Cumene considerations

Induced reactions involving hydrogen peroxide can be observed with hydrogen peroxide derivatives, as well. For instance, the reaction between cumene hydroperoxide and iron(IT), in the absence of oxygen, results in a considerable induced decomposition of the peroxy compound, while, in the presence of oxygen, a marked oxidation of iron(II) takes place s . 

An increase in cumene yield by 0.5% was thus achieved in the micro reactor [64]. Considering an annual production of 7 million tons, this is a considerable improvement regarding economics. 

Tn 1955 Russell showed that mixtures of cumene and small amounts of - Tetralin oxidized at rates considerably below the rates of oxidation of either of the pure hydrocarbons (26). Russell suggested that this decrease in rate was caused by the fact that tetralylperoxy radicals terminate oxidation chains much more readily than cumylperoxy radicals. The mixtures, therefore, oxidize at a lower rate than pure cumene because of... 

Reactivity ratios for all the combinations of butadiene, styrene, Tetralin, and cumene give consistent sets of reactivities for these hydrocarbons in the approximate ratios 30 14 5.5 1 at 50°C. These ratios are nearly independent of the alkyl-peroxy radical involved. Co-oxidations of Tetralin-Decalin mixtures show that steric effects can affect relative reactivities of hydrocarbons by a factor up to 2. Polar effects of similar magnitude may arise when hydrocarbons are cooxidized with other organic compounds. Many of the previously published reactivity ratios appear to be subject to considerable experimental errors. Large abnormalities in oxidation rates of hydrocarbon mixtures are expected with only a few hydrocarbons in which reaction is confined to tertiary carbon-hydrogen bonds. Several measures of relative reactivities of hydrocarbons in oxidations are compared. 

The same relation applies to Tetralin, cumene, alcohols, and all compounds oxidizing with a considerable chain length. For m-xylene the chain is short (A — 2). In this case, the simplifying hypotheses for long chains which neglect the initiator radicals (I ) and their peroxy (I02 ) forms are no longer valid (11). In particular the rate of oxygen con-... 

In structures with various reactional positions, consideration must be given to a composite action of the different recombinations possible. The values of kt are then intermediate between those of structures having only one reaction function. This is the case, for example, of pinane compared with cyclohexane and dimethylbutane, or else of p-cymene compared with cumene and toluene. 

The measured rate constants show some inconsistencies in relation to other work. The most noticeable is the low ratio of kceric/kphotoiysis at 30°C. for f erf-butyl hydroperoxide and cumene hydroperoxide compared with estimates, —5 to 10 for k /k2, obtained from studies of the induced decomposition of these hydroperoxides (22, 46, 48). The photolytic rate constant for cumene hydroperoxide is considerably larger than the termination constant for the oxidation of cumene containing cumene hydroperoxide as determined by the rotating sector (25, 26, 27, 28). It is not clear whether these differences represent some unappreciated features... 

Except for cumene (discussed below), no great accuracy is obtained in the rapid individual measurements of the low velocities employed, but the deviations are smoothed out considerably in the logarithmic straight lines, so that the resultant slopes are reasonably accurate (Table I and Figure 3). A fundamental drawback of the method is the limited range of conditions under which measurements can be made (except for cumene) the deviations observed are therefore regarded as a measure of the reproducibility of the method rather than of the accuracy of values obtained. 

Since the duration of the non-steady state can be shown (4) to be inversely related to kt1/2 and R 1/2, a high value of kt is unfavorable because it reduces both the time available for measurement and the rate to be measured. For this reason cumene, having a low kt, is the obvious choice for demonstrating the method. Cyclohexene and Tetralin, on the other hand, probably represent the limit of what can be measured their relatively high kt is partially offset by high values of kp [RH], which increase the velocities, hence also the accuracy obtainable at a given Rt The method proved ineffective with fluorene, which must be measured in solution (e.g., 1.8M in chlorobenzene which reduces its kp [RH] by a factor of 4 relative to Tetralin), sec-butylbenzene and cyclohexylbenzene, which probably combine a relatively low kp with considerably higher kt than for cumene (I). 

Other workers (4, 5, 6, 7) have made Al-deficient sieves by leaching aluminum from the lattice structure with EDTA or HC1. These zeolites have high thermal stability (4). Extraction of Al removes selectively the aluminic sites that are catalytically inactive. The number of sites of weak or medium acid strength drops to zero (6). Eberly and Kimberlin (7) investigated the catalytic properties of Al-deficient mordenite and found it to be considerably more active than conventional mordenite for cumene cracking. 

