Enthalpies of formation difference

From a consistent set of hydrogenation enthalpies in glacial AcOH, the cyclooctadi-enes decrease in stability 1,5- (29) < 1, 4- (56) < 1,3- (55a) with sequential differences of 13.0 (29, 55a) and 6.7 (55a, 56) kJmol-1. For comparison—despite our earlier enunciated skepticism about isomerization reactions performed in polar media (Z-BuOK in DMSO) — the following enthalpies of reaction, and thus enthalpies of formation, differences were found43 16.4 1.4 and 2.8 0.8 kJmol-1. Consistency, if not precise numerical agreement, is found for the energetics of the isomeric cyclooctadienes. 

This class of compounds is defined to have some of the three conjugated double bonds found in the ring and others not. This class includes the isomeric 3,3 -bis(cyclohexenylidenes), 100 and 101. Roth shows us that the two isomers have the same enthalpy of formation within ca 1 kJmol-1, a difference somewhat smaller than the 4 kJmol-1 found for the totally acyclic 1,3,5-hexatrienes, 79 and 80 respectively. Naively these two sets of trienes should have the same (E)/(Z) enthalpy difference. Given experimental uncertainties, we will not attempt to explain the difference69. We may compare 100 and 101 with phenylcyclohexane, 102, an isomeric species which also has the same carbon skeleton. There is nearly a 110 kJ mol-1 enthalpy of formation difference between the semicyclic and cyclic trienes. We are not surprised, for the word cyclic is customarily replaced by aromatic when in the context of the previous sentence. 

TABLE 2. Enthalpy-of-formation differences between gaseous primary, secondary and tertiary amines and the corresponding hydrocarbons (kJmol-1)... 

Table 3 presents the experimental enthalpies of formation of polynitrobenzenes and Table 4 presents the calculated additivity values and DSEs for these same compounds. Enthalpy-of-formation values have been determined experimentally for all three dinitrobenzene isomers in the gaseous state. The enthalpy-of-formation difference between the meta and para isomers is indistinguishable from 0. Conventional wisdom suggests that the para isomer should be destabilized relative to the meta because of adjacent positive charges in key ionic or polar resonance structures. Thus it seems that electronic effects due to meta/para dinitro substituent position are small. This small enthalpy-of-formation difference is similar to that for the meta and para dicyano, difluoro and dichloro benzenes, but does not mimic the ca 22 kJ mol 1 difference for the phthalic acids with which the... 

This observation was made by Coon, in Reference 90. To the best of our knowledge, there are no experimental data for the enthalpy-of-formation difference of simple nitroarenes and aryl nitrites such as PhN()2 and PhONO. Given earlier enunciated complications regarding measurements of alkyl nitrites, we are not surprised by this gap in our knowledge. 

The secondary isopropyl and icc-butyl hthium compounds, for which there are liquid-phase enthalpy of formation values, are two in a homologous series. Their enthalpy of formation difference, representing one methylene group, is ca —13 or —30 kJmoP depending on which enthalpy of formation is chosen for isopropyl lithium. By comparison, the methylene increment for liquid-phase 2-methylaIkanes is ca —25 kJmoP. ... 

There is a variation in the differences between the enthalpies of formation of RMgBr(soln) and RLi, depending on the class of hydrocarbon substituent. For the sole example, or the average of multiple examples, the enthalpy of formation differences between the organohthium and the corresponding solution-phase Grignard are methyl (259), primary (268), secondary/tertiary (280) and aryl (256)kJmoP. Generally, the... 

There are thermochemical data for only one nonmethyl aliphatic hydroxylamine, N,N-diethylhydroxylamine . The enthalpy of formation difference between it and A-methyl-hydroxylamine is 71.6 kJ mol . This is very nearly the same as the difference of 79.7 kJ mol between the corresponding primary and secondary alcohols, ethanol and 3-pen-tanol, where the N of the hydroxylamine is replaced by a CH. Thus, the formal reaction enthalpy of equation 3 is only 8.1 kJmol . 

By comparison of the neutralization energy of a variety of samples of acetaldoxime, it was deduced that there are two isomers with an enthalpy of formation difference of ca 8 kJmol . From an NMR study of aqueous acetaldoxime, a syn anti isomer ratio of 63 37 was found (where syn means the OH is on the same side as the H and opposite from the methyl and anti means the opposite locations). H it is assumed that... 

The liquid enthalpy of formation difference between 1-hexyl and 1-heptyl hydroperoxides is almost twice that of a normal enthalpy of formation methylene increment of about 25 kJmol . But which of these two, if either, is correct For hydrocarbon snb-stituents bonded to electronegative functional groups, the secondary isomers are more stable than the n-isomer. Accordingly, either the 1- or 4-heptyl hydroperoxide, or both, have an inaccurate enthalpy of formation because the primary isomer is reported to have the more negative enthalpy of formation. All of the enthalpies of formation for the Cg and C7 hydroperoxides cited in Reference 2 come from a single source. There is a reported value for the gas phase enthalpy of formation of fert-butyl hydroperoxide that is 11 kJ mol less negative than the value in Reference 2. 

