Alkanes relative reactivities

Chlorination of Higher Alkanes Relative Reactivity and Selectivity CHAPTER 3... 

CHLORINATION OF HIGHER ALKANES RELATIVE REACTIVITY AND SELECTIVITY... 

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... 

There is some increase in selectivity with functionally substituted carbenes, but it is still not high enough to prevent formation of mixtures. Phenylchlorocarbene gives a relative reactivity ratio of 2.1 1 0.09 in insertion reactions with i-propylbenzene, ethylbenzene, and toluene.212 For cycloalkanes, tertiary positions are about 15 times more reactive than secondary positions toward phenylchlorocarbene.213 Carbethoxycarbene inserts at tertiary C—H bonds about three times as fast as at primary C—H bonds in simple alkanes.214 Owing to low selectivity, intermolecular insertion reactions are seldom useful in syntheses. Intramolecular insertion reactions are of considerably more value. Intramolecular insertion reactions usually occur at the C—H bond that is closest to the carbene and good yields can frequently be achieved. Intramolecular insertion reactions can provide routes to highly strained structures that would be difficult to obtain in other ways. 

Alternatively, some conclusions can be derived from the relative reactivities of car-banions. For example, DePuy and colleagues13 made use of a clever method involving reactions of silanes with hydroxide ion to deduce acidities of such weak acids as alkanes and ethylene. The silane reacts with hydroxide ion to form a pentacoordinate anion that ejects a carbanion held as a complex with the hydroxysilane rapid proton transfer gives the stable silanoxide ion and the carbon acid (equation 5). 

In addition to the electronic difference between PR3 and PH3, bulkier ligands on the phosphine can change the reaction through their steric effect. Using the R = Bu on the anthraphos system, Haenel et al. calculated the available molecular surface (AMS) around the metal center as a measure of the space available to the alkane (13b). They correlated the AMS to the relative reactivities of the catalysts and the results show that two bulky tert-butyl groups on each P certainly limit the access to the metal center, and thus, may reduce the reactivity. Other theoretical studies on the pincer complexes showed that this steric contribution/ limitation plays a less important role than the activation barriers introduced by the catalyst itself (22), where the increase in energy barrier induced by the bulky 4Bu is smaller than the original barriers calculated... 

Table 16.12 compares the POCP values derived by Dement et al. (1996, 1998) and Andersson-Skold et al. (1992) to the MIR approach of Carter (1994). While the general trends in reactivities predicted by each approach are qualitatively similar, there are quantitative differences. For example, the POPC values for the simple alkanes relative to ethene are larger than the MIR values. This reflects in part the details of the mechanisms used in the calculations and the time scale over which the reactions are followed as well as differences in the assumed pollutant mix into which the VOC is injected, such as the VOC/NO ratio. 

Compare the relative reactivity of silanes and alkanes toward nucleophilic attack, hydrolysis, and halogenation. ... 

Alkanes are preferentially hydroxylated at the more nucleophilic C—H bonds, with relative reactivities tertiary secondary primary hydrogens = 7000 110 l.303 This reaction occurs with a high retention of configuration at the hydroxylated carbon atom, as shown by the selective formation of cis-9-decalol from the oxidation of cis-decalin with chromyl acetate in an acidic medium304 and the hydroxylation of chiral (+)-3-methylheptane (91) to chiral alcohol (92) with 72 to 85% retention of configuration.305... 

The (r-donor ability of the C—C and C—H bonds in alkanes was demonstrated from a variety of examples. The order of reactivity of single bonds was found to be tertiary C—H > C—C > secondary C H primary C H, although various specific factors such as steric hindrance can influence the relative reactivities. 

However, it is difficult to reconcile the observed relative reactivities of hydrocarbons with a mechanism involving electron transfer as the rate-determining process. For example, n-butane is more reactive than isobutane despite its higher ionization potential (see Table VII). Similarly, cyclohexane undergoes facile oxidation by Co(III) acetate under conditions in which benzene, which has a significantly lower ionization potential (Table VII), is completely inert. Perhaps the answer to these apparent anomalies is to be found in the reversibility of the electron transfer step. Thus, k-j may be much larger than k2 for substrates, such as benzene, that cannot form a stable radical by proton loss from the radical cation [Eqs. (224) and (225)]. With alkanes and alkyl-substituted arenes, on the other hand, proton loss in Eq. (225) is expected to be fast. 

