Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups / 0337 378

8. Thermochemistry of amines, nitroso, nitro compounds and related species 347

the sole azetidine for which enthalpy of formation data seemingly are available. The natural comparison is with phenethylcyclobutane, for which we estimate an enthalpy of formation of the liquid of 25 kJ mol 1 by assuming thermoneutrality of the liquid-phase transalkylation reaction 9.

EtCH(CH2)3 (l) C PePh (l) ! PhCH2CH2CH(CH2)3 (l) C n-C5H12 (l) 9

From this and υ6(lq), Hf(6, lq) is predicted to be 114 kJ mol 1 compared to the experimental value of 122 kJ mol 1. The comparatively large discrepancy is suggestive that azetidines, like cyclobutanes, enjoy unusual substituent effects.

We now consider N-methyl-2-(3-pyridyl)pyrrolidine (7), also known as the alkaloid nicotine. Parallelling our discussion of coniine, we find in Kharasch and Domalski the nearly century-old enthalpy of formation for the liquid of 39 kJ mol 1. Is this value plausible? With υ6(liquid, tert/ R, R1, R2) necessary to transform a tertiary amine into the corresponding hydrocarbon24, we would conclude that the enthalpy of formation of liquid 1-methyl-2-(3-pyridyl)cyclopentane (8, R D 3-Py) is ca 50 kJ mol 1. To estimate the last datum in another way, we assume that equation 10 is essentially thermoneutral.

We derive the desired enthalpy of formation to be ca 12 kJ mol 1. The origin of the nearly 40 kJ mol 1 discrepancy eludes us. Again the age of the data is suggestive of problems.

The fifth and final type of amine are bicyclic tertiary amines, e.g. the isomeric quinuclidine (9) and pyrrolizidine (10). Do the earlier-derived gaseous and liquid υ6(tert/ R, R1, R2) values remain valid? From the enthalpy of formation of gaseous bicyclo[2.2.2]octane (11) of 99 š 1.1 kJ mol 1 we calculate an enthalpy of formation of 6 š 3 kJ mol 1 for 9. From the archival enthalpy of formation of gaseous cis- bicyclo[3.3.0]octane (12) of 92.9 š 1.5 kJ mol 1, we deduce an enthalpy of formation of pyrrolizidine of 0 š 3 kJ mol 1. The literature values for these species are 4.2 š 1.3 kJ mol 1 and 3.9 kJ mol 1, respectively25. That the hydrocarbons have a 6 kJ difference while the nitrogen compounds are identical is a surprise this may reflect the uncertainty or error in the analysis, or the estimations of enthalpy of vaporization, both ours or from the literature. What about the liquid? Using temperature-uncorrected fusion enthalpies for solid 11 and 9, we derive enthalpies for the corresponding liquids of 138 and 49 kJ mol 1. The literature enthalpies of formation of liquid 10 and 12 are 136.0 š 1.3 and 48.3 š 3.1 kJ mol 1 resulting in a υ6 of 87.7 š 3.4 kJ mol 1, in agreement with our previously suggested difference.

Consider now the cis-2,8-dimethyl-trans-pyrrolizidine (13, X = N), with its trans-ring fusion and its gas-phase enthalpy of formation of 66.7 š 2.6 kJ mol 1. The enthalpy of formation of the corresponding hydrocarbon, 2,8-dimethyl-trans-bicyclo[3.3.0]octane (13, X =CH) is estimated to be 126 kJ mol 1 by assuming thermoneutrality of reaction 11.

An exchange quantity, υ6(tert/ sec, sec, sec), not previously calculated, is found to be ca 59 kJ mol 1 from this example. By comparison with other exchange quantities in Table 2 this value seems inaccurate26. However, we note that the cis/trans enthalpy difference of pyrrolizidine (ca 14 kJ mol 1 from Reference 25) is significantly less than the ca 26 kJ mol 1 we deduce for bicyclooctane. This should not surprise us because trivalent nitrogen has been known to accommodate strain by partial or total planarization27.

