Enthalpy of formation values

Let us now turn to the ring-contracted xylylene 3,4-dimethylenecyclobutene (121). Does Roth s preferred enthalpy of formation value of 336 kJmol-1 look plausible In the absence of both special strain and resonance energy contributions, the difference of the... 

A plot of A7/y°(SnR4, g) against A///TRII, g), where R is alkyl, is presented in Figure 3 where the enthalpy-of-formation values used are both experimentally determined (Table 3) and calculated. The calculated values are for tetramethyl and tetraethyl tin and methane. As stated above, the enthalpy-of-formation values for tetraethyl tin may be incorrect. That the calculated value for tetraethyl tin results in a better linear fit with tetrapropyl and tetrabutyl tin is further confirmation of this supposition. And, as discussed for the alkyl germaniums, the methyl deviations of methane and tetramethyl tin are too different for their measured values to fit a linear relationship such as equation 7. 

We have illustrated how standard enthalpy of formation values can be handled to yield data for practical conditions. The procedure always involves thermochemical cycles, relating the standard state processes with those observed in... 

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 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. ... 

The enthalpy of isomerization of the primary n-butyl lithium to ec-butyl lithium is ca - -2 kJmoP. From the two different enthalpies of formation of isopropyl lithium and the previously derived liquid-phase enthalpy of formation of n-propyl lithium, the isomerization enthalpy is either ca -hi5 klmoL or nearly zero. Considering both this result and the one earlier that also compares the two different enthalpy of formation values for isopropyl lithium, the value from Reference 7, —57.7 klmop, seems more plausible. 

Usable equilibrium constants were obtained only for Ar = p-CHs, m-CHs (i.e. p-Tol and w -To1) and p-Ph and were reported for approach to equilibrium from both the left and right sides of the equation with equimolar concentrations of reactants. The averaged A obsd values are 0.64, 0.86 and 3.78, respectively. The corresponding values of AH, estimated from equation 13 are 1.1, 0.37 and —3.3 klmoP. From equation 11, the enthalpy of formation of p-tolyl lithium is calculated to be ca 3 klmoP where the enthalpy of formation of p-tolyl bromide is 12.1 kJmor, as suggested in Reference . The enthalpy of formation value for p-tolyl lithium derived here is nearly identical to that in Table 1. Unfortunately, there is no measured enthalpy of formation of m-tolyl bromide. However, the difference between the enthalpies of formation of phenyl bromide and phenyl lithium (9.8 kJmoU ) must be about the same as the difference between the enthalpies of formation of the m-tolyl bromide and the m-tolyl lithium when the reaction is thermoneutral for equation 12. 

Unfortunately, we lack measured enthalpy of formation values for most organic iodides of interest here except for ethyl, n-propyl and phenyl iodides. From equation 14 and with phenyl iodide in its reference liquid state and with ethyl and propyl iodides in their reference gaseous states, the enthalpies of formation of ethyl lithium and of n-propyl lithium are calculated to be ca —54 and —74 klmoP, respectively. The former value is the same as those from Table 1 and the latter is compatible with one of the other values for n-propyl lithium derived in earlier sections. 

There remains to be discussed butanone oxime, for which there is an enthalpy of formation value only in the liquid phase. The enthalpy of reaction 23 (RR = —(CH2)5— R = Me, R = Et) is +11.8 kJmoH. The enthalpy of formation of butanone oxime would seem to be ca 12 klmoE too negative. 

Reaction 49 involves the isomerization of the 2,2-dinitro compound to its 1,1-isomer, which is of direct interest here. The latter is plausibly more strained and so its enthalpy of formation would be more positive. The phase change—from a liquid to a solid—would result in a more negative enthalpy of formation. If these two changes are assumed to cancel, we would predict an enthalpy of formation of 1-nitroacetaldehyde 0-(l,l-dinitroethyl)oxime of —151 kJmoU while the literature value is —165.5 kJmol . That these two values are close suggests that the two nitrolate enthalpy of formation values are at least self-consistent. 

