Enthalpy of formation measurement

For example, consider the near-planar nitrogen in l-azabicyclo[3.3.3]undecane (manxine), N. J. Leonard, J. C. Coll, A. H. -J. Wang, R. J. Missavage and I. C. Paul, J. Amer. Chem. Soc., 93, 4639 (1971). Regrettably, we know of no enthalpy of formation measurements on either this heterocycle or the corresponding hydrocarbon, manxane. 

There is a 30 kJ mol1 difference in the recommended enthalpies of formation of solid 2-aminobiphenyl in the two volumes by Pedley, cf the new measurements reported in W. F. Steele, R. D. Chirico, S. E. Knipmeyer and A. Nguyen, J. Chem. Thermodyn., 23, 957 (1991). This difference attests to the difficulty of making enthalpy-of-formation measurements on amines, a conclusion already intimated in the discussion of the naphthylamines. 

If the enthalpy of formation of 4-lithiobutyl methyl ether is interpolated between the values for the lithiopropyl and the lithiopentyl ethers to be —285 kJ moP, then the enthalpy of isomerization to the less stable 3-lithiobutyl methyl ether is - -10 klmoP, which is about half that of isomerization of n-butyl lithium to 5ec-butyl lithium (-1-21.3 kJmol ). However, a linear interpolation assumes the same strain energy for the 6-membered 4-lithiobutyl ether as for the above 5- and 7-membered cu-lithioalkyl methyl ethers. If it is less strained, then the isomerization enthalpy would be larger. How much of the isomerization enthalpy difference is due to other differences, such as intramolecular complexation and/or aggregation among the various species, is not known. Unfortunately, there is no enthalpy of formation measurement for the delithiated 7-methoxynorbornane. 

We close this section with a statement of surprise and disappointment. For no apparent reason there are no published enthalpy of formation measurements for 1-hydroxy-2,2,6,6-tetramethylpiperidine, the parent species for the above 1-hydroxylated piperidines and the hydroxylamine counterpart to probably the most famous nitroxide radical, TEMPO . 

An admittedly enigmatic result involves the thermolysis of solution phase di-f-butyl diperoxyoxalate and a cyclic counterpart, 7,7,10,10-tetramethyl-l,2,5,6-tetraoxacyclodecane-3,4-dione. While the former reaction was shown to be exothermic by 238.5 8.4 kJmol and the latter by 414.2 8.4 kJmol , the latter is slower by a factor of some 3000. The latter decomposition results in acetone, ethylene, CO2 and 3,3,6,6-tetramethyl-l,2-dioxacyclohexane. The enthalpy of formation measurements of the cyclic peroxyoxalate and the 1,2-dioxane are coupled if we knew the enthalpy of formation of 1,2-dioxane it would allow us to derive the enthalpy of formation of 7,7,10,10-tetramethyl-l,2,5,6-tetraoxacyclodecane-3,4-dione and the other way around. 

N. L. Allinger, op. cit., cited in Reference 24. Admittedly, there is an earlier enthalpy of formation measurement, that of T. L. Flitcroftand H. A. Skinner, Trans. Faraday Soc., 54,644(1958) from which we would likewise have derived a (5 (Ph, Cypr —C=CH) value of 33.7 4.8 kJmol-1. 

This reaction enthalpy probes the superstrain associated with the two, trigonal, bridgehead carbons beyond that found in the individual rings. As may simply be concluded by looking at the molecular structure, the m = n = 1 species is clearly highly strained (quantum chemical calculations show the additional structural feature of non-planarity77). Experimental enthalpy-of-formation measurements of bicyclobutene show this compound to be accompanied by an exothermicity of reaction 18 of 42 42 kJ mol 1. 

The enthalpies of solution of LiOH(cr) and Li2Si0g(cr) in 20 percent HF(aq) at 50 C were measured by Hatton et al. (2). The Li2Si0g sample was prepared from lithium carbonate and silica by fusion under vacuum at 1500 K. Analysis gave 66.74 percent Si02 and 15.12 percent Li. The corresponding calculated values are 66.79 and 15.43. Also present were 0.5 percent K and 0.06 percent Na. Corrections were made for impurities in the enthalpy of formation measurements. The results are given as follows ... 

Af,C-polymethyleneanilines are cyclic species of the general formula 1,2-C6H4NH (CH2) , such as 2,3-dihydro-1// -indole36 (indoline, n = 2) and 1,2,3,4-tetrahydroquino-line37 (n = 3). There are surprisingly few studies of cyclic anilines or their derivatives of this type for which there are enthalpy of formation measurements. The differences between the enthalpies of formation of the aniline and the corresponding hydrocarbon are essentially the same for both n = 2 and 3 ca 45 (lq) and 61 (g) kJ mol 1. 

Reaction calorimetry provides useful insights here even if direct enthalpy of formation measurements are absent. Liquid phase isomerization of the o- to p-tcrt-butylphenol has been shown to be exothermic hy 16.9 1.6 kJ mol [T. N. Nesterova, S. P. Verevkin, T. N. Malova and V. A. Pil shchikov, Zh. Prikl. Khim., 58, 827 (1985) Chem. Abstr., 103, 159918x (1985)] while the p- to m-isomerization in both the Uquid and gas phase is exothermic by ca 1 kJ mol [cf. V. A. Pil shchikov, T. N. Nesterova and A. M. Rozhnov, J. Appl. Chem. USSR, 54, 1765... 

See the discussion of the difficulties of enthalpy of formation measurements of fluorinated species in A. J. Head and W. D. Good, in Combustion Calorimetry (Eds. S. Sunner and M. Mansson), Pergamon, Oxford, 1979. Complications include corrosion of the calorimeter, inadequate mixing of the solution formed from combustion (a uniform final aqueous solution of HP is required) and the formation of any perfluorocarbon during combustion. 

The enthalpy of formation of P-Zrp4 has been measured by Greenberg et al. [61GRE/SET]. The enthalpy of formation measured in the study and its assoeiated uncertainty are selected by this review,... 

As noted in Section X.5, this value is reasonably consistent with the enthalpy of formation measurements of [1982BOR/BOR2] at 300 K. 

Alkyl substituent groups on an aromatic ring introduce new modes of interaction, and perhaps distortion, without the complicating effects of jr-elec-tron donation or withdrawal because alkyl groups are essentially electronically innocuous. Almost all of the thermochemical literature on substituted aromatic species relates to benzene derivatives. There are comparatively few enthalpy of formation measurements for substituted naphthalenes. It has been shown in those few cases that the difference between enthalpies of formation of the 1- and 2-isomers is generally small, and the difference between the naphthalene and benzene derivatives is largely independent of the substituent. For example, the enthalpies of formation of 1- and 2-bromonaphthalene are 174.3 5.6 and 175.6 2.3 kj/mol. These values differ from that of bromobenzene (105.4 4.1 kJ/mol) by ca. 70 kJ/mol, very close to the enthalpy of formation difference between the parent hydrocarbons. For other aromatic hydrocarbons, there are almost no substituted derivatives other than methylated ones. 

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