Enthalpies of Formation and Hydrogenation

The equilibration of allylamines like 176, which have an isolated C=C double bond, to conjugated enamines under basic conditions has been studied extensively  

With only one exception , that of l-(indol-3-ylethyl)-3-pyrroline (181), which is only partially transformed into its enamine 182 (equation 15), the equilibrium generally lies far on the side of the enamine l-, Af-dimethylamino-2-propene (183) is isomerized with potassium /ert-butoxide in DMSO, first to the Z-enamine (184) and then to the thermodynamically more stable -enamine (185, see equation 16) as was shown by H-NMR spectroscopy. 

Similarly, the heterocyclic amine 186 is isomerized to the heterocyclic enamine 187 (equation 17) with a AG of —4 kcal mol , which may be considered as the energy gain 

An enantioselective hydrogen migration of prochiral allylamines 188 and 190 to chiral enamincs 189 was realized by use of chiral cobalt catalysts (equation 18). 

In the base-catalysed equilibration of 191 and 192 (equation 19) one observes a contest between the conjugativc ability of a phenyl ring and a N-methylanilino group. The equilibrium lies again on the side of the enamine 192 to the extent of AG = —2.3(3) kcal mol determined for the dimethylamino group .  

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Figure 4. Diagram showing the enthalpy of formation and hydrogen capacity of known metal hydrides.

Effect of hydrocarbon enthalpy of formation and hydrogen to carbon ratio on performance with liquid oxygen as rocket propellants (sea level specific impulse, Pc = 1000 psia, equilibrium expansion, mixture ratio of maximum specific impulse). 

Table V. G. 2 - Effect of Hydrocarbon Enthalpy of Formation and Hydrogen to...

TABLE 17. Experimental enthalpies of formation and hydrogenation (in kcalmol ) of amines shown in Scheme 9... 

C, A-diphenyl nitrones (p-XC6H4CHN(0)Ph, X = NO2, Cl, H, Me and MeO) with N-phenylmaleimide to form 2,3,6-triaryl derivatives of l-oxa-2,6-diazabicyclo[3.3.0]octane-5,7-dione. No enthalpy of formation data are available for A-phenylmaleimide or for maleimide itself. However, it is available for the corresponding A-methyhnaleimide along with some other imides. The gas phase enthalpy of hydrogenation of this species (derived as the difference between its enthalpy of formation and that of N-methylsucc-inimide ) is 133.7 2.2 kJmoC. This value is essentially the same as for ethylene (derived as the difference between its enthalpy of formation and that of ethane) of 136.3 0.4 kJmoH. Therefore, let us assume the reaction of the above parent nitrone with ethylene to form the diphenylated isoxazolidine, shown in equation 12, has very much the same exothermicity as with A-phenylmaleimide, namely ca 82 kJmol . If so, the enthalpy of formation of 2,3-diphenylisoxazolidine would be 233 kJmol . Now, is this value plausible ... 

The reader will note that we avoided mention of the n 0 species and the difference of their enthalpies of formation and those with n = 1. For the gas phase Me2N(CH=CH) CHO series, the difference is 87.7 kJ mol-1 while for the H(CH=CH) H series it is 52.5 kJ mol-1. For the liquid phase Me2N(CH=CH) CHO series, the difference is 64.3 kJ mol-1, while for the solid phase Ph(CH=CH) Ph and PhN=CH(CH=CH) NHPh series, they are now 8 and 37.5 kJ mol-1. A pattern is not particularly obvious—then again we have included real amides and amidines, biphenyl that wants to be twisted and diatomic hydrogen. Perhaps we have taken chemical homology and analogy too far. 

From archival enthalpies of formation and of fusion, the estimated enthalpy of formation of solid 3,5-dibromotoluene is 28 9 kJ moF. This value, combined with 5 (OH/H), gives a predicted enthalpy of formation of the corresponding phenol of —175 kJ moF. A bromine atom and methyl group crowd the intervening OH, which could account for at least some of the ca 16 kJ moF difference between the predicted and experimental values, and we don t expect 2,4-dibromo-6-methylphenol to participate in intermolecular hydrogen bonding. The remainder of the difference is accounted for by the error bars. Altogether, the value is plausible. 

From the archival enthalpies of formation and of fusion for benzene and bromobenzene, the estimated enthalpies of formation of 1,3,5-tribromobenzene are (s) 72 kJ mol and (g) 151 kJ moH. From these values and the appropriate OH/H exchange increments, we would predict enthalpies of formation for 2,4,6-tribromophenol of —131 kJ mol for the solid and —28 kJ moC for the gas phase species. The predicted results are both ca 30 kJ moC more exothermic than the experimental ones. Since we don t expect the solid tribromophenol to participate in intermolecnlar hydrogen bonding in the same way as solid phenol does, on that basis the estimated valnes are seemingly too negative. 

Hine and coworkers showed that the dimethylamino group is by far one of the best known double-bond stabilizing substituents. This may be evaluated quantitatively from experimental enthalpies of formation and of hydrogenation presented in Table 17 and may be seen from the value of Hammett s ffp(NMe2)-constant which, at —0.83, has one of the largest negative values observed. Both enthalpy values mentioned show a stabilization of the enamine due to conjugation by — 5 to — 6 kcal mol with respect to allylamines which may be corrected in relation to olefinic systems to about —2.5 kc mol ... 

The free enthalpies of formation and therefore the equilibrium constants between paraffins, olefins and ions are known we can therefore calculate the olefin and ion concentrations. Given the operating conditions (high temperature and high hydrogen pressures), the quantities of olefins and ions are always negligible compared with those of paraffins.17... 

A recent analysis of thermochemical data based on relationships between enthalpies of formation and the unshielded core potential of a moiety X, V ( ), throws some light into the behavior of silicon-hydrogen bonds in methyls lanes. Consider first the case of carbon-hydrogen bonds. The enthalpy of Reaction 12, which is shown to be a linear function of V iZ ) can be expressed in terms of bond dissociation enthalpies. Equation 13. 

Fig. 6.40 Schematic illustration of the process whereby hydrogen can be transformed into ammonia, which is stored as Mg(NH3)6Cl2. The Mg(NH3)6Cl2 can be transported safely and when needed ammonia is released and decomposed into molecular hydrogen and nitrogen. The standard enthalpies of formation and decomposition are indicated [243]. If all...

Fig. 15 Enthalpy diagram for tetrahydroborates. The enthalpy for the hydrogen desorption is the difference between the enthalpy of formation and the enthalpy of the elemental hydride. In case of the desorption above the melting point, the heat of crystallization has to be considered [34]...

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