Just as there are tables of standard enthalpies of formation, values of the standard Gibbs free energy of formation, AfG, are listed [5—9] for very many compounds and these may be combined in a way analogous to eqn. (10)... 

For the formal deoxygenation (decomposition) reaction 5, there is an enthalpy of formation value for every alcohol that matches a hydroperoxide . Using our exemplary groups, R = 1-hexyl, cyclohexyl and ferf-butyl, the liquid enthalpies of reaction are —77.9, —75.0 and —65.6 kJmoR, respectively (there is no liquid phase enthalpy of formation reported for f-butyl peroxide from Reference 4). The secondary hydroperoxides enthalpies of reaction average —77 7 kJmoR. For the three instances where there are also gas phase enthalpies of formation, the enthalpies of reaction are almost identical in the gas and liquid phases. The 1-heptyl (—60.3 kJmoR ) and 1-methylcyclohexyl (—50.6 kJmoR ) enthalpies of reaction are again disparate from the 1-hexyl and tert-butyl. If the enthalpy of reaction 5 for 1-hexyl hydroperoxide is used to calculate an enthalpy of formation of 1-heptyl hydroperoxide, it is —325 kJmoR, almost identical to values derived for it above. The enthalpies of reaction for the liquid and gaseous phases for the tertiary 2-hydroperoxy-2-methylhex-5-en-3-yne are —78.2 and —80.9 kJmoR, respectively. For gaseous cumyl hydroperoxide, the enthalpy of reaction is —84.5 kJmoR. ... 

There are liquid enthalpy of formation values for the methyl ethers for R = Me, -Pr, w-Bu and w-decyl. Additional enthalpies can be extrapolated for the R = Et and w-Pen species from the linear regression analysis of the enthalpies of formation vs. number of carbons . However, the regression constants from this same analysis immediately reveal that any assumption of thermoneutrality for equation 12 is incorrect. The slope of —25.3 0.1 kJmol for the methyl ethers is much too different from the slope for the peroxide series for there to be a constant difference between enthalpies of formation for two compounds with the same R substitution (the two alcohol enthalpies of formation in equation 12 are constant). As expected, the derived enthalpies of reaction for equation 12 increase with increasing number of carbons 13.9, 21.4, 24.0 and 25.6 kJmol. ... 

In this section we discuss the thermochemistry of some species with the general formulas, RC(=0)00H, RC(=0)00R and RC(=0)00C(=0)R. These species have the generic names peracids (peroxycarboxyUc acids), peresters (percarboxylate esters) and acyl peroxides. The enthalpy of formation values are in Table 3. Three formal reactions that are discussed here are conceptually the same as in the earlier sections. Because now there is a carbonyl group present, we rewrite equations 5, 6 and 9 as equations 14, 15 and 16. 

Because the accuracy of the data for three of the diacyl peroxides is in question, we will attempt to derive enthalpies of formation for them from the reverse of equations 15 and 16. The enthalpy of reaction 15 for dibenzoyl peroxide, using enthalpy of formation values of unquestioned accurac)f, is —400.8 kJmor. This is the same as the ca —398 kJmol for the hquid non-aromatic diacyl peroxides discussed above. Using the solid phase enthalpy of reaction for dibenzoyl peroxide and the appropriate carboxylic acid enthalpies of formation, the calculated enthalpies of formation of bis(o-toluyl) peroxide and bis(p-toluyl) peroxide are —432.2 and —457.6 kJ moU, respectively. From the foregoing analysis, it would seem that the measured enthalpy of formation is accurate for the bis(p-toluyl) peroxide but is not for its isomer. The analysis for dicinnamoyl peroxide is complicated by there being two enthalpies of formation for frawi-cinnamic acid that differ by ca 12 kJmoU. One is from our archival source (—336.9 12 kJmoU ) and the other is a newer measurement (—325.3 kJmol ). The calculated enthalpies of formation of dicinnamoyl peroxide are thus —273.0 and —249.8 kJmoU. Both of these results are ca 80-100 kJmol less negative than the reported enthalpy of formation. 

Table 5.1 lists some enthalpy-of-formation values for Schottky and Frenkel defects in various crystals. 

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