348 Joel F. Liebman, Mary Stinecipher Campbell and Suzanne W. Slayden

IV. ANILINES AND OTHER AROMATIC AMINES

We now turn to the subject of aromatic amines. The number of classes of aromatic species vastly exceeds the number of classes of aliphatic and alicyclic amines. We somewhat arbitrarily consider mixed aromatic/ ‘ali-’ species such as N,N-dimethylaniline and N-methyldiphenylamine as aromatic and so they accompany the totally ‘aromatic’ triphenylamine instead of joining the totally ‘ali-’ species trimethylamine. However more numerous the compounds of potential interest, the number of relevant aromatic species for which there are appropriate thermochemical data is also few.

A. Aniline and N-Alkylated Anilines

The first compound of interest is aniline (14) itself. While drawings of its resonance structures permeate textbooks, the research literature acknowledges ambiguities as to its quantitation28. For example, there is a ca 38 kJ mol 1 spread of plausible resonance energies for aniline as defined by the exothermicity of reaction 12

generates a resonance stabilization energy of ca 14 and 11 kJ mol 1 for these two R groups. Further problems in defining resonance energies in aromatic amines arise as we consider N-alkylated anilines because we lack the enthalpy of formation of the reference aliphatic amines. For example, consider N,N-dimethylaniline (15). Equation 13 suggests the use of i-PrNMe2 or t-BuNMe2 but, regretfully, no enthalpies of formation have been reported for either species. We note that even the simpler question of the accuracy of the enthalpies of formation of N-alkylated anilines is problematic since these compounds are expected to oxidize and polymerize on standing, thus compromising the thermochemical measurement.

tities are less negative than their all-aliphatic counterparts. There are two experimental enthalpy-of-formation values in the literature30,31 for N-methylaniline (16) and calculating υ5(sec/ Ph, Me) from either of them results in values which are compatible with the trend noted in Table 2. Remembering from a previous section that υ5 is roughly the average of υ4 and υ6, we directly calculate υ5(sec/ Ph, Me) as ca 67 kJ mol 1 and thus the enthalpy of formation of N-methylaniline as ca 97 kJ mol 1. This result is compatible with the Hf derived from the enthalpy of reaction measurements31 which is not so surprising. The near thermoneutrality of that measurement provides an experimental rationalization for our previous estimate of υ5 from equation 8. In contrast, the value from the enthalpy of combustion measurements is quite different30.

A second assessment of the enthalpy of formation of 16 is made by extending equation 13 and its reaction enthalpy to include the N-methylated anilines 16 and 15.

8. Thermochemistry of amines, nitroso, nitro compounds and related species 349

Difference quantity υ14(Ph, Me) is defined independent of whether the amine be primary, secondary or tertiary.

We find the results 24 š 2 and 14 š 5 kJ mol 1 for 14 and 15, respectively. If equation 14 is accepted as a quantitative measure of resonance stability of anilines, then N,N-dimethylaniline has a lower resonance energy than the parent aniline. We would have expected the alkyl groups on nitrogen to stabilize the positive charge in the dipolar resonance structure and so, if anything, we would have expected N,N-dimethylaniline to have the higher resonance or stabilization energy. If υ14(Ph, Me) for 16 is the average of the other two, the resonance energy of 16 interpolates that of 14 and 15. The enthalpy of formation of the monomethylated species is thus calculated to be 96 š 5 kJ mol 1.

Consider N-ethylaniline (17) in concert with 16. The literature enthalpy of formation of 17 is 56.3š5.9 kJ mol 1. The calculated υ5(sec/ Ph, Et) of ca 48 kJ mol 1 is not incompatible with the values in Table 2 and their aromatic extensions calculated above. One can estimate the enthalpy of formation 17 by assuming methyl/ethyl difference quantities are reasonably constant. Two such are equations 15 and 16.

There are data only for R D H and R1 D H, resulting in difference quantities of 24.4 and 27.0 kJ mol 1. The enthalpy of formation of N-ethylaniline calculated using the average of υ15 and υ16 and each of the two literature values for N-methylaniline would be ca 59 and 70 kJ mol 1. The former value is closer to the literature value. Perhaps we should not be surprised because the same problems that may beset combustion measurements of the enthalpy of formation of N-methylaniline are likely to be applicable to N-ethylaniline, especially since measurements of both species appear in the same literature source.

