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 357

VI. NITROSO COMPOUNDS

Despite the plethora of amines and nitro compounds for which enthalpies of formation are available, there are few nitroso compounds for which we have the desired data. This difference, in part, reflects the comparative interest in the three classes of compounds. Amines are precursors of numerous bioactive compounds, chelating agents and dyes; nitro compounds are precursors of amines and many are components of explosives. By contrast, nitroso compounds are seemingly less important and therefore less likely to be available in the purity, quantities and structural diversity wished for by the thermochemically inclined scientist. In addition, we acknowledge the difficulty of synthesizing nitroso compounds by either reduction of the corresponding nitro compounds or by oxidation of the corresponding primary amines. Most importantly, perhaps, is that many putative nitroso compounds normally exist as the isomeric oximes64, while others are normally found as their azo-dioxy dimers65. As such, the energetics of tautomer and monomer/dimer interconversion are additional, nontrivial measurements that need to be made in order to acquire the desired nitroso ‘number’66. We note that few direct measurements have been reported since Batt’s chapter in this series8.

A. Nitrosobenzene and Its Methylated Derivatives

Through a collection of indirect measurements, the enthalpy of formation of gaseous nitrosobenzene (40) has recently been reevaluated67 to be 210 š 8 kJ mol 1, in interesting comparison to those values reported earlier68. The need for indirect, i.e. not ‘simple’, combustion and phase-change measurements is unavoidable for this species because nitrosobenzene is normally dimeric.

Although likewise unequivocally dimeric as solids, 2,6-dimethyl- and 2,4,6- trimethylnitrosobenzene (41 and 42) allow for relatively unequivocal thermochemical analysis of both the monomeric and dimeric gases. The desired enthalpies of formation67 of monomeric 41 and 42 are 207.3 š 12.3 and 140.6 š 12.5 kJ mol 1 respectively. Are these values for the enthalpies of formation of nitrosobenzene derivatives consistent with each other? The first comparison considers the difference of the enthalpies of formation of nitroso and nitro compounds, ArNO and ArNO2, υ47(Ar; NO, NO2):

For Ar D Ph, 2,6-Me2C6H3- and 2,4,6-Me3C6H2-, the three difference quantities equal 143 š 8, 131.2 š 2.3 and 134.2 š 2.9 kJ mol 1. Although within experimental error these three numbers are essentially equal, the spread between the methylated and non-methylated compounds may be significant.

The next comparison entails the difference of the enthalpies of formation of the nitroso and correspondingly methylated benzene, υ48(Ar; NO, Me):

For Ar D Ph, 2,6-Me2C6H3- and 2,4,6-Me3C6H2-, the three difference quantities equal69 160 š 8, 149.3 š 2.0 and 151.8 š 2.3 kJ mol 1. Again, within experimental error, these three numbers are essentially equal, exhibiting the same pattern in the spread of values. We recall the near constancy70 of the related difference υ(Ar; COOH, Me).

What about the difference υ49(Ar; NO, H).

For Ar D Ph, 2,6-Me2C6H3- and 2,4,6-Me3C6H2-, the three difference quantities equal69 127 š 8, 122.5 š 1.9 and 123.3 š 2.3 kJ mol 1. Within experimental error, these three

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

numbers are essentially equal and the spread between methylated and non-methylated species has decreased. That these difference quantities are essentially independent of methylation suggests weak interaction between the nitroso group and the rest of the molecule.

B. Nitrosobenzene and Amino Substitution

Direct calorimetric measurements on a monomeric nitroso compound with a strong interaction between the nitroso group and the rest of the molecule are possible for the 4- (N,N-dimethylamino)nitrosobenzene (43). We adopt here the recently measured values71 for the enthalpy of formation of 103.0 š 1.6 and 185.0 š 2.3 kJ mol 1 for the solid and gaseous species respectively72 and now examine the values for aforementioned υ47, υ48 and υ49. Using the enthalpy of formation of 4-(N,N-dimethylamino)nitrobenzene (44)

from Reference 71 as well73, υ47 is found to be 122.2 š 3.5 kJ mol 1. This value is significantly less than that found for the other nitroso compounds and suggests that

there is more nitroso/amino conjugative stabilization in 43 than nitro/amino conjugative stabilization in 44. We welcome the corresponding comparison with the 3-isomers of 43 and 44 where no such conjugative stabilization is customarily proposed. We cannot make use of υ48 because we lack enthalpy-of-formation data for the desired 4-N,N- trimethylaniline74. υ49 equals 84.0 š 3.2 kJ mol 1, suggestive of ca 40 kJ mol 1 of stabilization in 43.