Tanaka296 found the relative rates of oxidation of cycloalkanes by Co(III) acetate in acetic acid at 90°C to decrease in the order Cs >C6 > C7-Ci2. He concluded that the rate-controlling step did not involve C—H bond rupture but, instead, formation of a complex between the alkane and Co(III). The relative reactivities were attributed to steric hindrance in the formation of the complex, the structural features of which were not elaborated further. 

Systematic examination of the catalytic properties of dimeric complexes was initiated shortly after the identification of dinuclear iron sites in metalloenzymes. The first report of a reactive dimeric system came from Tabushi et al. in 1980, who examined the catalytic chemistry of [Fe3+(salen)]20, 1 (salen is N,N -(salicylaldehydo)-l,2-ethylenediamine) (12). They reported interesting stereoselectivity in the oxidation of unsaturated hydrocarbons with molecular oxygen in the presence of mercaptoethanol or ascorbic acid and pyridine as a solvent ([l]<<[alkane]<<[2-mercaptoethanol]). With adamantane as substrate, they observed the formation of a mixture of (1- and 2-) adamantols and adamantanone (Table I) (12). Both the relative reactivity between tertiary and secondary carbons (maximum value is 1.05) and final yield ( 12 turnovers per 12 hr) were dependent on the quantity of added 2-mercaptoethanol. Because autoxidation of adamantane gave a ratio of 3°/2° carbon oxidation of 0.18-0.42, the authors proposed two coexisting processes autooxidation and alkane activation. 

Both in vapor and in liquid phase reaction conditions, nonane is more reactive than heptane. The reactivity difference is however much more pronounced in the vapor phase. In USY zeolite micropores exposed to the vapor of the two n-alkanes, the heaviest alkane molecule is preferentially adsorbed, resulting in a higher apparent reaction rate. When the alkane mixture is fed in the liquid phase, the competing alkanes are adsorbed in a non-selective manner in the micro- and mesopores of USY. Consequently, in liquid phase conditions the relative reactivity of the n-alkanes corresponds to the relative intrinsic reactivities. 

Effect of Other Hydrocarbons. The complex role of C3H6 in this system has been analyzed in terms of its reactions with O, O3, and OH, and a similar analysis is applied to determining the role of other hydrocarbons in the smog chemistry. Relative rate constants for the reactions of a number of hydrocarbons with O, O3, and OH hydrocarbons are presented in Table I, together with the relative reactivities of these hydrocarbons based on NO-NO2 conversion rates observed in smog chambers. The relative OH rate constants correlate remarkably well with the reactivities for all the types of hydrocarbons listed in the table. By contrast, the O-atom and O3 rates correlate with the reactivities only for the olefinic compounds. For the aromatics, aldehydes, and alkanes in the table, the relative O-atom and O3 rate constants are negligibly small compared with the relative reactivities. The relative OH rate... 

Two-laser two-photon results revealed photoisomerization of the cation E,E-11 to its stereoisomer Z,E-11, which undergoes thermal reversion with a lifetime of 3.5 ps at room temperature. Absolute rate constants for reaction of styrene, 4-methylstyrene, 4-methoxystyrene and /i-methyl-4-methoxystyrene radical cation with a series of alkanes, dienes and enol ethers are measured by Laser flash photolysis [208]. The addition reactions are sensitive to steric and electronic effects on both the radical cation and the alkene or diene. Reactivity of radical cations follows the general trend of 4-H > 4-CH3 > 4-CH3O > 4-CH30-jff-CH3, while the effect of alkyl substitution on the relative reactivity of alkenes toward styrene radical cations may be summarized as 1,2-dialkyl < 2-alkyl < trialkyl < 2,2-dialkyl < tetraalkyl. 

We shall make constant use of these designations in our consideration of the relative reactivities of various parts of an alkane molecule. 

This sequence applies (a) to the various hydrogens within a single alkane and hence governs orientation of reaction, and (b) to the hydrogens of different alkanes and hence governs relative reactivities. 

Furthermore, competition experiments show that, under conditions where 3", 2, and P hydrogens show relative reactivities of 5.0 3.8 1.0, the relative rate per benzylic hydrogen of toluene is only 1.3. As in its attack on alkanes (Sec. 3.28), the more reactive chlorine atom is less selective than the bromine atom less selective between hydrogens in a single molecule, and less selective between hydrogens in different molecules. 

These results question the validity of many previous results on catalytic oxidation of light alkanes. One should reassess the data concerning the relative reactivity of the various alkanes [105] and selectivity. 

Fig. 11. Relative reactivities of methane and C3-C4 alkanes at different initial oxygen concentrations ft = (Q,0-Qf)/Q,o- (T = 800K, P = 30bar). ( )—methane, ( )—propane, (A)—butanes.

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