57.J. F. Liebman and A. Greenberg, Chem. Rev., 76, 311 (1976).

58.Were superstrain absent, the difference of the enthalpies of formation of bicyclo[n.1.0]alkanes and (n C 2)-membered rings would be independent of n. Accepting the enthalpies of formation of the bicycles from either Reference 16 or 56, we find for n D 2, 3 and 4 values of 130.3 š

1.5, 115.2 š 1.2 and 111.0 š 1.5 kJ mol 1. It is thus seen that some of the destabilization of bicyclo[2.1.0]pentene can be ascribed to this superstrain of the bicycle, the majority of the destabilization of the olefin arises from electronic factors.

59.D. W. Rogers, F. J. McLafferty, W. Fang and Y. Qi, Struct. Chem., 4, 161 (1993).

60.D. W. Rogers, F. J. McLafferty, W. Fang, Q. Yang and Y. Zhao, Struct. Chem., 3, 53 (1992).

61.D. W. Rogers, F. J. McLafferty and K. Channamallu, Struct. Chem., 3, 291 (1992).

62. The enthalpy of hydrogenation of the alkene was taken from Reference 54, and the enthalpy of formation of the requisite bicycloalkane was suggested by J. F. Liebman, M. S. Campbell and S. W. Slayden, in Supplement F2: The Chemistry of Amino, Nitroso, Nitro and Related Compounds (Ed. S. Patai), Wiley, Chichester, 1996.

63.D. W. Rogers, S. A. Loggins, S. D. Samuel, M. A. Finnerty and J. F. Liebman, Struct. Chem., 1, 481 (1990).

64.J. F. Liebman, L. A. Paquette, J. R. Peterson and D. W. Rogers, J. Am. Chem. Soc., 108, 8267 (1986).

65.We adopt the enthalpies of formation from Stull, Westrum and Sinke, the combustion measurements of G. Becker and W. A. Roth, Chem. Ber., 67, 627 (1934) and the structural assignments of K. Alder and G. Stein, Chem. Ber., 67, 612 (1934). The ˛- and ˇ-isomers were also called ‘endo’ and ‘exo’ in this last reference to reflect the stereochemistry of the terminal 5-membered ring: we note the exo-‘fusion’ of the norbornane units.

66.We consider this plausible assumption somewhat suspect. Admitting ignorance as to the difference in solvation energies of the two cyclic olefins and the methylcycloalkane, we note that summing the enthalpies of hydrogenation and formation of liquid methylenecyclobutane results in a value

of the enthalpy of formation of liquid methylcyclobutane of ca 29 kJ mol 1, in marked contrast to the archival value of 44.5 š 1.4 kJ mol 1. The source of the discrepancy is not apparent.

67.N. L. Allinger, H. Dodziuk, D. W. Rogers and S. N. Naik, Tetrahedron, 38, 1593 (1982).

68.Our further analysis of the enthalpy of formation of 2-methylbicyclo[2.2.1]heptene only worsens the disparity. That is, we find methylation of one doubly bonded carbon in gaseous cyclopropene, cyclopentene and cyclohexene is accompanied by a decrease in enthalpy of formation of 34, 38 and 38 kJ mol 1, i.e. 36 š 2 kJ mol 1. The recommended enthalpy of

formation of bicyclo[2.2.1]heptene (see Reference 60) is 90 kJ mol 1 and so we would predict an enthalpy of formation of its gaseous 2-methyl derivative of 90 36 ³ 54 kJ mol 1. Using our standard enthalpy of vapourization estimation protocol we would predict a phase-change enthalpy of 40 kJ mol 1 for this species, and so derive an enthalpy of formation of liquid 2-

604 Suzanne W. Slayden and Joel F. Liebman

That 2-methylenebicyclo[2.2.1]heptane and 2-methylbicyclo[2.2.1]heptene have the same carbon skeleton suggests some experiments that can directly give information as to their enthalpy- of-formation difference. The first suggested experiment is the measurement of hydrogenation to 2-methylbicyclo[2.2.1]heptane. On the assumption that the same exo/endo ratio is found and that the two olefins have the same enthalpy of solution, the difference of the enthalpies of hydrogenation equals the numerical negative of the enthalpies of formation. The second suggestion notes that deprotonation of both olefins yields the same carbanion: it is reasonable to assume that the two olefins have nearly identical enthalpies of solution here as well. Such a deprotonation study was performed on the isomeric 1- and 2-propenylbenzenes [E. Taskinen and N. Lindholm, J. Phys. Org. Chem., 7, 256 (1994)]. The last approach notes that protonation of both olefins gives the same carbocation. This would parallel the investigation of the thermochemistry of the acid-catalysed interconversion of 2-ethylbicyclo[2.2.2]oct-2-ene and 2- ethylidenebicyclo[2.2.2]octane [cf G. Van Binst and Y. Hera, Tetrahedron Lett., 3897 (1967)] that showed the latter to be more stable by 5.4 š 0.1 kJ mol 1. Alternatively, measurement of the low-temperature solution phase reaction enthalpies of these olefins with HCl to form 2- chloro-2-methylbicyclo[2.2.1]heptane [cf E. M. Arnett and N. J. Pienta, J. Am. Chem. Soc., 102, 3329 (1980)] would allow for direct numerical determination of the difference of enthalpies of formation. Relatedly, and even more directly, is to measure the (gas phase) proton affinities of the two olefins. In principle, this should give us the numbers that are the most trustworthy to subtract. Both numbers are available [cf the archival review by S. G. Lias, J. F. Liebman and