B. Primary Aromatic Amines Containing more than One Benzenoid Ring

Let us now turn to compounds with more than one benzenoid ring. The first species are the isomeric ˛- and ˇ-naphthylamines, 18a and 18b. The archival enthalpies of formation are found to be 157.6 š 6.9 and 133.8 š 5.1 kJ mol 1. The 24 š 9 kJ mol 1 difference of these two numbers is incompatible with the near-zero difference of the enthalpies of formation for the isomeric naphthols, methyland bromonaphthalenes32. Which or either naphthylamine has the ‘correct’ enthalpy of formation? The gas-phase enthalpies of formation of the naphthols differ from their single benzene ring analog, phenol, by 66 kJ mol 1 in close agreement with the difference between the methylnaphthalenes and toluene, 63, and between the brominated and parent hydrocarbons, 69 š 6 and 68 š 2 kJ mol 1 respectively. That is, it is plausibly asserted33 that the difference quantities υ17 are nearly constant and equal.

υ17 D Hf(1-NpX, g) Hf(PhX, g) D Hf(2-NpX, g) Hf(PhX, g) 17

Taking an average phenyl/naphthyl enthalpy difference of 66 š 3 kJ mol 1 results in a predicted enthalpy of formation of either naphthylamine of ca 153 kJ mol 1. Both naphthylamine values are problematic. The ˛-isomer just fits this estimate by invoking allowances from large error bars. Interestingly, using the older enthalpies of (solid)

350 Joel F. Liebman, Mary Stinecipher Campbell and Suzanne W. Slayden

formation and of sublimation of 18b (the same and sole references as for its isomer 18a), we find an enthalpy of formation of the gaseous species to be 150.9 š 6.8 kJ mol 1. Now, agreement between the enthalpy of formation of both 18a and 18b and with our expectation is confirmed34.

We now turn to derivatives of biphenyl. Our archives show a 12.8 š 6.3 kJ mol 1 difference in the enthalpies of formation of the 2- and 4-amine as solids35. Is this difference due to strain in the former species? One probe of the strain energy is to consider the enthalpies of reaction 12 for R D 2- and 4-PhC6H4. υ12 for the 2-isomer equals 1.5 š 2.6 kJ mol 1. We do not know what it is for the 4-isomer because we lack all phase-change enthalpy data for this species. Intuitively, this difference quantity should be 0 because no stabilizing or destabilizing effects are expected for this isomer. We thus conclude that 2-aminobiphenyl is essentially strain-free.

C. Secondary and Tertiary Aromatic Amines

In this section we discuss the phenylamines. The first comparison involves the series NH3, PhNH2 (14), Ph2NH (19) and Ph3N (20) and their corresponding enthalpies of formation of 45.94 š 0.35, 87.1 š 1.0, 219.3 š 3.0 and 326.8 š 4.1 kJ mol 1. It is interesting to note that the effects of sequential phenylation as manifested by the difference of Hf(NH3) and Hf(14) and of Hf(14) and Hf(19), are essentially equal, 133.0 š 1.1 and 132.2 š 3.2 kJ mol 1 respectively. This means that equation 18 is thermoneutral.

5.1 kJ mol 1, suggesting that triphenylamine is more stable than we would have thought and/or diphenylamine is correspondingly less stable36. Equivalently, disproportionation reaction 19 is found to be 25 kJ mol 1 exothermic.

is more sensibly endothermic37 by 8.1 š 2.1 kJ mol 1. We assert ‘sensibly’ because steric repulsion between the phenyls destabilizes the phenylmethanes and we would have thought also the phenylamines as well. Furthermore, the general ‘saturation’ of conjugative effects would have suggested decreasing amounts of additional stabilization as the number of phenyls in the phenylamine increase. Finally, we note that the corresponding methyl reactions

are exothermic by ca 9 kJ mol 1 for both reactions.

