Alkylaromatic hydrocarbons, reaction

The scope of oxidation chemistry is enormous and embraces a wide range of reactions and processes. This article provides a brief introduction to the homogeneous free-radical oxidations of paraffinic and alkylaromatic hydrocarbons. Heterogeneous catalysis, biochemical and hiomimetic oxidations, oxidations of unsaturates, anodic oxidations, etc, even if used to illustrate specific points, are arbitrarily outside the purview of this article. There are, even so, many unifying features among these areas. 

Opeida proposed the following empirical equation for the rate constant of the peroxyl radical reaction with C—H bonds of alkylaromatic hydrocarbons [34] ... 

VF Tsepalov. The Study of Elementary Reactions of Liquid-Phase Oxidation of Alkylaromatic Hydrocarbons. Doctoral Thesis Dissertation, Institute of Chemical Physics, Chernogolovka, 1975, pp 3-40 [in Russian],... 

As in the case of the reaction of hydroperoxide with the rr-bond of the olefin, the reaction of ROOH with the rr-bond of the aromatic ring occurs more rapidly than the attack of ROOH on the C—H bond of alkylaromatic hydrocarbon. 

In addition, phenols are formed by the reaction of hydroxyl radical addition to the aromatic ring of oxidized alkylaromatic hydrocarbon [56]. 

E0 = 40 kJ mol-1 at AH=0) is substituted by a few consecutive fast reactions with electron transfer. Russel [284-291] studied a few reactions of oxidation of alkylaromatic hydrocarbons in the presence of strong bases. He proved the chain mechanisms of these reactions. One of them includes a few stages with addition of dioxygen to carbanion. Another includes the electron transfer from carbanion to dioxygen. 

The side chain alkylation of alkyl benzenes is usually performed with alkenes as alkylating agents in the presence of strong-acid catalysts (256,257). The use of highly basic catalysts such as alkali metals, their hydrides, and sodium and potassium complexes for alkylation of alkylaromatic hydrocarbons has also been reported (256,257). The reaction mechanism proposed by Pines et al. (258) involves the addition of a benzylic carboanion to the alkene (Scheme 41). 

The increase in the electron acceptor concentration in a solution transforms the reaction of fluorene oxidation into regime when the first stage (deprotonation of fluorene) limits the chain process. For the oxidation of other alkylaromatic hydrocarbons, see Refs. [292 304]. 

Autoxidation of alkylaromatic hydrocarbons has been studied exten-- sively in recent years (1, 2, 3,11,12 13), and the mechanism (Reactions 1-4) is now well understood. 

The cyclic sequence of Reactions 5 and 6 can be accelerated by adding a species which promotes the formation of Co(III) from Co(II). In the oxidation of alkylaromatic hydrocarbons ozone (7), acetaldehyde (8), and methyl ethyl ketone (4) act as promoters in this way. 

Catalytic cyclization is a very important reaction, both commercially and theoretically. Here we review metal-catalyzed cyclization of alkylaromatic hydrocarbons, and we present reaction mechanisms that govern these reactions. 

The reaction mechanism for a very strong one-electron donor centre in the dehydrogenation of alkylaromatic hydrocarbons is similar to that proposed by Krause for ethylbenzene dehydrogenation [reactions (5) and (6)]. The mechanism for n-propylbenzene transalkylation and cyclization on the radical pathway has been suggested. ... 

Partial catalytic oxidation of alkylaromatic hydrocarbons is interesting both from the industrial and the scientific point of view. The industrial interest is due to the availability of these substances from the petrochemical industry and to a number of applications for the possible oxidation products. Conventional gas phase oxidation concerns the side chain and leads mainly to benzoic acid or even to destruction of the aromatic ring. Oxidation under mild conditions could cease the reaction at earlier stages and reduce the number of the products formed. However, the appropriate catalyst for such partial oxidation has not been found yet. 

Scheme 1. Expected reaction routes to the partial oxidation of alkylaromatic hydrocarbons.

Benzene introduced in reaction leads to appreciable activation of propane. As a result of the conversion of these mixtures, isopropylbenzene (IPB), propylene, alkylaromatic hydrocarbons (ArH) and hydrogen are formed. 