R.D. Levin, J. Phys. Chem. Ref. Data, 13, 695 (1984)] but we are thwarted in our comparison because the necessary ‘bracketing data’, which are effectively calibration experiments, are lacking for the exocyclic isomer.

69.In turn, our archive shows that 3-methylcyclopentene has been reported to be 7.2 š 2.1 kJ mol 1 more stable than the 4-isomer. Interestingly, hydrogenation calorimetry (cf Reference 67) shows the 1,3-isomer difference to be 14.1 kJ mol 1 but no data is available to compare the 3- and 4-isomers. We also note that 1-methylcyclohexene (as liquid) is found to be 6 š 4 kJ mol 1 more stable than its 4-isomer as shown by the archival enthalpy of formation of the former and the measurement of B. Lebedev and H. Smirnova, Macromol. Chem. Phys., 195, 35 (1994).

70.W. R. Maier and P. v. R. Schleyer, J. Am. Chem. Soc., 103, 1891 (1981); A. B. McEwen and

P.v. R. Schleyer, J. Am. Chem. Soc., 108, 3951 (1986).

71.Z.-H. Li and M. Jones, Jr., Tetrahedron Lett., 28, 752 (1987).

72.P. M. Warner, Chem. Rev., 89, 1067 (1989).

73.D. W. Rogers and F. J. McLafferty, Tetrahedron, 27, 3765 (1971). In particular, the enthalpy of hydrogenation of 1,2-dimethylcyclopropene is ca 180 kJ mol 1, a value considerably higher than the acyclic tetrasubstituted paradigm. We recall that cyclopropanes with trigonal carbons are additionally strained beyond that of the saturated ring (cf Reference 7), and so that this enthalpy of hydrogenation is large is not particularly surprising. Indeed, the value for 1,2- dimethylcyclopropene is quite small compared to the ca 220 kJ mol 1 for both the parent and 1-monomethylated cyclopropene that we derive from the data in Reference 57.

74.The reader may wonder why we did not present the energetics of reaction 14 in the text for the currently discussed n D 4 case. We acknowledge that we do not know the precise cis/trans composition of the resulting 1,2-dimethylcyclobutane but the difference of the enthalpies of these isomers is expected to be less than 10 kJ mol 1 by analogy with other dimethylcycloalkanes. As

probed by the deviation from thermoneutrality for the reaction 2 cyclo-[ CH2 n 1CHMe] ! cyclo-[1,2-(CH2)2 CHMe n 2] C cyclo-[ CH2 n ] for other cycloalkanes (ca 6 š 6 kJ mol 1 endothermic), steric repulsion is expected to be small. The enthalpy of formation of 1,2- dimethylcyclobutane is achievable from that of the parent and monomethylated cyclobutanes. Experimentally measured enthalpy-of-formation data for methylcyclobutane is limited to that of the liquid, but our estimation protocol for enthalpies of vapourization easily ameliorates that deficiency. From all of the above data and assumptions, we derive enthalpies of formation of gaseous 1,2-dimethylcyclobutane and 1,2-dimethylcyclobutene to be ca 60 and 50 kJ mol 1,

respectively. Accordingly, we find the desired reaction energy to be ca 50 kJ mol 1 endothermic. This value makes little sense the origin of the very large deviation from thermoneutrality evades us.

75.Although the data are from the recent source, Allinger and coworkers67, the reader may still be bothered by results for hydrogenation in different solvents being discussed in this section. There are two reasons for our policy. The first is that we lack the exquisite collection of species that

characterized our discussion of the acyclic alkenes in an earlier section. The second is that within an admittedly greater tolerance for discrepancies, the differences are seemingly quite small: recall our earlier discussion of the monosubstituted 1-methylcyclopentene and methylenecyclopentene.

76.P. K. Chou and S. R. Kass, J. Am. Chem. Soc., 113, 697 (1991).

77.W. J. Hehre and J. A. Pople, J. Am. Chem. Soc., 97, 6941 (1975). This olefinic non-planarity of derivatives of 35 is quite general; cf W. T. Borden, Chem. Rev., 89, 1095 (1989).