V. POLYAMINES

Earlier in this chapter we discussed the enthalpies of formation of both aliphatic and aromatic monoamines. We now turn to polyamines where an obvious question is whether the effect of multiple amino groups on enthalpies of formation is simply the additive effects

8. Thermochemistry of amines, nitroso, nitro compounds and related species 351

of the individual amino groups or whether there is any stabilization or destabilization in the molecule that results from the concurrent presence of more than one amino group.

A. Aliphatic Diamines

Let us start with aliphatic species and sequentially move the amino groups apart. We are seemingly thwarted immediately. There is no enthalpy of formation reported for methylenediamine (diaminomethane) CH2(NH2)2 (21), a species we recognize as the simplest polyaminoalkane. As such the exothermicity of reaction 22

cannot be directly determined so as to allow its use as a probe of intersubstituent effects. We are not surprised by the absence of the desired data. After all, this species has never been synthesized. However, in that the gas-phase enthalpies of formation of mono-, diand trimethylamine have the same enthalpy of formation to within 5 kJ mol 1, we are confident that we can learn about the enthalpy of formation of 21 by studying that of its N,N,N0,N0 -tetramethyl derivative, 22, with its experimentally measured gas-phase value of 17.7 š 1.8 kJ mol 1. In the absence of any steric or electronic effects, the following reaction clearly would be thermoneutral:

In fact, this reaction is exothermic by almost 45 kJ mol 1. This exothermicity is consistent with anomeric effect reasoning38. Steric effects on the enthalpy of reactions 22 and 23 are expected to be small. After all, reactions 24 and 25 the all-hydrocarbon counterparts of reactions 22 and 23 are exothermic by 11.5 š 0.9 and 7.7 š 1.5 kJ mol 1 and so differ in their exothermicities by but 4 kJ.

Let us assume that the difference in exothermicities between reactions 24 and 25 is the same as between 22 and 23. We thus conclude that reaction 22 will be exothermic by the exothermicity of reaction 23 and the difference of reactions 24 and 25. The net result is some 45 C 4 D 49 kJ mol 1. The enthalpy of formation of 21 is predicted39 to be ca21 kJ mol 1.

If two amino groups geminalto each other provide stabilization, what about vicinalto each other? The archetype here is 1,2-diaminoethane or ethylenediamine, (CH2)2(NH2)2

is thermoneutral to better than 1 kJ mol 1. It appears that vic-diamino substitution is stabilizing40 but not to as great an extent as gem-substitution. The enthalpy of formation of three C-alkylated ethylenediamines are known: the 1-Me, 1-Et and 1,1-Me2 derivatives with corresponding gas-phase values of 53.6š0.5, 74.0š0.8 and 90.2š0.7 kJ mol 1. Another natural comparison for ethylenediamines is with the correspondingly alkylated ethylamines wherein a CH2NH2 group is deaminated to form CH3. Thus the three compounds above are deaminated to isopropylamine, 2-butylamine and t-butylamine,

352 Joel F. Liebman, Mary Stinecipher Campbell and Suzanne W. Slayden

respectively. The enthalpy of formation changes are 30, 31 and 31 kJ mol 1, nearly the same as the 29.8 found for the non-alkylated ethylenediamine ! ethylamine transformation.

What about species where the two amino groups are further apart? Here the data are sparse. Consider now 1,4-diaminobutane (24) with a suggested41 gas-phase enthalpy of formation of 65 kJ mol 1. The amino groups are far apart in terms of the number of carbons that separate them, while the possibility of intramolecular interactions makes them potentially near. From summing Hf(n-hexane) and twice the exchange increment υ5( prim/ prim) we would have a value of only 57 kJ mol 1. The difference of 8 kJ mol 1 is suggestive of intramolecular hydrogen bonding.

Another interesting comparison involves N,N,N0,N0 -tetramethyl-2-butyne-1,4-diamine (25) for no hydrogen bonding is expected in any phase. The experimentally measured enthalpy of formation data are for the liquid. The following formal reaction can be written:

which we expect to be essentially thermoneutral. We accede to the fact that both amines are liquids and have decided against estimating the enthalpy of vaporization of both aminoalkynes. Instead, we estimate the enthalpy of condensation for acetylene to obtain the enthalpy of formation of the liquid. So doing, we find reaction 28 is endothermic by ca 9 kJ mol 1. This is a large discrepancy from our prediction, but there is reason to suspect enthalpy of formation values for acetylenic amines42.