C. Nitrosoarenols

What about -electron donor substituents on nitrosoarenes other than dimethylamino? Pedley gives us the enthalpies of formation for three hydroxy derivatives: the isomeric 4-nitroso-1-naphthol, 2-nitroso-1-naphthol and 1-nitroso-2-naphthol, species 45 47 respectively. Of the three species, only the first cannot have an intramolecular hydrogen

4-isomer. We recall from the discussion of the isomeric naphthylamines that 1- and 2- naphthol are of almost identical stability. This suggests that species 46 and 47 should be of comparable stability. Both expectations are sorely violated by the literature results: the enthalpies of formation of species 45, 46 and 47 increase in the order 20.3 š 4.9,

One possibility is that these species should be described as naphthoquinone oximes76 in which case species 45, now redrawn as 48, is recognized as a 1,4-naphthoquinone

8. Thermochemistry of amines, nitroso, nitro compounds and related species 359

derivative and so more stable than the 1,2-naphthoquinone derivatives, 49 and 50 respectively. Another possibility is an error in the measurement of the enthalpy of sublimation. Regardless of the resolution of the nitroso/oxime dichotomy, we cannot explain why the phase-change enthalpies of species 46/49 and 47/50 should differ by some 30 kJ mol 1. The seeming discrepancy in the sublimation enthalpies of species 46/47 and 47/50 is about the same as the discrepancy in the enthalpies of formation of these species as gases. The difference for the solids is much smaller and much more reasonable, 11.3 š 5.0 kJ mol 1.

There is also the possibility that the enthalpy of combustion is in error. Because there is no possibility of intramolecular hydrogen bonding, the 1,4-disubstituted species, 45/48, is expected to be the most ‘normal’. Consider the formal reaction

Accepting the above enthalpy of formation of nitrosobenzene and the archival values for the other species, this reaction is deduced to be ca 60 kJ mol 1 exothermic. This nitroso/hydroxy induced stabilization value is perhaps not unreasonably large for comparison, the corresponding nitroso/dimethylamino stabilization value in equation 51 is 43 š 10 kJ mol 1.

the early 20th century sources given by Kharasch2a. For the three solid-phase nitrosonaphthols/naphthoquinone oximes, we find the enthalpies of combustion from the former archive are 4827.2, 4873.6 and 4884.9 kJ mol 1 and from the latter archive are 4868.3, 4885.6 and 4882.6 kJ mol 1. If we assume the enthalpy of sublimation of species 45/48 is correct and we use the earlier enthalpy of formation, then reaction 50 is exothermic by only ca 20 kJ mol 1. Interestingly, Kharasch gives us the enthalpy of combustion of 4-nitrosophenol (51) (or is it benzoquinone oxime, 52?) which comes from the same primary source as its naphthalene siblings 45 50. Using these data, but benzene and naphthalene from Pedley, we find the mixed phase reaction

to be some 7 kJ mol 1 exothermic. This result is preeminently plausible as is the nearthermoneutrality of the mixed-phase quinone reaction

using the enthalpy of formation of the quinones from Pedley, and the requisite fusion enthalpy of p-benzoquinone from Pedley and Domalski respectively. A reinvestigation of the various enthalpies of formation and of sublimation presented in this section is in order77.

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

D. Aliphatic Nitroso Compounds

Despite the above enunciated difficulties in obtaining pure nitroso compounds for conventional calorimetric measurements, gas-phase chemists have been active in devising methods for obtaining the C NO bond enthalpy of monomeric nitroso species. In principle and in practice the desired enthalpy of formation of RNO may be obtained if we know the enthalpy of formation of both NO and the organic radical78. One such method consists of directly determining the rates of gas-phase reaction 54

in both directions79 and deconvoluting the desired enthalpy and entropy of reaction from the observed gibbs energy. Another method involves the direct photochemical cleavage of RNO with monochromatic photons and making assorted corrections from the threshold measurement to the thermochemical quantity of interest at 298 K.

The authors of primary Reference 80 present their own and selected literature values for the R NO bond enthalpies for the hydrocarbyl cases of Me, Et, t-Bu, allyl and benzyl, as well as mixed fluorinated, chlorinated methyl radicals. We now wish to compare nitroso species with the corresponding amino and nitro compounds. Choosing what we consider the most reliable and relevant nitroso compound data, and accompanying them with the corresponding radical data, we derive enthalpies of formation of gaseous nitrosomethane, 2-methyl-2-nitrosopropane and ˛-nitrosotoluene81 to be 65 š 2, 29 š 4 and 174 š 7 kJ mol 1. (By comparison, the earlier values recommended8 for nitrosomethane and 2-methyl-2-nitrosopropane were 70 and 42 kJ mol 1 respectively.)

