Reactivity of alkylbenzenes

Table 1.14. Effect of substituents (nature and number) on the reactivity of alkylbenzenes over Pt/Al203. Reaction conditions see Table 1.11.

The reactivity of dibenzyl is similar to those of alkylbenzenes, and it is therefore most probable that the nitrations of the latter substances were also influenced by mixing. ... 

Kinetic data are available for the nitration of a series of p-alkylphenyl trimethylammonium ions over a range of acidities in sulphuric acid. - The following table shows how p-methyl and p-tert-h xty augment the reactivity of the position ortho to them. Comparison with table 9.1 shows how very much more powerfully both the methyl and the tert-butyl group assist substitution into these strongly deactivated cations than they do at the o-positions in toluene and ferf-butylbenzene. Analysis of these results, and comparison with those for chlorination and bromination, shows that even in these highly deactivated cations, as in the nitration of alkylbenzenes ( 9.1.1), the alkyl groups still release electrons in the inductive order. In view of the comparisons just... 

Formation of AllylEsters. AEyl esters are formed by reaction of aEyl chloride with sodium salts of appropriate acids under conditions of controEed pH. Esters of the lower alkanoic, alkenoic, alkanedioic, cycloalkanoic, benzenecarboxyEc, alkylbenzene carboxyEc, and aromatic dicarboxyEc acids maybe prepared in this manner (25). More information can be found about the reactivity of aEyl compounds (see Allyl alcohol and mono allyl derivatives). 

In this work the relative reactivities of the alkylbenzenes (PhR) varied as follows (R = ) Me, 100 Et, 76 i-Pr, 44 f-Bu, 23. This relates to 85 % aqueous acetic acid at 24 °C and with 88 % acid at 25 °C the same relative rate of toluene to /-butylbenzene was obtained261, showing the hyperconjugative order to prevail for this reaction. 

Also other Type B and C series from Table II are consistent with the above elimination mechanisms. The dehydration rate of the alcohols ROH on an acid clay (series 16) increased with the calculated inductive effect of the group R. For the dehydrochlorination of polychloroethanes on basic catalysts (series 20), the rate could be correlated with a quantum-chemical reactivity index, namely the delocalizability of the hydrogen atoms by a nucleophilic attack similar indices for a radical or electrophilic attack on the chlorine atoms did not fit the data. The rates of alkylbenzene cracking on silica-alumina catalysts have been correlated with the enthalpies of formation of the corresponding alkylcarbonium ions (series 24). Similar correlations have been obtained for the dehydrosulfidation of alkanethiols and dialkyl sulfides on silica-alumina (series 36 and 37) in these cases, correlation by the Taft equation is also possible. The rate of cracking of 1,1-diarylethanes increased with the increasing basicity of the reactants (series 33). 

A large number of papers has been devoted to the influence of substituents upon the reactivity of benzene nucleus. Extensive studies concerning various benzene derivatives and catalysts from the platinum group metals have been published by H. A. Smith and his co-workers (for a summary see 36). The most consistent sets of data on alkylbenzenes are available from him and other groups of authors. Table VI summarizes the influence of the structure of a single alkyl group Table VII (94, 95, 97-103) summarizes the influence of the number and position of the methyl groups. Both series show very similar behavior on all metal catalysts, a decrease in rate with the size... 

This classification of reactivity can be compared with the values of kp obtained from the apparent oxidizabilities and from the termination constants (Table I). For ethers, the order of reactivity above is the same as the order of kp. Similarly, the rates of oxidation of alkylbenzenes (11, 12) lead to the following classification. 

Disproportionation (transalkylation) and positional isomerization usually take place simultaneously when either linear or branched alkylbenzenes are treated with conventional Friedel-Crafts catalysts or with Nafion-H. The reactivity of alkyl groups to participate in transalkylation increases in the order ethyl, propyl < isopropyl < tert-butyl.117 207 217... 

Alkylnaphthalenes are more reactive than alkylbenzenes and react at temperatures as low as 90°C. Sodium is a selective catalyst for side-chain monoalkylation. More elevated temperatures and prolonged reaction times allow the replacement in ethylation of all a-hydrogen atoms of alkylnaphthalenes with high selectivity.238... 

The photocatalytic oxidation of alkylbenzenes and alkenylbenzenes has been widely reported. The data concerning alkylbenzenes have shown that the reactivity of toluenes is low when compared to other monosubstituted benzenes (Somarani et al., 1995). The effect of adding a zeolite, which is an acid solid catalyst, by Ti02/UV on various 4-substituted toluenes was studied by Somarani et al. (1995). The compounds of interest were prepared at 0.03 M in solutions containing TiOz. The effect of a zeolite was also studied by adding HY-type zeolites with various Si/Al ratios. The solutions were irradiated with a 125-W mercury lamp emitting light at 330 nm. Samples were taken at 48 hours and analyzed by GC/mass spectroscopy (MS) to determine percent conversions of the toluenes to the desired products. 

In contrast to oxidations with Mn(III) acetate, the oxidation of alkylbenzenes by the stronger oxidant, Co(III) acetate, appears to involve only electron transfer. No competition from classical free radical pathways is apparent. Waters and co-workers,239,240 studied the oxidation of a series of alkylbenzenes by Co(III) perchlorate in aqueous acetonitrile. They observed a correlation between the reactivity of the arene and the ionization potential of the hydrocarbon which was compatible with the formation of radical cations in an electron transfer process. 

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