Benzene relative reaction rates

Let us illustrate this with the example of the bromination of monosubstituted benzene derivatives. Observations on the product distributions and relative reaction rates compared with unsubstituted benzene led chemists to conceive the notion of inductive and resonance effects that made it possible to explain" the experimental observations. On an even more quantitative basis, linear free energy relationships of the form of the Hammett equation allowed the estimation of relative rates. It has to be emphasized that inductive and resonance effects were conceived, not from theoretical calculations, but as constructs to order observations. The explanation" is built on analogy, not on any theoretical method. 

In a polluted or urban atmosphere, O formation by the CH oxidation mechanism is overshadowed by the oxidation of other VOCs. Seed OH can be produced from reactions 4 and 5, but the photodisassociation of carbonyls and nitrous acid [7782-77-6] HNO2, (formed from the reaction of OH + NO and other reactions) are also important sources of OH ia polluted environments. An imperfect, but useful, measure of the rate of O formation by VOC oxidation is the rate of the initial OH-VOC reaction, shown ia Table 4 relative to the OH-CH rate for some commonly occurring VOCs. Also given are the median VOC concentrations. Shown for comparison are the relative reaction rates for two VOC species that are emitted by vegetation isoprene and a-piuene. In general, internally bonded olefins are the most reactive, followed ia decreasiag order by terminally bonded olefins, multi alkyl aromatics, monoalkyl aromatics, C and higher paraffins, C2—C paraffins, benzene, acetylene, and ethane. 

In acid-catalyzed reactions of cyanides with arenes (Gattermann reaction), the yield of products and relative reactions rates are found to correlate with the acid strength of the media.20 The reaction of either NaCN or trimethylsilyl cyanide (TMS-CN) with benzene in superacid gave the benzaldimine, which upon hydrolysis yielded benzaldehyde (eq 25). The reactions were found to give reasonable yields (> 40%) of product only... 

Cyclohexene is relatively weakly bound to the ruthenium surface, as compared with the other reaction intermediates benzene and cyclohexadiene. It, therefore, can desorb before further hydrogenation. A measure of the selectivity to cyclohexene is the relative reaction rate, r , given by the expression ... 

Rather than attempting absolute rate determinations, the rates of the reactions were determined relative to the rates of benzene by competition experiments. The CH2ONO2 reactant ion was generated by electron bombardment at lU eV from ethyl nitrate and was allowed to react competitively at about 5 x 10 5 Torr with a mixture of two aromatic species, with the relative peak heights of the two possible product ions (corrected for mass discrimination) giving the relative reaction rates for two aromatic substrates. 

Table 2.2 Relative reaction rates in nitration of benzene derivatives...

Studies of electrophilic substitutions on arenes are reported in which the experimental conditions allow a direct comparison of the relative reaction rates. For example, the relative reactivities of benzene and toluene toward halogenation, acetylation, sulfonation, nitration, and methylation have been determined. In all cases, electrophilic aromatic substitution was more rapid with toluene. For example, bromina-tion of toluene is some 600 times faster than that of benzene. Such studies have led to the classification of substituents as ring activators or deactivators, depending on whether the substituted arene reacts faster or slower than benzene itself. Thus, the methyl group of toluene is a ring activator. 

Among the methods for evaluating electrophile strength, a useful approach involves comparing relative reaction rates with benzene and toluene [20]. More reactive electrophiles are expected to be less selective in competition reactions between the two arenes. As noted by Stock and Brown [21],... 

The involvement of x-complexes in S Ar reactions was first proposed by Dewar to explain relative reaction rates for some conversions [48]. For example, the relative stabilities of arene x-complexes (with HCl) have been shown to correlate with the relative rates of nitration (Table 1.3) [49]. The x-complex for m-xylene is estimated to be only about twice as stable as that for benzene. The relative rates of nitration for these two arenes are similar, suggesting a role of the x-complex in the rate-determining step of the nitration. In contrast, chlorination exhibits a markedly greater rate of reaction with m-xylene compared to benzene. This suggests that the rate-determining step for chlorination involves a transition state resembling the a-complex. Thus, the importance of x-complexes varies among different S Ar reactions. 

Aromatic compounds (eqs 16 and 17) are acylated by PhCOCI in the presence of a Lewis acid such as AICI3, TiCU, BF3, SnCU, ZnClz, or FeClz, or of a strong acid such as polyphos-phoric acid or CF3SO3H. Metallic A1 or Fe and iodine (in situ formation of a Lewis acid) can also act as a catalyst. Various solvents that have been used to perform this reaction are CSz, CH2CI2, 1,2-dichloroethane, nitrobenzene, and nitromethane. PhCOCI is less reactive than aliphatic carboxylic acid chlorides (with benzene in nitromethane the relative reaction rates are Ph-COCl MeCOCl = 6 100). As for all electrophilic substitutions, the rate and the regioselectivity of the acylation closely depend on the nature and on the position of the substituents on the aromatic system (eqs 16 and 18 ). The nature of the solvent can also exert a strong influence. ... 

Benzene rings have a dramatic effect on SnI reaction rates. This depends on the position of the ring relative to the leaving group. Consider the following reactions. 

Relative reactivity wiU vary with the temperature chosen for comparison unless the temperature coefficients are identical. For example, the rate ratio of ethoxy-dechlorination of 4-chloro- vs. 2-chloro-pyridine is 2.9 at the experimental temperature (120°) but is 40 at the reference temperature (20°) used for comparing the calculated values. The ratio of the rate of reaction of 2-chloro-pyridine with ethoxide ion to that of its reaction with 2-chloronitro-benzene is 35 at 90° and 90 at 20°. The activation energy determines the temperature coefficient which is the slope of the line relating the reaction rate and teniperature. Comparisons of reactivity will of course vary with temperature if the activation energies are different and the lines are not parallel. The increase in the reaction rate with temperature will be greater the higher the activation energy. 

At 0.9 °C the rate of bromination of biphenyl relative to benzene was approximately 1,270, compared to 26.9 in the presence of mineral acid, and this latter value is fairly close to that obtained with 50 % aqueous dioxan. The possibility that the positive brominating species might be protonated bromine acetate, AcOHBr+, was considered a likely one since the reaction rate is faster in aqueous acetic acid than in water, but this latter effect might be an environmental one since bromination by acidified hypobromous acid is slower in 50 % aqueous dioxan than in... 

Not only are there substrates for which the treatment is poor, but it also fails with very powerful electrophiles this is why it is necessary to postulate the encounter complex mentioned on page 680. For example, relative rates of nitration of p-xylene, 1,2,4-trimethylbenzene, and 1,2,3,5-tetramethylbenzene were 1.0, 3.7, and 6.4, though the extra methyl groups should enhance the rates much more (p-xylene itself reacted 295 times faster than benzene). The explanation is that with powerful electrophiles the reaction rate is so rapid (reaction taking place at virtually every encounter between an electrophile and substrate molecule) that the presence of additional activating groups can no longer increase the rate. ... 

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