Enthalpies of destabilization

The comparisons we will make within each family involve experimental enthalpies of formation and the derived enthalpy of destabilization (DSE). If there are no intramolecular interactions in the nitrosubstituted parent compound, equation 58 would be thermoneutral. 

TABLE 11. Figure of insensitiveness (FI) compared to enthalpy of destabilization (DSE, kJ mol-1)... 

Again, one may take the difference of the enthalpies of hydrogenation of the diene and the sum of those for the two monoenes. Doing this separately for 14 and 15, we find the reaction enthalpies for the Z- and -dienes are —1.9 1.2 and —1.8 1.1 kJmol-1. These values are effectively zero. A stabilizing—or destabilizing—interaction was not expected for nonconjugated acyclic dienes and none was found. 

Summarizing all of the above, it would appear that 1,3- and 1,4-cyclohexadiene have nearly identical enthalpies of formation. Does this mean that the 1,3-isomer is destabilized and/or that the 1,4-isomer is stabilized Let us accept an enthalpy of formation of ca 105 kJmoD1 for the enthalpy of formation of both isomers. In the absence of any stabilization or destabilization, we would expect the cyclohexene disproportionation reaction 25... 

Except for the still thermochemically uninvestigated 1,2-bismethylenecyclopropane44, 57, all bismethylenecycloalkanes can further be divided into two categories—those in which the exo-methylene groups are on adjacent carbons and those further apart. The two isomeric bismethylenecyclobutanes have been studied. Roth presents an enthalpy of formation for the 1,2-isomer, 58, of 204.2 kJmol-1. In the absence of any additional strain-induced destabilization or conjugative/delocalization-induced stabilization, we would expect the disproportionation reaction 27 of methylenecyclobutane (59)... 

While we know of no experimental thermochemical data for 123, Roth informs us that the enthalpy of formation of 124 is 259 kJmol-1. There are no experimental thermochemical data for 125 either, but it is easy to estimate the desired enthalpy of formation. We may either use the standard olefin approach with ethylene, 1,3-butadiene and (E)-l,3,5-hexatriene (i.e. with CH2=CH2, 33 and 79) or linearly extrapolate these three unsaturated hydrocarbons. From either of these approaches, we find a value of ca 225 kJ mol-1. Cross-conjugation costs some 35 kJ mol-1 in the current case. Interestingly, the directly measured cross-conjugated 1,1-diphenylethylene (126) is only ca 10 kJmol-1 less stable than its directly measured conjugated (E)- 1,2-isomer (40) despite the expected strain effects that would additionally destabilize the former species. 

From the enthalpies of formation from Roth for the fulvenes and from Pedley for the other hydrocarbons in equations 37 and 38, we find the former reaction is exothermic by 12 kJmol-1 while the latter is endothermic by 12 kJmol-1. Ionic resonance structures analogous to 128 are expected to be of less importance for the ring alkylated species than for the parent species 103 negatively charged carbon is destabilized by adjacent... 

Is this value of 330 kJmol-1 plausible Were pentalene a normal polyene, we would anticipate an enthalpy of formation of ca 4.52,5 + 5.5 or ca 235 kJmol-1. There is thus ca 100 kJ mol-1 of destabilization. Is this due to antiaromaticity since we recognize pentalene as a derivative of planar [8]annulene We think not, for there are two five-membered rings in pentalene each contributing ca 30 kJmol-1 of strain apiece104. 

Had the compound been less stable than we would predict the discrepancy would have been easier to explain. One could argue that the gem- dimethyl groups would have resulted in destabilization because of buttressing . It is tempting to argue that the triene was contaminated by polymer and/or peroxide, both of which have lower enthalpies of formation. But we have no documentation of this. 

We now mm to derivatives of biphenyl. Our archives show a 12.8 6.3 kJmol-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 = 2- and d-PhCgFU. 2 f°r the 2-isomer equals 1.5 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. 

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 = 6) and twice the exchange energy, S5. The predicted value is 21 kJmol-1, suggestive of at least 7 kJmol-1 of strain. Do not forget that 27 should enjoy stabilization as befits its being a vie-diamine. This destabilization is twice that of piperidine (3, n = 6) as relatedly defined by its measured enthalpy of formation and that estimated by summing the enthalpy of formation of cyclohexane and 1 -85. Equivalently, disproportionation reaction 30 is found to be thermoneutral. 

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... 

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 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 ... 

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

TABLE 8. Calculated destabilization enthalpies of polynitrotoluenes (kJmol )... 

Figure 6. Specific enthalpy of denaturation for native CBH I, plotted as a function of the overall observed as the enzyme molecule is progressively destabilized by increasing the pH. Dot-centered circles represent the specific enthalpy in the absence of cellobiose the straight line is a linear least-squares best fit to these data points, plus the empirically derived intersection point (reference 2, see Discussion) represented by the crossed circle at upper right. The squares represent enthalpies measured at pH 4.80 and pH 8.34 in the presence of the indicated concentrations of cellobiose.

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