The last acyclic diamine we will discuss is 1,6-diaminohexane or hexamethylenediamine (26). How much interaction is there between the two amino groups? The desired enthalpy of formation is available in both condensed phases43, 205(s) and 164(lq). If all hydrogen bonding and any other intersubstituent interaction were negligible, then reaction 29 would be thermoneutral.

For both phases, thermoneutrality is found to within 2 kJ mol 1. Intramolecular hydrogen bonding in gaseous 26 is thus unlikely.

B. Alicyclic Diamines

The simplest isolable species that fits this description is piperazine (27) with an enthalpy of formation44 of 29.4 kJ mol 1. Can we reliably estimate this value in terms of the conceptually simpler acyclic amines, acyclic polyamines, alicyclic amines or other heterocycles? One may estimate it simply as the sum of the enthalpy of formation of cyclohexane (2, n D 6) and twice the exchange energy, υ5. The predicted value is 21 kJ mol 1, suggestive of at least 7 kJ mol 1 of strain. Do not forget that 27 should enjoy stabilization as befits its being a vic-diamine. This destabilization is twice that of piperidine (3, n D 6) as relatedly defined by its measured enthalpy of formation and that estimated by summing the enthalpy of formation of cyclohexane and 1Ð υ5. Equivalently, disproportionation reaction 30 is found to be thermoneutral.

This should not be taken for granted. After all, the related oxygen disproportionation reaction involving tetrahydropyran and 1,4-dioxane (28, cyclo-1,4-X(CH2)4 with X D CH2 and O respectively)

is endothermic by ca 7 kJ mol 1.

8. Thermochemistry of amines, nitroso, nitro compounds and related species 353

To compensate for the ignored vicinal interactions, consider now the formal reactions involving six-membered ring species (equations 32a, X D NH, Y D NH2; 32b, X D O, Y D OH; 32c, X D CH2, Y D Me):

Individually preserved are the numbers of CH2 and Me; NH and NH2 (primary and secondary amines); O and OH (ether and alcohol) groups. The ‘C’, ‘N’ and ‘O’ cases are ca thermoneutral, endo by 14 and endothermic by 23 kJ mol 1 respectively. We conclude piperazine is strained and that piperazine interpolates its carbon and oxygen analogs, 2(n D 6) and 28(X D O).

The last comparison for piperazine involves a 1,4-diheterocyclohexane that has two different types of heteroatoms, morpholine (28, X D NH). We find the following dispro-

is endothermic45 by ca 2 kJ mol 1. The species with one nitrogen and oxygen apiece is the average of those with two.

We now turn to 1,4-diazabicyclo[2.2.2]octane (29). While the two nitrogens have long been thought to interact considerably from the vantage point of ionization energies and proton affinities46, it is not obvious that would also be deduced from examination of enthalpies of formation. The simplest probe of this interaction is the enthalpy of the following nitrogen transfer disproportionation reaction involving the bisaza 29 and the carbocyclic 11:

This reaction is essentially thermoneutral in the gas phase as would have been expected in the absence of vic- and transannular interactions47. It would thus appear that conclusions drawn from neutral and ion thermochemistry need not be compatible48.

Consider now the monocyclic/acyclic + bicyclic exchange reaction

This reaction is endothermic by ca 18 kJ mol 1. It is tempting to identify the source of the destabilization as the three boat cyclohexanes (or, more properly, piperazines 29) on the right49.

C. Aromatic Diamines

We should naturally start with the three isomeric phenylene or benzene-diamines, 30o, 30m and 30p. The literature is littered with confusing results. Our prejudice is to choose the enthalpies of formation 0.3š 4.2, 7.8š 4.2 and 6.4š 4.2 kJ mol 1 because they all come from the same source: Pedley recommends the same values for the o- and m-isomer

measurements, we abstain from attempting to evaluate these numbers. Hoping not to prejudice the experimentalist, we may nonetheless predict the enthalpies of formation for all three phenylenediamines as solids, liquids and gases free of any complicating intramolecular interactions. Assuming thermoneutrality for reaction 36

values of 2.6, 13.6 and 91.6 kJ mol 1, respectively, are found.

