Enthalpy of hydrogenation

First derivatives of the total energy with respect to nuclear coordinates. See also Gradient Theory. 

Literature values of Ahydf are numerous and accurate. There are about 500 known enthalpies of hydrogenation, many of which have an experimental uncertainty of 1 kJ mol or less. For example, measurements of Ahyd (cyclohepta-1,3,5-triene) using three different techniques in experiments almost a half-century apart, have an arithmetic mean experimental uncertainty and a range, when corrected for solvent effects and temperature differences, of 1.1 kJ mol . This level of accuracy is important now that advances in computational methods are such that the old standard thermochemical accuracy of I kcal mol (4.2 kJ mol ) no longer suffices. 

Ahyd values are suitable as standards for evaluation and comparison of computational procedures, and for parameterization of empirical or semi-empirical computational methods.  

The methods of calculating stabilization or strain enthalpies cited in the introduction are open to criticism, largely on the choice of a reference state. Each method has, however, the virtues of simplicity and of basing theoretical constructs on enthalpies one can measure as distinct from things (ring currents, etc.) that we suppose are correlated with enthalpy. Stability is, in the final analysis, a thermodynamic property. 

A reliable Ahyd// and a reliable enthalpy of formation of either the reactant or the product of hydrogenation leads to Af// of the other participant in the reaction because Af//(H2) = 0 at 298 K by definition. Computed thermochemical properties are valid for a single, isolated molecule, hence they should be compared to experimental measurements made on the sample in the ideal gas state. Heat capacities are the most difficult of the common thermochemical properties to calculate, consequently temperature corrections are questionable if the temperature range is large. The most valid comparison is between calculated Ahyd// at 298 K and measured Ahyd//(gas, 298), 

There are enthalpies of formation for several unsaturated organomagnesium bromides as well as for species that are their saturated counterparts. How do the enthalpies of the formal hydrogenation reaction (equation 10) of the organomagnesium bromides compare with those for the corresponding hydrocarbons  

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If has long been known that the enthalpy of hydrogenalion of benzene (49.8 kcal moU Conant and Kistiakowsky, 1937) is not the same as three times the enthalpy of hydrogenation of cyclohexene (3 x 28.6 kcal moU ). Evidently, the double bonds that w e write in the Kekule structure of benzene... 

The actual value of the enthalpy of hydrogenation of 1,3-butadiene is —243 k,l rnol Both are hydrogenated to the same product, u-biitaiie hence the enthalpy diagram (Fig. 7-4) shows that bnta-1,3-diene is 11 kJ rnol. lower in enthalpy than it ought" to be on the basis of the reference standard, bnt-l-ene. 

Media Type Material Hydride Form H2 Capacity (wt%) Energy Density (kj/kg Hydride) Enthalpy of Hydrogenation (kj/mol H2)... 

The addition is exothermic. Unfortunately, no experimental heats of hydrosilation have been published. For ethylene, the enthalpy of hydrogenation is -AH = 32.82 kcal/mole (/). The enthalpy of hydrosilation is... 

Depending on amide and hydroperoxide, the equilibrium constants at room temperature vary from 0.1 to 20 L mol-1. The enthalpy of hydrogen bond hydroperoxide and amide is around 20 to 24 kJ mol-1. Three types of hydrogen bonds are formed ROOH 0 = C<, ROOH N<, and NH 0(H)0R. 

The simplest nonconjugated, acyclic diene is 1,4-pentadiene (1), with its enthalpy of formation of 105.6 kJmol-1. The obvious question is whether the two double bonds are truly independent. If they are, then the enthalpy of hydrogenation of one double bond as in (the identical) reactions 4a and 4b would be precisely one half of that of the hydrogenation of both as in reaction 5. 

