Rate constants hydrocarbons

The above expressions are empirical approaches, with m and D. as parameters, for including an anliamionic correction in the RRKM rate constant. The utility of these equations is that they provide an analytic fomi for the anliamionic correction. Clearly, other analytic fomis are possible and may be more appropriate. For example, classical sums of states for Fl-C-C, F1-C=C, and F1-C=C hydrocarbon fragments with Morse stretching and bend-stretch coupling anhamionicity [M ] are fit accurately by the exponential... 

A different kind of shape selectivity is restricted transition state shape selectivity. It is related not to transport restrictions but instead to size restrictions of the catalyst pores, which hinder the fonnation of transition states that are too large to fit thus reactions proceeding tiirough smaller transition states are favoured. The catalytic activities for the cracking of hexanes to give smaller hydrocarbons, measured as first-order rate constants at 811 K and atmospheric pressure, were found to be the following for the reactions catalysed by crystallites of HZSM-5 14 n-... 

The relative basicities of aromatic hydrocarbons, as represented by the equilibrium constants for their protonation in mixtures of hydrogen fluoride and boron trifluoride, have been measured. The effects of substituents upon these basicities resemble their effects upon the rates of electrophilic substitutions a linear relationship exists between the logarithms of the relative basicities and the logarithms of the relative rate constants for various substitutions, such as chlorination and... 

Efficiency of Intermediate Formation. The variation of the efficiency of a primary intermediate with conversion of the feed hydrocarbon can be calculated (22). Ratios of the propagation rate constants ( 2 / i) reactor type (batch or plug-flow vs back-mixed) are important parameters. 

Hydrocarbon resins based on CPD are used heavily in the adhesive and road marking industries derivatives of these resins are used in the production of printing inks. These resins may be produced catalyticaHy using typical carbocationic polymerization techniques, but the large majority of these resins are synthesized under thermal polymerization conditions. The rate constants for the Diels-Alder based dimerization of CPD to DCPD are weU known (49). The abiHty to polymerize without Lewis acid catalysis reduces the amount of aluminous water or other catalyst effluents/emissions that must be addressed from an environmental standpoint. Both thermal and catalyticaHy polymerized DCPD/CPD-based resins contain a high degree of unsaturation. Therefore, many of these resins are hydrogenated for certain appHcations. 

Fig. 14. Energy-gap dependence of the rate constant of intersystem ST conversion for 1. aromatic hydrocarbons and 2. their totally deuterated substitutes.

That the reaction with a lower rate constant is taking place preferentially and that the rate increases during the reaction are phenomena that can also occur with parallel reactions. As an example, Wauquier and Jungers (48), when studying competitive hydrogenation of a series of couples of aromatic hydrocarbons on Raney-nickel, have observed these phenomena for the couple tetraline-p-xylene (Table I). The experimental result was... 

Rate Constants k (mmole min g ) of Isolated Reactions, and Relative Reactivities S from Competitive Reactions Obtained in the Hydrogenation of Aromatic Hydrocarbons... 

It has been proposed that aromatic solvents, carbon disulfide, and sulfur dioxide form a complex with atomic chlorine and that this substantially modifies both its overall reactivity and the specificity of its reactions.126 For example, in reactions of Cl with aliphatic hydrocarbons, there is a dramatic increase in Ihe specificity for abstraction of tertiary or secondary over primary hydrogens in benzene as opposed to aliphatic solvents. At the same time, the overall rate constant for abstraction is reduced by up to two orders of magnitude in the aromatic solvent.1"6 The exact nature of the complex responsible for this effect, whether a ji-coinplex (24) or a chlorocyclohexadienyl radical (25), is not yet resolved.126- 22... 

Because of the importance of hydroperoxy radicals in autoxidation processes, their reactions with hydrocarbons arc well known. However, reactions with monomers have not been widely studied. Absolute rate constants for addition to common monomers are in the range 0.09-3 M"1 s"1 at 40 °C. These are substantially lower than kL for other oxygen-centered radicals (Table 3.7). 454... 

The N+ relationship, as discussed above, is a systematization of experimental facts. The equation of Scheme 7-4 has been applied to nearly 800 rate constants of over 30 electrophiles with about 80 anionic, neutral, and even cationic nucleophiles covering a range of measured rate constants between 10-8 and 109s 1 (Ritchie, 1978). Only about a dozen rate constants deviated from the predicted values by more than a factor of 10, and about fifty by factors in the range 5-10. It is therefore, very likely that this correlation is not purely accidental. Other workers have shown it to be valid for other systems, e.g., for ferrocenyl-stabilized cations (Bunton et al., 1980), for coordinated cyclic 7r-hydrocarbons (Alovosus and Sweigart, 1985), and for selectivities of diarylcarbenes towards alkenes (Mayr, 1990 Mayr et al., 1990). On the other hand, McClelland et al. (1986) found that the N+ relationship is not applicable to additions of less stable triphenylmethyl cations. 

