Dehydrogenation enthalpy

One might have thought that phenylcyclohexene might be contaminated by polymer and/or biphenyl, both formed by free-radical processes. In both cases, the apparent dehydrogenation enthalpy would be smaller, not larger, than for the alkylcyclohexenes. 

The first step was confirmed to be exothermic (AH —7.5 kj moH ) which makes a reversible rehydrogenation under reasonable physical conditions almost impossible. The reaction enthalpy for the second step was determined to be about 32 kJ mol H2. The experimental data have been confirmed by first-principles DFT calculations of the dehydrogenation enthalpies, showing that the first step of dehydrogenation is indeed exothermic [187, 188]. 

Thus, the addition of Si, increases the equilibrium pressure by >10 times while lowering the dehydrogenation enthalpy from 190 to 120 kJ (mol Hz). ... 

Suleimenov and Ha have used high-level G2 and CBS-Q ab initio methods to study the thermochemical properties of gaseous polysulfanes (H2S , =l-6) [4]. The enthalpy of formation were calculated from atomisation energies and from enthalpies of dehydrogenation reactions such as shown in Eq. (1) ... 

Among the various synthetic procedures for polysilanes is the Harrod-type dehydrogenative coupling of RSiH3 in the presence of Group 4 metallocenes (Reaction 8.1) [5,6]. One of the characteristics of the product obtained by this procedure is the presence of Si—H moieties, hence the name poly(hydrosilane)s. Since the bond dissociation enthalpy of Si—H is relatively weak when silyl groups are attached at the silicon atom (see Chapter 2), poly(hydrosilane)s are expected to exhibit rich radical-based chemistry. In the following sections, we have collected and discussed the available data in this area. 

These values of A Hr are standard state enthalpies of reaction (aU gases in ideal-gas states) evaluated at 1 atm and 298 K. 7VU values of A are in kilojoules per mole of the first species in the equation. When A Hr is negative, the reaction hberates heat, and we say it is exothermic, while, when A Hr is positive, the reaction absorbs heat, and we say it is endothermic. Tks Table 2-2 indicates, some reactions such as isomerizations do not absorb or liberate much heat, while dehydrogenation reactions are fairly endothermic and oxidation reactions are fairly exothermic. Note, for example, that combustion or total oxidation of ethane is highly exothermic, while partial oxidation of methane to synthesis gas (CO + H2) or ethylene (C2H4) are only slightly exothermic. 

Earlier in this chapter we talked about the enthalpies of formation of singly and then multiply substituted cyclopropanes and the correspondingly substituted benzenes. It is now time to talk about multiple cyclopropanes and the corresponding benzene derivatives. In Section II.D, the enthalpies of formation of the isomeric (cis)- and (trans)-1,1 2, 1 "-ter-cyclopropyl (9a and 9b) were estimated to be 208 kJ mol 1. It was also asserted that the difference of this value and the enthalpy of formation of bicyclopropyl (8) was nearly identical to the difference of the enthalpies of formation of bicyclopropyl and cyclopropane. Let us now rewrite the oligomerization/dehydrogenation reactions 11a and lib as reactions 31aand31b ... 

Corma and co-workers152 have performed a detailed theoretical study (B3PW91/6-31G level) of the mechanism of the reactions between carbenium ions and alkanes (ethyl cation with ethane and propane and isopropyl cation with ethane, propane, and isopentane) including complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved. Reaction enthalpies and activation energies for the various elemental steps and the equilibrium constants and reaction rate constants were also calculated. It was concluded that the interaction of a carbenium ion and an alkane always results in the formation of a carbonium cation, which is the intermediate not only in alkylation but also in other hydrocarbon transformations (hydride transfer, disproportionation, dehydrogenation). 

Relative Adsorption Coefficients (zr), Free Energies (A F°), Enthalpies (—AH °), and Entropies (AS°) of Adsorptive Exchange Rate Constants (K), Activation Energies (e), and the h Parameters in Catalytic Dehydrogenation. AF°, AH°, and t cal./mole AS° e.u. Ai Rate of Reactant Supply, andmo Reaction Rate ml. Substance Vapor/min., K-ml./(ml. min.). All Valuesfor N.T.P. Original Data Are Reduced to the Same Units. 

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