In order to estimate the enthalpies of formation of nitroso compounds from the corresponding amine or nitro compounds, we derive difference quantities υ55 and υ56. The

For R D Me, t-Bu and PhCH2, υ55(NO, NH2; R) equals 88š2, 90š4 and 86š7 kJ mol 1. For R D Me, t-Bu and PhCH2, υ56(NO, NO2; R) equals 139 š 2, 146 š 5 and 143 š 7 kJ mol 1. Ignoring all error bars, the range of values is smaller for the υ55 and thus we conclude that estimation from the corresponding amine is more reliable. Nonetheless, in that both sets of values of υ55 and υ56 are so comparably independent of R makes us conclude that the NH2, NO and NO2 groups are surprisingly similar.

We conclude this section with discussion of the question of whether these enthalpies of formation for MeNO, t-BuNO and PhCH2NO are ‘plausible’? Should we wish to use solely experimentally measured quantities, the thermochemically instructive comparison of hydrogen and alkyl derivatives14 is complicated in the current case by the fragility of HNO and the seeming nonexistence of H NO2. Comparison with aryl derivatives seems precarious because we are convinced of conjugation with the aromatic ring. As is recalled from organic chemistry classes, amino is strongly -electron donating and nitro is strongly-electron withdrawing. There is thus no way that nitroso can mimic both groups82. Indeed, the difference of the enthalpies of formation of aniline and nitrosobenzene is 123 š 8 kJ mol 1, some 30 kJ mol 1 higher than found for the above saturated nitroso and amino compounds. By contrast, the 142 š 8 kJ mol 1 difference for nitrosobenzene and nitrobenzene is nearly the same as for the above aliphatic species. This is compatible

8. Thermochemistry of amines, nitroso, nitro compounds and related species 361

with the conclusion83 that nitrobenzene lacks net resonance stabilization by its conjugation of the nitro group and aromatic ring, while aniline is stabilized with its amino group. Acknowledging seeming consistency, we close with the admission that lacking a trustworthy calorimetric bench mark for any RNO species where R is a saturated (tetracoordinate) carbon allows for the possibility that the current values may be replaced by still newer values, to be prominently featured in the next amino/nitroso/nitro Patai supplement ‘F3’).

VII. AROMATIC NITRO COMPOUNDS

A. The Roles of Resonance and Steric Effects: Molecular Families and Reference States

Numerous aromatic nitro compounds have explosive properties, and thus it is important to understand the role that enthalpy of formation has on the sensitivity and long-term stability of these compounds. We will examine three nitro-substituted aromatic families for which thermochemical data can be found in the literature2,84: derivatives of nitrobenzene, aniline and toluene. The choice of these three families allows us to compare the various electronic effects exerted by the parent functional group. The parent compounds differ electronically with respect to the aromatic ring in that:

(a) the nitro group of nitrobenzene is both - and -electron withdrawing,

(b) the amino group of aniline is -electron donating but -electron withdrawing,

(c) the methyl group of toluene has only a slightly inductive -electron donating effect into the aromatic ring.

Ionic or dipolar valence-bond resonance structures of nitrobenzene (53a,b) and aniline (54a,b) illustrate the difference of -electron density in the ring of these two compounds85.

Because of the opposite nature of the interactions between the ring and either the nitro or the amino substituent, let us assess the stabilization energies of nitrobenzene and aniline. In equation 13, the resonance stabilizing energy of aniline was defined as the exothermicity of a reaction involving arbitrary reference states. By analogy to equation 13, we may write equation 57 for nitrobenzene and the same arbitrary reference states, R D i-Pr or t-Bu.

For these two R groups the reaction is endothermic by 22 and 26 kJ mol 1 and so nitrobenzene is seriously destabilized relative to our chosen reference states. Resonance energy most generally arises from stabilizing interactions. That nitrobenzene is planar and has hindered rotation around the C N bond attests to interactions between the ring and the nitro group. However, there are also powerful inductive interactions between the

stabilization and the ‘true’ resonance energy of aniline is less ameliorated than that of nitrobenzene.

8. Thermochemistry of amines, nitroso, nitro compounds and related species 363

TABLE 4. Calculated destabilization enthalpies of the polynitrobenzenes (kJ mol 1)

a We have insufficient information to decide between the two enthalpies of formation of hexanitrobenzene (from Reference 90) and so give destabilization enthalpies corresponding to each source.

dinitrobenzenes are isoelectronic. The difference between the ortho and meta/para isomers is ca 26 kJ mol 1 and may be attributed to steric interaction. One reason that the ortho steric interaction is so great is because the strain is partially relieved by rotation of a nitro group from planarity with the ring, thereby losing conjugation and raising the energy of the system87. However, in the solid state the para isomer is more stable than the meta, paralleling the difference found for the phthalic acids. To result in such a difference, the lattice energy of the para isomer must be greater than that of the meta isomer88.

The solid-phase enthalpy-of-formation data for the 1,2,4- and 1,3,5-trinitrobenzenes are wildly discrepant. Whichever of them are compared, the 1,2,4-trinitrobenzene isomer is less stable than the 1,3,5-isomer. The dominant destabilization of the 1,2,4-isomer is probably due to the ortho dinitro interaction. We would welcome enthalpy-of-formation data on the 1,2,3-trinitrobenzene isomer.