354 Joel F. Liebman, Mary Stinecipher Campbell and Suzanne W. Slayden

There are many possible diaminobiphenyls, but only for the 4,40 -species [a.k.a. benzidine (31)] are there enthalpy-of-formation data from this century51. Consider now the formal all-solid transamination reaction

This reaction is endothermic by 12 kJ mol 1. The formal monoamine ‘conproportionation’ reaction 38

is endothermic by 8 kJ mol 1. What is the instability of solid 31 due to: physical state, structural or experimental effects? Both of these reaction enthalpies seem quite insignificant compared to that involving acetanilide and the corresponding diacetyl derivative of benzidine (32) in reaction 39.

We now consider heteroaromatic diamines with the condition that an amino group is not ˛ to a heterocyclic nitrogen. The only thermochemical data we can find are for 2,8- diaminoacridine for which the solid-phase enthalpy is 127 š 7 kJ mol 1. In the absence of significant substituent and solid state effects, thermoneutrality is expected for the conproportionation reaction 40 that produces diaminoarenes from monoamine derivatives.

From data in our archives, we derive an exothermicity for the diaminoacridine synthesis by ca 4 š 21 kJ mol 1. The large uncertainty obscures and thus compromises any thermochemical conclusion we should wish to make.

D. Alicyclic Triamines

The first species we will discuss is 1,3,5-trimethylhexahydro-1,3,5-triazine (33, X D NMe). The gas-phase trimerization enthalpy of N-methylmethylenimine (34), reaction 41,

From gas-phase ion molecule reactions we obtain54 the necessary enthalpy of formation of 34, 44 š 8 kJ mol 1, and so derive Hf(33, g) as equal to ca 46 kJ mol 1. Is this number plausible? We start with the trimerization enthalpy. Intuitively, this number for CDN bonded species should interpolate the trimerization enthalpy of species with CDC and CDO bonds. More precisely, since it is N and not C that is methylated, the trimerization enthalpy of 34 should be close to that of CH2NH. As such, this reaction enthalpy should interpolate that of CH2O and CH2CH2. Our thermochemical archive includes the enthalpies of formation of these latter two substances and of their trimers, 1,3,5-trioxane (33, X D O), and cyclohexane (alternatively identified as 33, X D CH2 and 2, n D 6), from which derive the desired reaction enthalpies 140.1 and 276.0 kJ mol 1 respectively. The reaction enthalpy for the X D NMe case interpolates that of X D CH2 and X D O as expected from size and electronegativity reasoning.

8. Thermochemistry of amines, nitroso, nitro compounds and related species 355

for X D O, CH2 and NMe. We find these reactions are ca 27 kJ mol 1 endothermic, 3 endothermic and 64 exothermic, respectively. The nitrogen value is out of line. We acknowledge that the enthalpy of formation of CH2NH is a subject of major dispute55 and so, by inference, is that of its N-methylated derivative.

From liquid-phase hydrogenation studies56, we can derive the enthalpy of formation of the perhydro-tris[1,2-a: 3,4-c: 5,6-e]pyrrolo-1,3,5-triazine and of perhydro-tris[1,2-a: 3,4- c: 5,6-e]pyrido-1,3,5-triazine (35 with n D 5 and 6, respectively). Correcting for medium effects in directly measured enthalpies of solution, hydrogenolysis reaction 43

(CH2)n−2

N

(CH2)n−2

(35)

is found to be exothermic by 105 and 127 kJ mol 1 respectively for n D 5 and n D 6. This results in liquid-phase enthalpies of formation of 35 for n D 5 and 6 respectively of18 and 132 kJ mol 1 respectively. These values are not particularly consistent with each other. The 5,6 ring fusion in 35 (n D 5) and the 6,6 ring fusion in 35 (n D 6) should not result in any significant strain effects. Furthermore, the H H repulsion arising from inter-5-membered ring interactions should be comparable to the corresponding 6- membered ring interactions. What about that of the monocyclic 33 (X D NR)? Let us make the comparison of liquid-phase thermochemical studies of either 35 with that of its gas-phase monocyclic analog 33 (X = NMe) by use of formal reaction 44