The sulfenic acids have been found to be extremely active radical scavengers showing rate constants of at least 107 m"1 s 1 for the reactions with peroxyl radicals at 333 K17. It has also been suggested that the main inhibiting action of dialkyl sulfoxides or related compounds in the autoxidation of hydrocarbon derives from their ability to form the transient sulfenic acids on thermal decomposition, i.e.17... 

Low energy ion-molecule reactions have been studied in flames at temperatures between 1000° and 4000 °K. and pressures of 1 to 760 torr. Reactions of ions derived from hydrocarbons have been most widely investigated, and mechanisms developed account for most of the ions observed mass spectrometrically. Rate constants of many of the reactions can be determined. Emphasis is on the use of flames as media in which reaction rate coefficients can be measured. Flames provide environments in which reactions of such species as metallic and halide additive ions may also be studied many interpretations of these studies, however, are at present speculative. Brief indications of the production, recombination, and diffusion of ions in flames are also provided. 

Collisional Detachment. Reactions of negative ions in flames not containing hydrocarbons have not been widely studied, although OH -ion formation is important in flames containing high electron concentrations. The rate constant k l of the reaction... 

An interesting approach to studies of the effects of coordination on the reactivity of lithium polydienes in hydrocarbon solvents was developed by Erussalimski and his colleagues 151 154 The polymerization of lithium polyisoprene in hexane is accelerated by the addition of TMEDA152), the rate levels off at a value of R = [TMEDA]/[li-thium polyisoprene] of 8, its final value giving the absolute rate constant of propagation of the polyisoprene coordinated with TMEDA, namely 0.17 M7l s at 20 °C. 

Square brackets around a molecular species indicate atmospheric concentration. The rate constants k times the reactant concentration product refers to the rates of the chemical reactions of the indicated number. The photolytic flux term /l4 refers to the photodissociation rate of N02 in Reaction R14, its value is proportional to solar intensity.]. RO2 stands for an organic peroxyl radical (R is an organic group) that is capable of oxidizing NO to NO2. Hydrocarbons oxidize to form a very large number of different RO2 species the simplest of the family is methylperoxyl radical involved in R5, R6 and R8. 

Explicit mechanisms attempt to include all nonmethane hydrocarbons believed present in the system with an explicit representation of their known chemical reactions. Atmospheric simulation experiments with controlled NMHC concentrations can be used to develop explicit mechanisms. Examples of these are Leone and Seinfeld (164), Hough (165) and Atkinson et al (169). Rate constants for homogeneous (gas-phase) reactions and photolytic processes are fairly well established for many NMHC. Most of the lower alkanes and alkenes have been extensively studied, and the reactions of the higher family members, although little studied, should be comparable to the lower members of the family. Terpenes and aromatic hydrocarbons, on the other hand, are still inadequately understood, in spite of considerable experimental effort. Parameterization of NMHC chemistry results when NMHC s known to be present in the atmosphere are not explicitly incorporated into the mechanism, but rather are assigned to augment the concentration of NMHC s of similar chemical nature which the... 

The results of chain transfer studies with different polymer radicals are compared in Table XIV. Chain transfer constants with hydrocarbon solvents are consistently a little greater for methyl methacrylate radicals than for styrene radicals. The methyl methacrylate chain radical is far less effective in the removal of chlorine from chlorinated solvents, however. Vinyl acetate chains are much more susceptible to chain transfer than are either of the other two polymer radicals. As will appear later, the propagation constants kp for styrene, methyl methacrylate, and vinyl acetate are in the approximate ratio 1 2 20. It follows from the transfer constants with toluene, that the rate constants ktr,s for the removal of benzylic hydrogen by the respective chain radicals are in the ratio 1 3.5 6000. Chain transfer studies offer a convenient means for comparing radical reactivities, provided the absolute propagation constants also are known. 

One rather unfortunate aspect of the M + hydrocarbon (and M + OX) reactions mentioned thus far is that the products of the reactions were not detected directly, but were instead inferred via the pressure and temperature dependencies of the measured rate constants for metal reactant consumption and by comparison to ab initio calculations. Exceptions are the reactions of Y, Zr + C2H4 and C3H6, for which the Weisshaar group employed the 157 nm photoionization/mass spectrometry technique to identify the products of the reaction as those resulting from bimolecular elimination of H2.45 47 95... 

Quenching rate constants for dienes and quadricyclenes have similar sensitivities to the electronic and structural features of the excited aromatic hydrocarbon. However, during this process quadricyclene isomerizes to nor-boraadiene with a quantum yield of 0.52, whereas dienes usually remain unchanged/10 Hammond has suggested that vibrational energy which is partitioned to the acceptor upon internal conversion of the exciplex can lead to isomerization(10a,103) ... 

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