The propagation step, Eq. (4), is much slower than Eq. (3) as an example, its rate constant kp is 0.18 M 1 sec-1 for cumene at 303K. Values of kp can vary considerably for different substrates, as shown by the oxidation rates of substituted toluenes (8). With respect to toluene, taken as 1.0, the reactivity of 4-nitrotoluene toward ROO is 0.33 and that of / -xylene is 1.6. A homolytic process like the fission of the C-H bond should be essentially apolar, but data for substituted toluenes correctly suggest that the hydrogen radical abstraction is favored by electron-donor substituents and that in the transition state the carbon atom involved has a partial positive charge. The difference in kp between different molecules or different groups of the same molecule is the reason of the selectivity of autoxidation. 

The cracking of cumene has received considerable attention in recent years as a reaction typical of one class of cracking reactions, namely dealkylation of aromatics. Among the studies of cumene cracking found in the literature there are several attempts to determine the kinetics of... 

Figure 6.16 displays the temperature profile and liquid-phase molar fractions for cumene and DIPB. It may be observed that the temperature is practically constant over the reactive sections with a first plateau at 200 °C and a second one at 210 °C. The top temperature is at 198 °C while the bottom temperature climbs to 242 °C. The explanation may be found in the variation of concentrations for cumene and DIPB in the liquid phase. The maximum reaction rate takes place on the stages where propylene is injected. The cumene concentration increases rapidly and reaches a flat trend corresponding to the exhaustion of the propylene in liquid phase. It may be seen that the amount of DIPB increases considerably in the second reaction zone. This variation is very different from that with a cocurrent PFR. The above variations suggest that the productivity could be improved by providing several side-stream injections and/or optimizing the distribution of catalyst activity. 

Proof that radicals are formed by treatment of an alkyl halide with an alkyllithium was first presented in 1956 45 The degenerate exchange between BuBr and BuLi in cumene over an extended period was found to be accompanied not only by the formation of butane, butene and (in 43% yield) octane but also considerable quantities of the cumene dimer 32. 

The results can be divided into studies of the coke deposition rate as a function of the amount and type of reactants injected, and studies of the activity (cumene conversion) vs. the cumulative coke level Because of space considerations, only the former is discussed. 

The systems involving bromine atoms also show the fast drop-off as AH deviates from 0, especially marked in the toluene to cumene series, which implies a small value of the intrinsic barrier. There is a further problem that there is a substantial equilibrium isotope effect of about a factor of 26b accompanied in the aliphatic cases by considerable opportunity for reversibility, thus A H/fcD values in the endothermic attack of Br- on aliphatic RH are unlikely to be less than 2. Correspondingly, isotope effects in the reverse directions can be inverse. This has not been observed, but the trend shown in the last several reactions, in which isotope effects as low as 1.0 are observed suggests this possibility. We attach no meaning to (fcH/ D)max for abstraction from HBr because of the failure of the one-dimensional model. Both chlorine and bromine then fit the scheme of highly variable isotope effects associated with low intrinsic barriers. 

Unlike acetone, diethyl ketone cleaves well even in cumene 22>. 2-Pentanone triplet undergoes considerable a-cleavage in competition with rapid intramolecular hydrogen abstraction, as judged by the buildup of an efficient triplet quencher 23>. Biacetyl is the only likely candidate for that quencher and is the major product of a-cleavage of methyl ketones at temperatures low enough that decarbonylation of the acetyl radical is slow. Isopropyl, tert-butyl, and benzyl ketones all cleave quite efficiently and various reports have appeared on the CIDNP spectra of products derived from the radicals 24>. 

The nature of hydrogen abstraction process is normally indicated by the fact that the hydrogen-supplying solvent undergoes dehydrogenative coupling to a considerable extent but always in a poorer yield as the aniline formation would predict. Thermolysis of phenylazide 6 in p-xylene 7 affords aniline 8 a) and syn-di-p-tolylethane (P) 61>. Thermolysis of p-methoxyphenylazide (70) in cumene 11) gave the aniline 12 and bicumyl 13) 23>. 

Noncarbonium Ion Type Catalysis. Richardson (80) reported that cation-exchanged Y zeolites show considerable activity for the dehydrogenation of cumene to a-methylstyrene and that the activity parallels the cation strength to some extent. 

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