The results are not particularly consistent here either. This last reaction, exothermic by ca 206 kJ mol 1, is not consistent with either result in equation 43. We strongly doubt the principal source of error is the difference of choice of phases employed57. The difference of the hexahydrotriazine enthalpies of formation, 18 132 D 114 kJ mol 1, is comparable to three times the difference of liquid pyrrolidine and piperidine, 132 kJ mol 1. (The difference for other pairs of corresponding 5- and 6-membered ring species includes 166 kJ mol 1 for cyclopentane and cyclohexane, and 162 for perhydroindene and decalin.) The origin of the ca 20 and 50 kJ mol 1 discrepancies remains enigmatic.

E. Tetramines

The first tetramine we will discuss is the acyclic N,N0-bis-(2-aminoethyl)propane- 1,3-diamine (36) with an accompanying gas-phase enthalpy of formation58 of 0.0 š 3.3 kJ mol 1. In the absence of intramolecular hydrogen bonding and any vic- and more distant diamine effects, this value could be estimated by summing the enthalpy of formation of n-undecane, 2Ðυ4 and 2Ðυ5. The calculated value is 17 kJ mol 1. Because the calculated enthalpy of formation is more negative than the experimentally measured

356 Joel F. Liebman, Mary Stinecipher Campbell and Suzanne W. Slayden

quantity, it suggests that the gaseous tetramine 36 is destabilized. It is unclear in what way 36 could be destabilized.

The next tetramine we will discuss is the monocyclic 1,4,8,11-tetraazacyclotetradecane (37) with an accompanying gas-phase enthalpy of formation58 of 18.0 š 3.3 kJ mol 1. Summing the most recent value for the enthalpy of formation of its carbocyclic analog, cyclotetradecane59 (278 kJ mol 1), and 4Ðυ5 results in a predicted value of 22 kJ mol 1. Ignoring any consequence of intramolecular hydrogen bonding and vic- and other diamine effects, we find agreement is satisfactory.

We now turn to one of the longest known, simplest and most symmetric polyamines 1,3,5,7-tetraazaadamantane (a.k.a. hexamethylene tetramine) (38). Is this species strained? A related but simpler question asks whether the ‘parent carbocycle’ adamantane (39) is strained. The answer to this latter question depends on the author. Some argue that its structural rigidity results in unavoidable geometric distortions and thus strain60 while others have used adamantane as a strainless reference state for deriving strain of other hydrocarbons61. Perhaps a fairer question62 to ask is whether 38 is more or less strained than 39. In the absence of any additional interactions, the enthalpy of formation of 38 may be estimated as the sum of that of 39 and 4Ðυ6. We find that the tetramine is stabilized relative to the hydrocarbon by 38 kJ mol 1. The degree of stabilization is surprisingly large. However, we observe that the tetramine has an extensive collection of stabilizing gem-diamine units; there are six N C N units found in it. As with other stabilizing interactions, optimal stabilization by two geminal amino groups requires a specific orientation of the substituents. The acyclic diamine has considerably more flexibility than the tetramine to approximate this orientation while the tetramine lacks this flexibility. Additionally, stabilizing effects tend to saturate. As such, parallelling equation 23, we consider the formal reaction

six independent gem-diamino groups: it is exothermic by only 153 kJ mol 1, a large and impressive number.

This reaction conserves the number of >CH , CH2 and tertiary amine groups, as well as both boat and chair six-membered rings. What are not conserved are the number of vicinal and geminal diamino groups. The left-hand side has 6 of the former and the right-hand side has 6 of the latter. We find the reaction is exothermic by ca 45 kJ mol 1 for gas-phase species and 32 kJ mol 1 for solids. Our prejudice for geminal stabilization being more significant than vicinal is validated, although the degree of stabilization is less than we would have anticipated from the acyclic species discussed earlier in this section63.