Benzene Reaction Rate

Benzene is a reactant in the BTX system. There are two possible avenues for how it may be converted into a product. The first occurs when benzene reacts with ethylene to form toluene by reaction 1. The rate of benzene conversion in this reaction is thus written with respect to both reactants, which is given by 

Note that the term is negative indicating that benzene is consumed, kj is the rate constant for reaction 1 and Cg and Cg are the concentrations of benzene and ethylene, respectively. The subscript number 1, associated with r j, signifies that this expression relates to reaction 1. Note there is no special significance to the form of the rate expression given here. The rate of reaction for benzene might assume a number of different forms. By mass balance, one mole of benzene and half a mole of ethylene react to form one mole of toluene. 

It is also possible for benzene to simultaneously decompose, in an autocatalytic reaction, given by reaction 3. For every two moles of benzene that react by reaction 3, one mole of diphenyl and hydrogen are produced. The rate of reaction for benzene is again expressed in terms of the reactants that participate in this reaction. In this instance, benzene is the sole reactant, and the following rate expression for this reaction is obtained as a result  

Compared to reaction 1, reaction 3 is second order overall. 

The overall reaction order may be obtained by summing the exponents in the rate expression. Reaction 1 has order of 1.5, whereas reaction 3 is second order overall. 

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There are several kinetic and product studies on Cl atom gas phase reactions of hydrocarbons [16-42], Though the various kinetic studies show more consistency on Cl-I-toluene reaction rates [41], the reactions of Cl- -benzene has been shown to be quite contradictory. Indeed, six existing published studies on the Cl -l- benzene reaction rates vary by more than 5 orders of magnitudes, ranging from 1.3 x 10 to 1.5 X 10 cmV(molecules) [16,17,19,24,27,42],... 

Kinetic reaction rate constants increase with the number of ethyl groups alkylated on the benzene ring. For example, the relative rate constant for alkylation of EB is roughly twice that for the alkylation of benzene. Reaction rate constants continue to increase with each successive alkylation reaction until a limitation is reached, such as steric hindrance. The formation of penta-EB and hexa-EB proceeds very slowly for this and other reasons so that only trace quantities are formed. 

This expression is the negative complement of the benzene reaction rate for reaction 1. 

A point in case is provided by the bromination of various monosubstituted benzene derivatives it was realized that substituents with atoms carrying free electron pairs bonded directly to the benzene ring (OH, NH2, etc) gave 0- and p-substituted benzene derivatives. Furthermore, in all cases except of the halogen atoms the reaction rates were higher than with unsubstituted benzene. On the other hand, substituents with double bonds in conjugation with the benzene ring (NO2, CHO, etc.) decreased reaction rates and provided m-substituted benzene derivatives. 

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. 

The reaction rate is increased by using an entraining agent such as hexane, benzene, toluene, or cyclohexane, depending on the reactant alcohol, to remove the water formed. The concentration of water in the reaction medium can be measured, either by means of the Kad-Eischer reagent, or automatically by specific conductance and used as a control of the rate. The specific electrical conductance of acetic acid containing small amounts of water is given in Table 6. 

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. 

The PMBs, when treated with electrophilic reagents, show much higher reaction rates than the five lower molecular weight homologues (benzene, toluene, (9-, m- and -xylene), because the benzene nucleus is highly activated by the attached methyl groups (Table 2). The PMBs have reaction rates for electrophilic substitution ranging from 7.6 times faster (sulfonylation of durene) to ca 607,000 times faster (nuclear chlorination of durene) than benzene. With rare exception, the PMBs react faster than toluene and the three isomeric dimethylbenzenes (xylenes). 

The reaction rates of toluene and benzene with i-propyl chloride in nitromethane fit a third-order rate law ... 

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. 

Recently, a kinetic study has been made of the substitution of diazotised sulphanilic acid in the 2 position of 4-substituted phenols under first-order conditions (phenol in excess) in aqueous buffer solutions at 0 °C131a. A rough Hammett correlation existed between reaction rates and am values, with p about -3.8 however, the point for the methoxy substituent deviated by two orders of magnitude and no explanation was available for this. The unexpectedly low p-factor was attributed to the high reactivities of the aromatic substrates, so that the transition state would be nearer to the ground state than for reaction of monosubstituted benzene derivatives. 