Using the combination of molecular oxygen, acetaldehyde and N-hydroxyphthalimide (the last compounds is a component - in addition to a metal complex - of the catalyst in the Ishii oxidation reaction), it is possible to oxidize alkylaromatic hydrocarbons and cycloalkanes in acetonitrile at room temperature [40a]. The reaction occurs via a radical mechanism depicted in Scheme II.5. 

The oxidation of alkylaromatic hydrocarbons proceeds particularly easily in the presence of both cobalt and bromide ions (a so-called cobalt-bromide catalysis ). Carboxylic acids are the final products of the reaction. For example, terephthalic acid is selectively formed from p-xylene, the whole process being used in the industrial production of the acid [Ik, 19]. Despite the large number of works on cobalt-bromide catalysis, its mechanism has long remained speculative. 

Water-soluble macromolecular metal complexes based on terminally functionalized ethylene oxides and ethylene oxide-propylene oxide block copolymers have been used as catalysts for hydroformylation, hydrogenation, Wacker oxidation of imsaturated compounds, hydroxylation of aromatic compounds, oxidation of saturated and alkylaromatic hydrocarbons, metathesis, Heck reaction, and some asymmetric reactions. 

Without going into detail we simply note that oxidation of alkylaromatic hydrocarbons with polymer-immobilized catalysts can be considered as a system of successive-parallel reactions including mononuclearic hydroxylation to phenols, oxidation of phenols, oxidative dehydrogenation of hydroquinones, oxidative decomposition of quinones and thermal and catalytic decomposition of peracids. 

The determination of peroxides has two goals one is to monitor peroxide concentration used as initiator and catalysts and the other is to detect formation of hazardous peroxides formed as autoxidation products in ethers, acetals, dienes, and alkylaromatic hydrocarbons. A sample is dissolved in a mixture of acetic acid and chloroform. The solution is deaerated and potassium iodide reagent is added and let to react for 1 h in darkness. The iodine formed in reaction is measured by absorbance at 470 nm and result calculated to active oxygen in the sample. The method can determine hydroperoxides, peroxides, peresters, and ketone peroxides. Oxidizing and reducing agents interfere with flic determination. 

A complete mechanism for the autoxidation of alkylaromatic hydrocarbons by cobalt(n) in acetic acid has not been established,25 6 although a complex rate law has been determined for tetralin. 22 The reaction most likely proceeds by a fiiee radical chain mechanism in which the purpose of the cobalt ions is to provide a hi h steady state concentration of free radicals by catalysis of the decomposition of THP. The free radical nature of the autoxidation of tettalin with the colloidal CoPy catalysts is supported by experiments which showed inhibition of the reaction by 2,6-di-rerr-butylphenol and 2,6-di-rm-butyl-4-methylphenol, and by a shortening of the induction period and increase of the reaction rate when azobis(isobut nitrile) was added to the reaction mixture as a free radical initiator. 

FC Knopf, T-H Pang, KM Dooley. Catalyzed reactions of alkylaromatic hydrocarbons dissolved in supercritical fluids. Prepr— ACS Div Fuel Chem 32(3) 359-367, 1987. 

The above examples are mostly related to either acidic forms of zeolite-like materials or supported metals and complexes. The basic forms of zeolites are known to exhibit interesting properties in side chain alkylation of alkylaromatic hydrocarbons as well as in diverse condensation reactions (Knoevenagel, Michael, etc.). One example was given in the paper by Corma et al. [163] related to the condensation of benzaldehyde with diethyl malonate on... 

In most cases, this is precisely the reaction that limits chain propagation and determines the oxidation rate. Since the strength of the O—bond in hydroperoxide is independent of the structure of the alkyl substituent R and even of the replaconent of R by H, then reaction (2) is exothermic for hydrocarbons with Z)r h < roo— 365 kJ/mol (olefins, alkylaromatic hydrocarbons) and endothermic for hydrocarbons with )r h > 365 kJ/mol (paraffinic and naphthenic hydrocarbons). The activation energy of this reaction is related by a linear correlation to At-n... 

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