Alkane Reactions

The most extensive studies of alkane reactions in aqueous media are on the oxygenation reaction. In fact, nature has used monooxygenase (found in mammalian tissue) and other enzymes to catalyze the oxidation of alkanes to give alcohols in aqueous environments at ambient... 

The nitroxide radical (from processes 5 and 6 and attack by other radicals on the parent piperidine) is found in photo-oxidizing PPH samples in concentrations of M. x 10 M (initial piperidine level 5 x 10-3M) up to the embrittlement point of the PPH film (7.). Nitroxides are well known to scavenge carbon centered radicals (but not peroxyl radicals) in both polymers and liquid alkanes (reaction 7) (10, 8). In the liquid phase k7 is... 

Determination of OH relative rate constants for compounds that photolyze significantly in actinic radiation requires a nonphotolytic source of OH. Three such OH sources are H202-N02-C0 mixtures (Campbell et al., 1975, 1979 Audley et al., 1982), the thermal decomposition of H02N02 in the presence of NO (Barnes et al., 1982), and 03-hydrazine reactions (Tuazon et al., 1983) or 03 alkane reactions in the dark (Finlayson-Pitts et al., 1993). However, in these cases, the reactant must not react with 03, HOz, or H202, and care must be taken in interpreting the data since these systems have the potential of being rather complex. Indeed, the rate constants derived have not always agreed well with literature values. Until the... 

Second, the room temperature rate constants increase with increasing size and complexity of the alkane and are of the order of 10-11 cm3 molecule-1 s-1 for the largest alkanes. To put this in perspective, a diffusion-controlled reaction, i.e., one that occurs on every collision of the reactants, is of the order of (3-5) X 10 10 cm3 molecule-1 s-1. Thus for the larger alkanes, reaction occurs in approximately one in 10 collisions, which is quite a fast process. 

The kinetics and mechanisms of nitrate radical reactions with alkanes and a variety of other organics relevant to the atmosphere are discussed in detail in two excellent reviews by Wayne et al. (1991) and Atkinson (1991). The kinetics of the N03-alkane reactions are summarized in Table 6.3, where it can be seen that, with the exception of methane, they are in the range 10 lX-10 lf cm3 molecule-1 s-1. 

The products of the abstraction path are easily predictable, based on our understanding of the fates of alkyl radicals produced in alkane reactions (see Sections C and D). For example, in the case of toluene, the... 

Reaction with OH is, however, reasonably rapid as might be expected and is of the same order of magnitude as the OH-alkane reactions. Table 6.22, for example, shows the room temperature rate constants for the reactions of OH at 298 K with some alkyl nitrates. With 2-butyl nitrate as an example, the lifetime with... 

This technique was quickly adopted by others and it was soon found by F.O. Rice and co-workers that the pyrolysis of many organic compounds at 800 to 1000°C removed metallic mirrors, implicating the formation of free radicals. The cleavage of larger free radicals into smaller radicals and olefins under these conditions, was also proposed (equation 22), as well as chain reactions in which radicals abstract hydrogen from alkanes. Reactions of alkyl halides with metal atoms in the gas phase were also found by M. Polanyi and co-workers to yield alkyl radicals (equation 23). 

Propane s greatest use is not as a fuel but in the petrochemical industry as a feedstock. As an alkane, it undergoes typical alkane reactions of combustion, halogenation, pyrolysis, and oxidation. Pyrolysis or cracking of propane at several hundred degrees Celsius and elevated pressure in combination with metal catalysts result in dehydrogenation. Dehydrogenation is a primary source of ethylene and propylene ... 

In most cases, the two types of mechanisms, the bond shift and cyclic mechanisms, are not exclusive but parallel pathways. With increasing molecular weight, the contribution of the cyclic mechanism increases and may become dominant. The pure selective mechanism on iridium is a unique exception. Hydrogenolysis, however, is the characteristic transformation on this metal. The nature of possible surface intermediates in metal-catalyzed alkane reactions, the role of electronic and geometric effects in their formation, and the relation of isomerization and hydrogenolysis have been reviewed.163... 