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... 

Kinetic studies at 25 °C showed that for benzene, toluene, o-, m-, and p-xylene, /-butylbenzene, mesitylene, 4-chloroanisole, and p-anisic acid in 51 and 75 % aqueous acetic acid addition of small amounts of perchloric acid had only a slight effect on the reaction rate which followed equation (100). At higher concentrations of perchloric acid (up to 0.4 M) the rate rose linearly with acid concentration, and more rapidly thereafter so that the kinetic form in high acid concentration was... 

Addition of up to a tenfold molar excess of hydrogen chloride did not appreciably alter the reaction rate. Orton and Bradfield227 obtained the same kinetic form for the chlorination of formanilide, acetanilide, benzanilide, and benzene-sulphonanilide in 99 % aqueous acetic acid at 20 °C reaction rates were higher than previously obtained with the less aqueous medium, and this medium effect has been subsequently found to be general. 

The most valuable and comprehensive kinetic studies of alkylation have been carried out by Brown et al. The first of these studies concerned benzylation of aromatics with 3,4-dichloro- and 4-nitro-benzyl chlorides (these being chosen to give convenient reaction rates) with catalysis by aluminium chloride in nitrobenzene solvent340. Reactions were complicated by dialkylation which was especially troublesome at low aromatic concentrations, but it proved possible to obtain approximately third-order kinetics, the process being first-order in halide and catalyst and roughly first-order in aromatic this is shown by the data relating to alkylation of benzene given in Table 77, where the first-order rate coefficients k1 are calculated with respect to the concentration of alkyl chloride and the second-order coefficients k2 are calculated with respect to the products of the... 

The lower reaction rates obtained with this catalyst permitted measurements of the reaction rates of benzene and toluene with a range of alkyl halides including /-propyl and /-butyl bromides, the rate being followed in some cases by the... 

Finally, rates of mercuration have been measured using mercuric trifluoro-acetate in trifluoroacetic acid at 25 °C450. The kinetics were pure second-order, with no reaction of the salt with the solvent and no isomerisation of the reaction products rate coefficients (10 k2) are as follows benzene, 2.85 toluene, 28.2 ethylbenzene, 24.4 i-propylbenzene, 21.1 t-butylbenzene, 17.2 fluorobenzene, 0.818 chlorobenzene, 0.134 bromobenzene, 0.113. The results follow the pattern noted above in that the reaction rates are much higher (e.g. for benzene, 690,000 times faster than for mercuration with mercuric acetate in acetic acid) yet the p factor is larger (-5.7) if the pattern is followed fully, one could expect a larger... 

The synthesis of chaparrinone and other quassinoids (naturally occurring substances with antileukemic activity) is another striking example [16a-c]. The key step of synthesis was the Diels-Alder reaction between the a,/l-unsaturated ketoaldehyde 1 (Scheme 6.1) with ethyl 4-methyl-3,5-hexadienoate 2 (R = Et). In benzene, the exo adduct is prevalent but it does not have the desired stereochemistry at C-14. In water, the reaction rate nearly doubles and both the reaction yield and the endo adduct increase considerably. By using the diene acid 2 (R = H) the reaction in water is 10 times faster than in organic solvent and the diastereoselectivity and the yield are satisfactory. The best result was obtained with diene sodium carboxylate 2 (R = Na) when the reaction is conducted 2m in diene the reaction is complete in 5h and the endo adduct is 75% of the diaster-eoisomeric reaction mixture. 

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. ... 

Nametkin and co-workers hrst reported the alkylation of benzene derivatives with allylchlorosilanes in the presence of aluminum chloride as catalyst. " 2-(Aryl)propylsilanes were obtained from the alkylation of substituted benzenes (Ph—X X = H, CL Br) with allylsilanes such as allyldichlorosilane and allyltrichlo-rosilane.The yields ranged from 34 to 66% depending upon the substituents on the benzene ring, but information concerning reaction rates and product isomer distribution was not reported. 

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