To do so, one can take the enthalpy of formation of n -hexane from Pedley, and with the phase independence assumptions in Reference 7, employ the enthalpies of hydrogenation of 1-hexene and 1,5-hexadiene from References 11 and 12 respectively. Alternatively13, one can forget about the first quantity altogether and simply take the difference of the enthalpies of hydrogenation of the diene and twice that of the monoene. This reaction is endothermic by 1.1 1.8 kJ mol-1, a value statistically indistinguishable from the absence of any interolefin interaction in the diene. Relatedly, for the isomeric 1,4-hexadienes 14 and 15, equation 8 may be used. 

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. 

Starting with the n = 4 case, the desired polymer can be obtained by polymerization of either cyclobutene (16, n = 4) or butadiene. Using the cyclobutene polymerization enthalpy from Reference 16 and of the enthalpy of formation of monomer from Pedley, we find the enthalpy of formation of [— CH=CH—(CH2)2—] is 12 kJmol-1. We conclude that the enthalpy of hydrogenation is —116 kJmol-1. 

For the n = 5 case there is the unique starting material of cyclopentene (16, n = 5) and polymerization enthalpy16 from which the enthalpy of formation of [—CH=CH—(CH2)3— ] is found to be —14 kJmol-1. The enthalpy of hydrogenation is thus ca —121 kJmol-1. Likewise, for n = 6, 7 and 8, the respective enthalpies of hydrogenation of [—CH=CH—(CH2)n-2] are seen to be ca —83, —120 and — 121 kJmol-1. Except for the n = 6 case, the various enthalpies of hydrogenation are around —120 kJmol-1, a value comparable to those found for numerous simple internal olefins reported in References 11 and 14. We can think of no reason why the n = 6 case should be so different from the others17. 

This is the solvent and vaporization-corrected enthalpy of hydrogenation of the diene from Reference 23. There is no need in the current context for the enthalpy of formation of the hydrogenation product 2,3-di-f-butylbutane (or more properly named 2,2,3,4,5,5-hexamethylhexane), unlisted in our archives, and derived in Reference 23 by molecular mechanics. 

We can narrow the difference from 10 kJmol-1 even further once it is remembered that in the comparison of meso-bisallene, 27, and (Z, Z)-diene, 29, there are two extra alkylallene and alkylolefin interactions for which a stabilization of ca 3 kJ mol-1 for the latter was already suggested. Admittedly, comparison with the corresponding 1,5-cyclooctadiyne suggests strain-derived anomalies. From the enthalpy of hydrogenation, and thus derived enthalpy of formation, of this diyne from W. R. Roth, H. Hopf and C. Horn, Chem. Ber., 127, 1781 (1994), we find 1/2S (bis-allene, bis-acetylene) equals ca — 80 kJ mol-1. We deduce that the discrepancy of this last 5 quantity from the others is due to strain in the cyclic diyne. 

We now make the a posteriori obvious suggestion that determinations of enthalpies of hydrogenation be made. 

We recall that Fang and Rogers, op. cit., measured the enthalpy of hydrogenation of the acyclic trienes in a nonpolar solvent instead of acetic acid as earlier reported. However, they did not remeasure the Z- and E-isomers separately but instead assumed the earlier measured difference is correct. Said differently, they assumed that the effect on the enthalpy difference of the Z- and -hexatriene is essentially independent of solvent. This is plausible but remains untested. 

The desired enthalpy of formation of fulvene and of its 6-methyl derivative were determined by Roth by measurement of the appropriate enthalpy of hydrogenation. The facile polymerization of this compound precludes conventional bomb calorimetry. 

As documented by Pedley, only enthalpies of combustion and sublimation have been reported for 6,6-diphenylfulvene. We recommend the measurement of the enthalpy of hydrogenation to form a-cyclopentyl diphenylmethane to acquire a more precise enthalpy of formation. 

We suspect fewer problems would have arisen had Oth and coworkers (see Reference 97) decided to perform enthalpy of hydrogenation measurements on [18]annulene. Nonetheless, we note that Oth s suggested value for the enthalpy of formation of benzo-l,3,5-cyclooctatriene is within 2 kJ mol of that estimated summing Roth s enthalpy of formation of 1,3,5-cyclooctatriene and Liebman s (cited in Reference 68) benzoannelation constant. 

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