There are much fewer data for the dialkyl peroxides. The gas phase enthalpy of formation difference between the diethyl and dibutyl peroxides of about 40 kJ moH per methylene group is about twice that of the normal methylene increment of ca 21.6 kJmoH. The 219 kJmoH enthalpy of formation difference between the di-fert-butyl and di-fert-amyl peroxide is so large as to be incredible. 

Admittedly, we are not surprised that the decomposition enthalpies of styrene and 2-vinylnaphthalene peroxides are close to each other, as are those of the isomeric isopropenyl species. There is a rather rehable constant enthalpy of formation difference between phenyl and naphthyl derivatives, and as a corollary, a near-equaUty of the enthalpies of formation of 1- and 2-naphthyl derivatives, cf. the combined calculational and calorimetric studies of M. V. Roux, M. Temprado, R. Notario, S. P. Verevkin, V. N. Emel yanenko, D. E. DeMasters and J. F. Liebman, Mol. Phys., 102, 1909 (2004) and references cited therein. It is perhaps more surprising that the a-methyl group on the unsaturated moiety (vinyl -> isopropenyl) causes such a small change. 

The second approximation refines the first by noting there is cis-destabilization of the central cyclopropane in either methyltercyclopropyl, either by the two attached cyclopropyls or by the methyl and one of the attached cyclopropyls. Approximating the destabilization by the enthalpy of formation difference of cis- and /ran.s-1,2-dimethylcyclo-propane or of cis- and irons-1,2-diethylcyclopropane (20b vs 20c, with X = Me or Et) gives us an estimated enthalpy of formation of either tercyclopropyl of ca 208 kJ mol 1. The difference of enthalpies of formation of tercyclopropyl (9) and bicyclopropyl (8) is some 79 kJ mol 1, within the error bars of that earlier given for bicyclopropyl (8) and cyclopropane49 (2, X = H). 

As noted above, it is long-established that cyclopropane has olefinic character. If this cyclopropane-ethylene analogy were strictly thermochemically correct, then the difference of the enthalpies of formation of vinyl-X and cyclopropyl-X would not depend on the affixed group X. Equivalently, the enthalpy of formation difference quantity <517 (Vi, Cypr, X) would be independent of X (equation 17). 

More recently, the similarity of phenyl-X and vinyl-X was noted19, and were this analogy strictly thermochemically correct as well, then the enthalpy of formation difference quantity <5,8(Ph, Vi X) would also be independent of X as well (equation 18). 

One can alternatively derive enthalpy of formation difference values explicitly for the liquid phase (equation 22) ... 

Equation 37 and the accompanying analysis documents that cyclobutane is essentially normal, i.e. cyclobutene and cyclobutane are not particularly different in terms of their strain energy. One might therefore expect that the enthalpy of formation difference of bicy-clo[2.1.0]pentane (17) and bicyclo[2.2.0]hexane (54) of 34.0 kJ mol1 (the above n = 2 case) would approximately equal that of the unsaturated bicyclo[2.1,0]pent-2-ene (41) and bicy-clo[2.2.0]hex-2-ene (55). However, the difference is ca 83 kJmoT1 for the latter pair101. The... 

This would only be achieved if the (Z)/(E)-enthalpy of formation difference of fi-methylstyrene and /5-t-butylslyrcnc were the same. For the former, we find the desired difference to be ca 13 2 kJ mol 1, while for the latter the difference is found41 to be 33.2 9.3 kJmol-1. Consider reaction 30 that interrelates the above two -alkylated styrenes ... 

We know of no experimental measurements for the enthalpy-of-formation difference of (Z)- and ( )-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... 

The enthalpy of this reaction is the same, within a numerical constant, as the difference of enthalpies of formation of C2H2XYZ and CH3XYZCH3. Were there a more or less constant enthalpy of formation difference between methyl and phenyl derivatives (an assumption employed in [27] that looked better in kilocalories than in kilojoules), the enthalpy of this reaction would be the same (again to an additive constant) as the difference of the enthalpies of formation of C2H2XYZ and C6H5XYZC6H5, the defining relationship in the experimental Dewar-Breslow approach [4—6]. 

Consider now the enthalpies of formation of our diverse indane heterocycles and corresponding one-ring species formed by de-benzoannelation (Table 1). De-benzoannelation resulting in aromatic one-ring species is generally more favorable than processes involving non- or antiaromatic species. The enthalpy of formation difference for aromatic species is typically 50-60 kJ mol-1, for nonaromatic species typically ca. 30 kJ mol-1, and for antiaromatic species ca. 24 kJ mol-1. The difference of the enthalpies of formation between the benzoannelated and one-ring dithiol-2-thiones is —11 kJ mol-1. No explanation for this discrepancy is apparent. 

A practicable approach to studying the enthalpy-of-formation differences in eq. 1 recognizes that the gaseous and liquid enthalpies of formation for compounds in a homologous series are linearly correlated with the number of carbon (nc) atoms in the molecules. For compounds in the gaseous state... 

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