Aromatic compounds, hydroxylation

Infrared The IR spectra of phenols combine features of those of alcohols and aromatic compounds. Hydroxyl absorbances resulting from O—H stretching are found in the 3600-cm region, and the peak due to C—O stretching appears around 1200-1250 cm . These features can be seen in the IR spectrum of p-cresol, shown in Figure 24.3. 

Stereocontrolled oxidation of aromatic compounds (hydroxylation or dihy-... 

Whatever the routes of formation, these oxidizing species, and in particular OH radicals, are known to react rapidly with most organic solutes. In aromatic compounds, hydroxylation of aromatic moiety occurs and successive oxidation / addition steps lead to ring opening. Resulting aldehydes and carboxylic acids (formed also in the reactions with aliphatics) are further decarboxylated, and finally produce CO2. 

The aromatic core or framework of many aromatic compounds is relatively resistant to alkylperoxy radicals and inert under the usual autoxidation conditions (2). Consequentiy, even somewhat exotic aromatic acids are resistant to further oxidation this makes it possible to consider alkylaromatic LPO as a selective means of producing fine chemicals (206). Such products may include multifimctional aromatic acids, acids with fused rings, acids with rings linked by carbon—carbon bonds, or through ether, carbonyl, or other linkages (279—287). The products may even be phenoUc if the phenoUc hydroxyl is first esterified (288,289). 

Nitrations are highly exothermic, ie, ca 126 kj/mol (30 kcal/mol). However, the heat of reaction varies with the hydrocarbon that is nitrated. The mechanism of a nitration depends on the reactants and the operating conditions. The reactions usually are either ionic or free-radical. Ionic nitrations are commonly used for aromatics many heterocycHcs hydroxyl compounds, eg, simple alcohols, glycols, glycerol, and cellulose and amines. Nitration of paraffins, cycloparaffins, and olefins frequentiy involves a free-radical reaction. Aromatic compounds and other hydrocarbons sometimes can be nitrated by free-radical reactions, but generally such reactions are less successful. 

A chlorohydrin has been defined (1) as a compound containing both chloio and hydroxyl radicals, and chlorohydrins have been described as compounds having the chloro and the hydroxyl groups on adjacent carbon atoms (2). Common usage of the term appHes to aUphatic compounds and does not include aromatic compounds. Chlorohydrins are most easily prepared by the reaction of an alkene with chlorine and water, though other methods of preparation ate possible. The principal use of chlorohydrins has been as intermediates in the production of various oxitane compounds through dehydrochlorination. 

Peroxytnfluoroacetic acid is used tor numerous oxidations of saturated hydrocarbons and aromatic compounds It oxidizes alkanes, alkanols, and carboxylic acids with formation of hydroxylation products [29] Oxidation of cyclohexane with peroxytnfluoroacetic acid proceeds at room temperature and leads to cyclohexyl trifluoroacetate in 75% yield, 1-octanol under similar conditions gives a mixture of isomeric octanediols in 59% yield, and palmitic acid gives a mixture of hydroxypalmitic acids in 70% yield [29]... 

The most commonly employed reagent for the hydroxylation of aromatic compounds is that consisting of ferrous ion and hydrogen peroxide. The suggestion that hydroxyl radicals are intermediates in this reaction was first made by Haber and Weiss, who proposed the following radical-chain mechanism for the process ... 

Udenfriend et al. observed that aromatic compounds are hydroxyl-ated by a system consisting of ferrous ion, EDTA, ascorbic acid, and oxygend Aromatic and heteroaroinatic compounds are hydroxylated at the positions which are normally most reactive in electrophilic substitutions. For example, acetanilide gives rise exclusively to the o-and p-hydroxy isomers whereas quinoline gives the 3-hydroxy prod-uct. - The products of the reaction of this system w ith heterocyclic compounds are shown in Table XIII. 

Hydrazinopyridazines such as hydralazine have a venerable history as anti hypertensive agents. It is of note that this biological activity is maintained in the face of major modifications in the heterocyclic nucleus. The key intermediate keto ester in principle can be obtained by alkylation of the anion of pi peri done 44 with ethyl bromo-acetate. The cyclic acylhydrazone formed on reaction with hydrazine (46) is then oxidized to give the aromatized compound 47. The hydroxyl group is then transformed to chloro by treatment with phosphorus oxychloride (48). Displacement of halogen with hydrazine leads to the formation of endralazine (49). ... 

Aromatic compounds can be hydroxylated, benzoxylated and acetoxylated and the isomer distributions and substituent effects indicate that the reaction is an electrophilic substitution132. Very little kinetic work has been carried out so that the nature of the electrophile is in some doubt. 

One 7i-bond of an aromatic ring can be converted to a cyclohexadiene 1,2-diol by reaction with enzymes associated with P. putida A variety of substituted aromatic compounds can be oxidized, including bromobenzene, chlorobenzene, " and toluene. In these latter cases, introduction of the hydroxyl groups generates a chiral molecule that can be used as a template for asymmetric syntheses. " ... 

Rieske-type clusters are found in aromatic-ring hydroxylating dioxygenase systems (20). These enzymes catalyze the conversion of different aromatic compounds into cis-arene diols ... 

Phenol is the starting material for numerous intermediates and finished products. About 90% of the worldwide production of phenol is by Hock process (cumene oxidation process) and the rest by toluene oxidation process. Both the commercial processes for phenol production are multi step processes and thereby inherently unclean [1]. Therefore, there is need for a cleaner production method for phenol, which is economically and environmentally viable. There is great interest amongst researchers to develop a new method for the synthesis of phenol in a one step process [2]. Activated carbon materials, which have large surface areas, have been used as adsorbents, catalysts and catalyst supports [3,4], Activated carbons also have favorable hydrophobicity/ hydrophilicity, which make them suitable for the benzene hydroxylation. Transition metals have been widely used as catalytically active materials for the oxidation/hydroxylation of various aromatic compounds. 

Nishioka MG, CC Howard, DA Conros, LM Ball (1988) Detection of hydroxylated nitro aromatic and hydroxy-lated nitro polycyclic aromatic compounds in ambient air particulate extract using bioassay-directed fractionation. Environ Sci Technol 22 908-915. 

Nadeau LJ, Z He, JC Spain (2003) Bacterial conversion of hydroxylamino aromatic compounds by both lyase and mutase enzymes involves intramolecular transfer of hydroxyl groups. Appl Environ Microbiol 69 2786-2793. 

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