Since the experimentally observed primary radical distribution in alkane reactions does not quantitatively correspond to that calculated from the quenching diameters of the constituent alky] groups within the molecule, it can be seen that additional evidence is thereby provided... 

Protonated alkanes (C H2 , 3 ) also play a significant role in alkane reactions. Saturated hydrocarbons can be protonated to alkonium ions, of which the methonium ion CH5+, 422 [Eq. (3.119)] is the parent, and formation of these pentacoordinate carbocations involves two-electron three-center (2e—ic) bonds. The dotted lines in the structure symbolize the bonding orbitals of the three-center bonds [Eq. (3.119)] their point of junction does not represent an additional atom. 

To the best of our knowledge, there are no published data on the reactions of alkanes with H2 over palladium single crystals. Deuterium exchange reactions with various alkanes over palladium films were extensively investigated by Kemball et al. (39-42). Because those results were already discussed in several excellent review articles (43-46), we do not intend to dwell upon analyzing all the data at greater length. We only specify here the basic characteristics of Pd films exhibited in D2/alkane reactions ... 

Summing up, it is conceivable that both the state of surface topography as well as the presence of other elements on the Pd surface are decisive for its activity in alkane reactions. However, two things seem obvious and unquestionable. First, all these catalytic variations result from the fact that hydrogenolysis, rather than isomerization, is sensitive to changes in... 

Cyclopropane is generally more reactive than other alkanes. Reactions that open the cyclopropane ring release 115 kJ (27.6 kcal) per mole of ring strain, which provides an additional driving force for these reactions. 

The last of the three papers, by the Schleyer group,was a study of the bond separation reaction for linear alkanes (Reaction 3.5) ... 

In this chapter, functionalization reactions are emphasized which are of genuine practical utiliQr, but others that are noteworthy and of potential synthetic significance are also discussed. Apart from alkane reactions, some intermolecular functionalization reactions which operate on unactivat C—H bonds, e.g. the side chain of a steroid, are examined. Intramolecular cases are covered elsewhere, and general reviews of the area are available. ... 

There are three general classes of mechanism most often encountered in alkane reactions (i) radical (ii) electrophilic and (iii) carbenoid. The C—bond-lneaking st in (i) and (ii) are shown in equations (2) and (3). Carbenoid reactions can go either by direct insertion into the C—bond (equation 4), which tends to happen when the carbene in question has singlet character, or by a two-stqi process (equations 5 and 6), in which H-atom abstraction precedes collapse of the radical pair, a pathway which is characteristic of triplet carbenes. ... 

Marks has reviewed this area. Basset has bound organometallic species to a silica surface and seen interesting alkane reactions that go via initial CH bond activation this step probably proceeds by a sigma bond metathesis pathway since the metals involved are d . Snbseqnent reactions inclnde CC bond cleavage and carbon skeletal rearrangements. 

Peroxyl Radicals Secondary peroxyl radicals, as are found in most lipid acyl chains, recombine rapidly (2k = 10 -10 M s ) (192, 362) to form a variety of products, including alcohols and ketones (Reaction 67) (361, 362, 366), ketones and alkanes (Reaction 68) (60, 292), or acyl peroxides and peroxyl radicals (Reaction 69) (264, 367, 369). The alcohols thus produced are indistinguishable from H abstraction products of an original LO, but the ketones and dialkyl peroxides are unique to recombination reactions. As any R3OO and RO released from Reaction 68 or Reaction 69a react further, peroxyl radical recombinations also have the potential for propagating lipid oxidation (Section 3.1.4). 

The correlation of k or Egct with bond strengths are the most commonly used for estimation purposes but correlations with other properties of the molecules are possible. For example, it is possible to correlate the rate constant for OH + alkane reactions with the ionization potential of the alkane [10] and, for other types of reaction, it has been suggested that both electron affinities and ionization potentials of the species involved may correlate with the kinetics parameters [51]. However, because of the great sensitivity of the rate constant to small errors in Eact, correlations aimed at predicting Eact are very rarely used. 

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