Kinetics benzene alkylation

Relative rates of alkylation of toluene and benzene using a mixture of nitro-sonium hexafluorophosphate, nitromethane (or acetonitrile) and aliphatic amine as the alkylations agent have been determined at 25 °C as follows360 1.5 (ethyl-amine), 2.5 (i-propylamine) and 3.5 (benzylamine) nothing more as yet is known about the kinetics of alkylation with these new alkylating reagents. 

Friedel-Crafts alkylation of benzene,220 221 toluene,222para-xylene,220 and naphthalene223 with benzyl alcohols have been studied over Nafion-silica nano-composite catalysts, including the kinetics of alkylation.221,223 In most cases, 13% Nafion-silica showed the highest activity, testifying again to the much higher accessibility of the active sites. Complete conversion of para-xylene was found in the presence of triflic acid, and it was the only reaction when ether formation as side reaction did not occur. 

The phenomenological Horiuti Boreskov Onsager equations allow in some cases a first approximation to be made for the kinetic description of catalytic transformations in systems that involve numerous parallel trans formation channels. Consider how these equations can be applied with the process of benzene alkylation with ethylene as an example. 

There are relatively few kinetic data on the Friedel-Crafts reaction. Alkylation of benzene or toluene with methyl bromide or ethyl bromide with gallium bromide as catalyst is first-order in each reactant and in catalyst. With aluminum bromide as catalyst, the rate of reaction changes with time, apparently because of heterogeneity of the reaction mixture. The initial rate data fit the kinetic expression ... 

The next and only other major kinetic study was carried out by Jensen and Brown184, who used aluminium chloride as catalyst, nitrobenzene as solvent, benzene- and p-toluene-sulphonyl chlorides as sulphonylating agents and benzene, chlorobenzene, alkyl- and polyalkylbenzenes as aromatic substrates184. 

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... 

Quite different kinetics are exhibited by the anaerobic oxidation of alkyl-benzenes by cobaltic acetate in a 95 % acetic acid medium , viz. 

As in the alkylation reaction, the reactive intermediate in Friedel-Crafts acylation can be a dissociated acylium ion or a complex of the acid chloride and Lewis acyl.49 Recent mechanistic studies have indicated that with benzene and slightly deactivated derivatives, it is the protonated acylium ion that is the kinetically dominant electrophile.50... 

In all that has gone before a tacit assumption has been made that the proportions of alternative products formed in a reaction, e.g. o-, m- and p-isomers, are determined by their relative rates of formation, i.e. that the control is kinetic (p. 42). This is not, however, always what is observed in practice thus in the Friedel-Crafts alkylation of methyl-benzene (Me o-/p-directing) with benzyl bromide and GaBr3 (as Lewis acid catalyst) at 25°, the isomer distribution is found to be ... 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

The effect of crown ethers on the alkylation of sodium diethyl n-butylmalonate by 1-bromobutane has been studied by Zaug et al. (1972). The absence of a common-ion rate depression in dimethylformamide (DMF) pointed to an ion pair being the kinetically active species. The addition of dicyclohexyl-18-crown-6 (a mixture of [20] and [21]) accelerates the alkylation in both benzene and tetrahydrofuran (THF) (Table 24). The rates reach a plateau, indicating that at a crown-ether concentration of 0.5 M the ion pair is fully converted to the crown ether-separated ion pair which is slightly less reactive than the uncomplexed ion pair in DMF. The rate constant in pure dimethoxyethane (DME) is equal to that observed in THF or benzene... 

Mixed coupling between naphthalene and alkyl benzenes has also been demonstrated (Table 10, numbers 10-13). The relative yield of mixed coupling products increases with the basicity of the alkyl benzene with mesitylene 19%, with tetramethylbenzene 42%, and with pen-tamethylbenzene 64%. This suggests an electrophilic reaction between naphthalene cation radicals and alkylbenzenes. The mixed coupling reaction of phenan-threne with anisole has been studied kinetically [163]. 

The form of potential energy curve deduced by Olah from kinetic evidence on the nitration of benzene, and some alkyl- and halo-benzenes, by nitronium ions derived from NOJ BIV is shown in Fig. 18. In this diagram, position D is associated with a localized structure analogous to that of Fig. 16 and 19b. 

Corma, A., Martinez-Soria, V., and Schnoeveld, E. (2000) Alkylation of benzene with short-chain olefins over MCM-22 zeolite catalytic behaviour and kinetic mechanism, f Catal, 192, 163-173. 

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