Alkanes, temperature dependence

Klatte, S.J. and Beck, T.L., Molecular-dynamics of tethered alkanes—temperature-dependent behavior in a high-density chromatographic system, 7. Phys. Chem., 91, 5727, 1993. 

Sulfur reacts with alkanes to either dehydrate (eq. 1), oxidize, forming carbon disulfide and hydrogen sulfide (eq. 2), or cyclize, forming thiophenes (eq. 3). The products of alkane sulfurization depend on the temperature, the time at the temperature, and the stmcture of the hydrocarbon (1). 

This key paper was followed by a flurry of activity in this area, spanning several years." " "" A variety of workers reported attempts to deconvolute the temperature dependence of carbene singlet/triplet equilibria and relative reactivities from the influence of solid matrices. Invariably, in low-temperature solids, H-abstraction reactions were found to predominate over other processes. Somewhat similar results were obtained in studies of the temperature and phase dependency of the selectivity of C-H insertion reactions in alkanes. While, for example, primary versus tertiary C-H abstraction became increasingly selective as the temperature was lowered in solution, the reactions became dramatically less selective in the solid phase as temperatures were lowered further. Similar work of Tomioka and co-workers explored variations of OH (singlet reaction) versus C-H (triplet reaction) carbene insertions with alcohols as a function of temperature and medium. Numerous attempts were made in these reports to explain the results based on increases in triplet carbene population... 

As Table 20 shows, the yields of the Rh(II)-promoted reaction are temperature-dependent. Furthermore, competitive experiments between pairs of alkanes revealed a marked dependence on the alkoxy group of the diazoester and on the perfluoroalkyl carboxylate part of the catalyst. The observed relative selectivities have been taken as evidence for the importance of lipophilic interactions between carbenoid and alkane. 

Piacente, V., Pompili, T., Scardala, P., Ferro, D. (1991) Temperature dependence of the vaporization enthalpies of n-alkanes from vapour-pressure measurements. J. Chem. Thermodyn. 23, 379-396. 

J. As with the alkane - water systems, the interaction parameters for the aqueous liquid phase were found to be temperature - dependent. However, the compositions for the benzene - rich phases could not be accurately represented using any single value for the constant interaction parameter. The calculated water mole fractions in the hydrocarbon - rich phases were always greater than the experimental values as reported by Rebert and Kay (35). The final value for the constant interaction parameter was chosen to fit the three phase locus of this system. Nevertheless, the calculated three-phase critical point was about 9°C lower than the experimental value. 

This mechanism clearly implicated alkane complexes as precursors to C-H activation but the IR absorptions of [Cp Rh(CO)Kr] and [Cp Rh(CO)(C6Hi2)] were not resolved and were presumed to be coincident. The temperature dependent data gave values of AH = 18 (or 22) kj mol for the unimolecular C-H (or C-D) activation step representing a normal kinetic isotope effect, kn/fco 10- However, an inverse equilibrium isotope effect (K /Kq 0.1) was found for the slightly exothermic pre-equilibrium displacement of Kr by CoHn/C Dn implying that C6Dj2 binds more strongly to the rhodium center than does C Hn-... 

Because fluorescence plays a very small role in the depopulation of alkane excited molecules the sum of the quantum yields of the chemical decompositions in the thermally activated and nonactivated channels is practically unity 4>(S Sx) + 4>(S T ) is 1. Using Eq. (3), the temperature dependencies of the product yields formed in the activated and nonactivated channels have the following forms ... 

TABLE 6.2 Rate Constants and Temperature Dependence" 11 for Reaction of OH Radicals with Some Alkanes... 

Alkane isomerization equilibria are temperature-dependent, with the formation of branched isomers tending to occur at lower temperatures (Table 4.1). The use of superacids exhibiting high activity allows to achieve isomerization at lower temperature (as discussed below). As a result, high branching and consequently higher octane numbers are attained. Also, thermodynamic equilibria of neutral hydrocarbons and those of derived carbocations are substantially different. Under appropriate conditions (usual acid catalysts, longer contact time) the thermodynamic... 

In this article (Part I) we have comprehensively reviewed the structural implications of the vibrational spectroscopic results from the adsorption of ethene and the higher alkenes on different metal surfaces. Alkenes were chosen for first review because the spectra of their adsorbed species have been investigated in most detail. It was to be expected that principles elucidated during their analysis would be applicable elsewhere. The emphasis has been on an exploration of the structures of the temperature-dependent chemisorbed species on different metal surfaces. Particular attention has been directed to the spectra obtained on finely divided (oxide-supported) metal catalysts as these have not been the subject of review for a long time. An opportunity has, however, also been taken to update an earlier review of the single-crystal results from adsorbed hydrocarbons by one of us (N.S.) (7 7). Similar reviews of the fewer spectra from other families of adsorbed hydrocarbons, i.e., the alkynes, the alkanes (acyclic and cyclic), and aromatic hydrocarbons, will be presented in Part II. 

This measure, however, pertains to the normal boiling point rather than to ambient conditions. The deficit of the entropy of the liquid solvent relative to the solvent vapour and to a similar non-structured solvent at any temperature, such as 25 °C, has also been derived (Marcus 1996). An alkane with the same skeleton as the solvent, i.e., with atoms such as halogen, O, N, etc. being exchanged for CH3, CH2, and CH, etc., respectively, can be taken as the non-structured solvent. Since the vapour may also be associated, the temperature dependence of the second virial coefficient, B, of the vapour of both the solvent and the corresponding alkane, must also be taken into account. The entropy of vaporization at the temperature T, wherep P°, is given by ... 

A characteristic feature of the hydrophobic interaction is that it is dominated by entropy effects. Both the temperature dependence of alkane solubilities in water126,127) and direct calorimetric measurements128 show that Iihf is close to zero at room temperature. Some calorimetric data for heats of solution of hydrocarbons in water are shown in Table 3.2. A further noticeable feature is that Iihf is temperature dependent due to the rather large heat capacity, Cp F, associated with the hydrophobic interaction. From a systematic calorimetric study of a series of compounds with rather short alkyl chains129 it was found that... 

An important class of model compound study that we have not discussed here is the determination of transfer free energies mentioned above. Though the study of transfer of amino acids and their analogs from water into various organic compounds provides a wealth of information about various interactions, the current data base includes only values AG° (usually relative to glycine) and not values for AH0, AS0, and ACp, and thus is not suitable for the temperature-dependent information required within the context of this review. The thermodynamics from liquid hydrocarbon, crystalline cyclic dipeptide, and alkane gas dissolution are summarized in Table I. 

Fig. 35 Temperature dependence of lamellar spacing (upper half) and angle of chain tiltwith respect to the lamellar normal (lower half) for extended-chain crystals of end-deuterated alkane C216H385D49 grown from toluene solution at 70 °C. SAXS spectra were recorded during heating from 60 °C to the melting point. Data for heating at 1 °C/min (solid diamonds) and 6 °C/min (open squares) are shown for comparison (from [84] by permission of American Chemical Society)...

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