Hydrocarbon thermodynamic stability

It has been found that there is often a correlation between the rate of deprotonation (kinetic acidity) and the thermodynamic stability of the carbanion (thermodynamic acidity). Because of this relationship, kinetic measurements can be used to construct orders of hydrocarbon acidities. These kinetic measurements have the advantage of not requiring the presence of a measurable concentration of the carbanion at any time instead, the relative ease of carbanion formation is judged from the rate at which exchange occurs. This method is therefore applicable to very weak acids, for which no suitable base will generate a measurable carbanion concentration. 

As thermodynamic stability indexes for the hydrocarbon ions, pA R+ and pA a values [(4) and (5)] have been widely applied for the carbocation and carbanion, respectively, in solution. Here K + stands for the equilibrium constant for the reaction (6) of a carbocation and a water molecule stands for the equilibrium constant for the reaction (7) of a hydrocarbon with a water molecule to give the conjugate carbanion. The equilibrium constants are given by (8) and (9) for dilute aqueous solutions. Obviously, the reference system for the pKn+ scale is the corresponding alcohol, and... 

Attempts to optimize the stability and SOD activity of C-substi-tuted (R = methyl and fused cycloalkyl) [Mn(II)[15]aneN5)Cl2] complexes 95 have shown that increasing the number of hydrocarbon substituents greatly increases the kinetic stability of the complex toward dissociation via protonation (Fig. 19). (443). There is also some enhancement of thermodynamic stability. The trans-fused endohexano Mn(II) complex 96 has a faster dismutation rate constant (9.09 X 107M-1 s1, pH 7.4) and a 10 times higher thermodynamic stability than the unsubstituted complex. 

This indicates a lack of dynamic cohesion within the adducts i.e. the substrate has considerable freedom for reorientation within the receptor. The apparent reason for an absence of mechanical coupling is the nearly cylindrical symmetry of cucurbituril, which allows the guest an axis of rotational freedom when held within the cavity. Hence, the bound substrates show only a moderate increase in tc relative to that exhibited in solution. No relationship exists between values and the thermodynamic stability of the complexes as gauged by K (or K, cf. Tables 1 and 2). It must be concluded that the interior of cucurbituril is notably nonsticky . This reinforces previous conclusions that the thermodynamic affinity within adducts is chiefly governed by hydrophobic interactions affecting the solvated hydrocarbon components, plus electrostatic ion-dipole attractions between the carbonyls of the receptor and the ammonium cation of the ligands. 

The thermodynamic stability of isomeric hydrocarbons is determined by burning them to CO2 and H O and comparing the heat evolved per mole -AH combustion). The more stable isomer has the smaller -AH) value. Trans alkenes have the smaller values and hence are more stable than the cis isomers. This is supported by the exothermic AH negative) conversion of cis to irons isomers by ultraviolet light and some chemical reagents. 

Despite their thermodynamic stability, dimetallacyclopropanes proved to be quite reactive when treated with appropriate reagents, specifically, unsaturated hydrocarbons and protic acids. Again, the nature of the metals and coligands distinctly influence the reactivity. 

Whereas step 1 is stoichiometric, steps 2 and 3 form a catalytic cycle involving the continuous generation of carbenium ions via hydride transfer from a new hydrocarbon molecule (step 3) and isomerization of the corresponding carbenium ion (step 2). This catalytic cycle is controlled by two kinetic and two thermodynamic parameters that can help orient the isomer distribution, depending on the reaction conditions. Step 2 is kinetically controlled by the relative rates of hydrogen shifts, alkyl shifts, and protonated cyclopropane formation, and it is thermodynamically controlled by the relative stabilities of the secondary and tertiary ions. (This area is thoroughly studied see Chapter 3.) Step 3, however, is kinetically controlled by the hydride transfer from excess of the starting hydrocarbon and by the relative thermodynamic stability of the various hydrocarbon isomers. 

Ion radicals of conjugated acyclic or aromatic hydrocarbons (butadiene or naphthalene) are typical examples of the species with a released unpaired electron. They are named ir-elec-tron ion radicals and have a spin distribution along the whole molecular contour. An important feature of such species is that all the structural components are coplanar or almost coplanar. In this case, spin density appears to be uniformly or symmetrically distributed over the molecular framework. Spin-density distribution has a decisive effect on the thermodynamic stability of ion radicals. In general, the stability of ion radicals increases with an enhancement in delocalization and steric shielding of the reaction centers bearing the maximal spin density. 

It is generally accepted that the soft-core RMs contain amounts of water equal to or less than hydration of water of the polar part of the surfactant molecules, whereas in microemulsions the water properties are close to those of the bulk water (Fendler, 1984). At relatively small water to surfactant ratios (Wo < 5), all water molecules are tightly bound to the surfactant headgroups at the soft-core reverse micelles. These water molecules have high viscosities, low mobilities, polarities which are similar to hydrocarbons, and altered pHs. The solubilization properties of these two systems should clearly be different (El Seoud, 1984). The advantage of the RMs is their thermodynamic stability and the very small scale of the microstructure 1 to 20 nm. The radii of the emulsion droplets are typically 100 nm (Fendler, 1984 El Seoud, 1984). 

Total rt-Electron Energy and the Thermodynamic Stability of Benzenoid Hydrocarbons... 

The cage rearrangements required knowledge of the thermodynamic stability of hydrocarbons. Experimental data were not available. This is why I became interested in computations in the mid-1960s. The computers... 

Barker, C., 1982. Geochemical considerations of deep earth gas including the thermodynamic stability and migration of hydrocarbons in the mantle. In Deep Source Gas Workshop, METC, Morgantown, WV, May 3-4. 

The most striking feature is the high thermodynamic stability of bicyclo[3.3.0]octane, or pentalane structures. Hydrocarbons of this type should be present in the products of acid-catalyzed reactions of terpene and in petroleum chemistry. 1-Methylbicyclo[3.3.0]octane was isolated from petroleum by the workers of the API Research Project 6 (38). 

For hydrocarbons the thermodynamic stability can be measured by means of the free energy of formation from the elements, i.e. carbon and hydrogen. All hydrocarbons become unstable above 500°C. The mechanism of conversion is quite complicated and can be modified by the presence of catalysts. In the case of thermal decomposition the primary products are converted into compounds of increasing stability [ 39] (Figure 1)... 

Another system for which thermodynamic data have been obtained in some detail is the Tp Rh(CNneopentyl)(R)H system studied by Jones. Here, the relative thermodynamic stabilities of a number of adducts were obtained by measuring both the competitive kinetic selectivity for two types of C-H bond (AAGt in Fig. 2) as well as the barrier for reductive elimination of free alkane from each adduct (AG and AG in Fig. 2). The free energies for the latter were obtained from kinetic studies of the reductive elimination of hydrocarbon in benzene. A summary of the AG° values, calculated equilibrium constants, and relative metal-carbon bond strengths are given in Table 4 [26]. For DC H for benzene, see ref. 

Both the electric arc and the plasma jet have been used for the pyrolysis of coal. Acetylene is the principal hydrocarbon product, its yield being three times more in a hydrogen atmosphere than in an argon atmosphere. Since the thermodynamic stability of acetylene decreases rapidly below about 1600 K, the product gases must be quenched rapidly in order to prevent the decomposition of acetylene (Sect. 2). 

Kenney J.F., Kutcherov V.A., Bendeliani N.A., Alekseev V.A. (2002) The evolution of multicomponent systems at high pressures VI.The thermodynamic stability of the hydrogen-carbon system the genesis of hydrocarbons and the origin of petroleum. Proc. Natl Acad.Sci. USA 99,10976—81. 

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