The compound with the maximum steric interactions is hexanitrobenzene (HNB) (55) with nitro groups at every ring position. The X-ray crystal structure89 showed the nitro groups were rotated by 53° around their C N bond axes. This gives a distance between the oxygens of 2.88 A˚ instead of 1.55 A˚ as would be found in a completely planar model. The calculated90 DSE is over 300 kJ mol 1. The alternate nitro groups around the ring are easily hydrolyzed by atmospheric moisture to form 1,3,5-trinitro-2,4,6-trihydroxybenzene, also known as trinitrophloroglucinol. In addition, in the presence of light one of the nitro groups rearranges to form pentanitrophenylnitrite, the otherwise unprecedented solid-phase aryl nitrite91.

O O

O N O

N N

O O

O O

N N

O N O

O O

(55)

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

C. Nitroanilines

The enthalpies of formation for nitrated anilines are listed in Table 5 and their destabilization energies are given in Table 6. Also included are the related compounds with additional amino groups, diaminotrinitrobenzene (DATB, 56) and triaminotrinitrobenzene (TATB, 57). Their destabilization energies are calculated from equations 59 and 60:

8. Thermochemistry of amines, nitroso, nitro compounds and related species 365

As was the case for dinitrobenzene, the meta and para nitroaniline isomers have essentially the same gaseous enthalpy of formation. In the gaseous phase, it is surprising to find that despite the more ‘attractive’ quinonoid resonance structures92 for the para isomer (58) than for the meta (59) the meta and para nitroaniline have essentially the same gasphase enthalpy of formation. In the solid and liquid states the intermolecular stabilization lowers the enthalpy of formation of the para isomer relative to the meta. Interestingly, the gas-phase intramolecularly hydrogen-bonded ortho isomer is of comparable stability to its isomers. In contrast, it is considerably less stable than its isomers in the solid state because it can form fewer intermolecular hydrogen bonds. All isomers of nitroaniline are more stable than calculated by additivity.

All the solid dinitroaniline isomers have been thermochemically studied. The most stable isomer is 2,4-dinitroaniline, which results in the only positive DSE in the isomeric set. Progressively less stable are the 2,6-, 2,5-, 3,5-, 3,4- and 2,3-dinitroanilines. The two least stable isomers experience dinitro steric interactions. The large stability difference between these two is surprising because it appears there is a possibility in the 2,3-isomer for nitro/amino hydrogen bonding. Perhaps the steric effect between the nitro groups requires the 2-nitro group to rotate out of the plane of the ring and thus presents a poorer position for hydrogen bonding. That the species are in the solid phase complicates our understanding. The exact ordering of the 2,4-, 2,6- and 2,5-isomers, all of which have intramolecular hydrogen bonding between the amino and the nitro groups, depends on the relative stabilizing contributions from the ‘competing’ meta and para orientations of the amino and nitro groups. The 3,5-isomer has no steric destabilizing effect but it also lacks intramolecular hydrogen-bonding derived stabilization.

Trinitroaniline (TNA) has but one isomer (the long known 2,4,6-species) with a reported enthalpy of formation. Its positive DSE, and hence relative stabilization, is due to hydrogen

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

bonding and the favorable positioning of the three nitro groups. TNA has also been called picramide because it is a derivative of picric acid93 (60), and has found use as a heatresistant explosive94.

N

O O

(60)

The tetranitroaniline is destabilized by steric interaction between three neighboring nitro groups. It has been used as an explosive booster (a moderately sensitive explosive between the detonator and the main charge that magnifies the shock from the detonator to start the detonation in the more insensitive main charge)94.

Pentanitroaniline (PNA) has five neighboring nitro groups and is destabilized even more than the tetranitro compound by repulsive steric interactions. The 3- and 5-nitro groups are easily hydrolyzed as observed earlier for HNB. PNA was studied as an initiating explosive (a sensitive explosive that will decompose to hot gaseous products explosively by a hot wire or weak shock)98.

Formal sequential addition of amino groups to 2,4,6-trinitroaniline gives 1,3-diamino- 2,4,6-trinitrobenzene (DATB, 56) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB, 57). TATB is more stable than expected from the additivity calculation. The ability to have hydrogen bonding with three amino groups both intraand inter-molecularly in the crystal stabilizes the molecule. The molecule that results is thermally stable and used as an explosive in situations where a very insensitive explosive is needed.

D. Nitrotoluenes

The enthalpies of formation for nitrated toluenes are listed in Table 7 and their calculated destabilization energies are given in Table 8. One polynitrotoluene, 2,4,6- trinitrotoluene, is the well-known explosive TNT (61).

N

O O

(61)