78.The reader is reminded of the suggested interpolation of norbornyne between cyclobutyne and

cyclopentyne discussed in Greenberg and Liebman53.

79. The origins of the 9 kJ mol 1 error and the much larger discrepancy between the two literature results is not obvious. However, we do note that for both naphthalene and 1,2,3,4- tetrahydronaphthalene, much better known hydrocarbons with the same carbon skeleton as the m D n D 4 species, our primary archive cites alternative enthalpy-of-formation measurements that deviate by ca 10 and 40 kJ mol 1 from those recommended by this source.

80.P. D. Staneke, S. Ingemann, P. Eaton, N. M. M. Nibbering and S. R. Kass, J. Am. Chem. Soc., 116, 6445 (1994).

81.Our analysis accepts the archival enthalpy of formation of cubane, although we admit there

is some 40 kJ mol 1 disagreement over this value; see D. R. Kirklin, K. L. Churney and

E.S. Domalski, J. Chem. Thermodyn., 21, 1105 (1989).

82.An example of the problem with species with multiple four-membered rings is seen in the

inorganic chemical literature involving P< in lieu of CH<. The standard form of elemental phosphorus is P4 with 60° P P P angles. Trivalent phosphorus ‘enjoys’ angles of somewhat over 90°. So, why is cubic P8 with P P P angles of 90° less stable than P4? Theorists have found this last question of considerable interest. See the nearly contemporaneous papers:

(a) M. W. Schmidt and M. S. Gordon, Inorg. Chem., 24, 4503 (1985).

(b) G. Trinquier, J.-P. Daudey and N. Komiha, J. Am. Chem. Soc., 107, 7210 (1985).

(c) R. Alrichs, S. Brode and C. Ehrhardt, J. Am. Chem. Soc., 107, 7260 (1985).

(d) K. Raghavachari, R. C. Haddon and J. S. Binkley, Chem. Phys. Lett., 122, 219 (1985).

83.J. Kiplinger, F. R. Tollens, A. G. Marshall, T. Kobayashi, D. R. Lagerwall, L. A. Paquette and

J.E. Bartmess, J. Am. Chem. Soc., 111, 6915 (1989).

84.The standard enthalpy of formation of dodecahedrane remains unmeasured. However, standard enthalpies of formation of a bis-carbomethoxy derivative of the isomeric pagodane, and pagodane are available in the literature: H.-D. Beckhaus, C. Ruchardt,¨ D. R. Lagerwall, L. A. Paquette,

F.Wahl and H. Prinzbach, J. Am. Chem. Soc., 116, 11775 (1994), from which these authors suggested a value of 94 š 5 kJ mol 1 for the desired quantity.

85.This problem does not arise in the cubene study because one can transform the ‘proper’ or ‘directly synthesized’ product ‘all-syn’-tetracyclo[4.4.0.02,5 .07,10]deca-3,8-diene, species 28, to its all-anti-isomer without particularly compromising our model. No such option exists for the ‘decacyclodocosadiene’ product that would be formed from the related, formal dodecahedrene/ethylene reaction.

86.(a) J. F. Liebman, in Molecular Structure and Energetics: Studies of Organic Molecules (Eds.

J.F. Liebman and A. Greenberg), VCH, Deerfield Beach, 1986.

(b) P. George, C. W. Bock and M. Trachtman, in Molecular Structure and Energetics: Biophysical Aspects (Eds. J. F. Liebman and A. Greenberg), VCH, New York, 1987.

87.Y.-R. Luo and J. L. Holmes, J. Phys. Chem., 96, 568 (1992) and J. Phys. Org. Chem., 7, 403 (1994).

88.From the tables of recommended values in Pedley, we find gaseous (E)-1,2-diphenylethylene 16.2 š 2.6 kJ mol 1 more stable than its (Z)-isomer, while E. L. Eliel and J. J. Engelsman, J. Chem. Educ., in press, recommend a difference of 22.8 š 2.8 kJ mol 1. These values are not altogether that different. However, it is quite seemingly accidental that these values are as close as they are because the recommended enthalpies of sublimation of the (E)-isomer differ by nearly

13 kJ mol 1 in the two sources while individually professing error bars less than 0.5 kJ mol 1.

89.Furthermore, given the phenyl/vinyl comparison, we also note interesting features of the energetics of cross-conjugated vs conjugated 2-vinylbutadiene and 1,3,5-hexatriene (see Reference 8).

90.S. E. Stein, D. M. Golden and S. W. Benson, J. Phys. Chem., 81, 314 (1977).

91.We have enthalpy-of-formation data for both 1,1- and (Z)-1,2-bis(p-tolyl)ethylene as solids. What is lacking are the corresponding data for the diphenylethylenes, or equally useful, the enthalpies of fusion of these two hydrocarbons.