Oxidation side reaction

Replacement of methionine by norleucine is a common way to eliminate unwanted side reactions (oxidation and alkylation) that can be observed especially when using a Boc protection strategy. Owing to the similarity of Met and Nle side chains, the exchange usually has no significant influence on the structure of peptides. 

Among the best-known nonderivatizing solvent systems is a combination between copper, alkali, and ammonia termed Schweizer s reagent. Solutions of cuprammonium hydroxide have been used for both analytical and industrial cellulose dissolution. Regenerated fibers with silk-like appearance and dialysis membrane have been (and partially continue to be) industrial products on the basis of cellulose dissolution in cuprammonium hydroxide. The success of this solvent is based on the ability of copper and ammonia to complex with the glycol functionality of cellulose as shown inO Fig. 11. Because of the potential side reactions (oxidation and crosslinking, Norman compound formation), alternatives to both ammonia as well as copper have been developed. Cuen and cadoxen are related formulations based on the use of ethylene diamine and cadmium, respectively. The various combinations of alkali, ammonia. 

Side reactions. Oxidative cyclization of unsaturated y9-dicarbonyl compounds that have two a-hydrogens will give products that still have one a-hydrogen and can be oxidized further. If the product is oxidized at a rate competitive with that of the starting material, mixtures of products will be obtained. For instance, oxidative cyclization of 27 affords 36% of 28 and 10% of dienone 30 formed by further eno-lization and oxidation of 28 to radical 29 and then to dienone 30 (Scheme 8) [16]. Competitive oxidation of the product is usually not a problem in intermolecular addition reactions because a vast excess of the oxidizable substrate, such as acetone... 

A second problem with Sarett oxidation is the difficulty in isolating the products from a pyridine solution. An advantage of the technique, as mentioned above, is that alkenes, ketals, sulfides, and tetrahydropyranyl ethers are oxidized much slower than alcohols and rarely give competitive side reactions. Oxidation of secondary alcohols proceeds in good yield, but oxidation of primary aliphatic alcohols often gives low yields of the aldehyde. 2 Benzylic and allylic alcohols give good yields, however. 

When liquid hydrogen fluoride is used as catalyst, alkylation may be accomplished in satisfactory yield (except with ethylene) at room temperatures. Higher temperatures can be used with hydrogen fluoride than with sulfuric acid because it is only with the latter that a side reaction, oxidation of the alkene, must be avoided. 

Carbon atoms of neutral polysaccharides may be converted into carboxyl groups by oxidation with dinitrogen tetraoxide, and the mechanism, at least for cellulose, possibly involves an intermonomeric contribution to the transition state by adjacent C-6 groups. Cellulose " " and ivory-nut mannan have been oxidized in this way, and, in both products, keto groups are formed at secondary carbon atoms in side reactions. Oxidation with dinitrogen tetraoxide has also been employed in the pro-... 

Direct preparation of an aziridine from an alkene is possible by reaction of the alkene with a nitrene or metal nitrenoid species. Nitrenes can be generated thermally or photochemically from azides, although their reaction with alkenes to give aziridines is often low yielding and is complicated by side reactions. Oxidation of iV-amino-phthalimide or related hydrazine compounds (e.g. with Pb(OAc>4 or by electrolysis) and reaction with an alkene has found some generality. The metal-catalysed reaction of nitrenes with alkenes has received considerable study. A variety of metal catalysts can be used, with copper(II) salts being the most popular. For example, styrene was converted to its A-tosyl aziridine 72 by reaction with [A-(tosyl)imino]phenyliodinane (PhI=NTs) and copper(II) triflate (5.75). ... 

Reactions Equation (2) is a simplified description of the passivation reaction. In fact, however, the phenomenon of passivity depends on various reactions of the substrate, the oxide film and side reactions. Oxide growth, corrosion, redox processes of the oxide, and redox reactions of species in the electrolyte have to be distinguished. 

As reported above, the autooxidation is a quite general and important reaction. Beside the co-oxidation with olefins, which may be an undesired side reaction, oxidation of thiols by molecular oxygen represents a simple method of transforming these unstable compounds characterized by a... 

Spin trapping Aqueous/ solid Short-lived radicals nuy be studied. Possibility for assignment of radicals. Only a relative measure of radicals. Introduction of foreign substances with a risk of side reactions (oxidation, reduction). Verification by different spin traps or alternative analytical methods needed for identification. 

Among the problems associated with oxidative polymerization is the abundance of possible side reactions. Oxidation of the simple, unsubstituted heterocycles results in poorly soluble, poorly processable materials so structural... 

This is a way to do this procedure without having to use one of those crazy tube furnaces stuffed with thorium oxide or manganous oxide catalyst [21]. The key here is to use an excess of acetic anhydride. Using even more than the amount specified will insure that the reaction proceeds in the right direction and the bad side reaction formation of dibenzylketone will be minimalized (don t ask). 18g piperonylic acid or 13.6g phenylacetic acid, 50mL acetic anhydride and 50mU pyridine are refluxed for 6 hours and the solvent removed by vacuum distillation. The remaining residue is taken up in benzene or ether, washed with 10% NaOH solution (discard the water layer), and vacuum distilled to get 8g P2P (56%). 

You will recognize the side chain oxidation of p xylene to terephthahc acid as a reaction type discussed previously (Section 11 13) Examples of other reactions encoun tered earlier that can be applied to the synthesis of carboxylic acids are collected m Table 19 4... 

A second mole of halogen adds with greater difficulty oxidative side reactions can be minimised by halogenating an ester instead of the free alcohol (26). 

The reaction is very exothermic. The heat of reaction of propylene oxidation to acrolein is 340.8 kJ /mol (81.5 kcal/mol) the overall reactions generate approximately 837 kJ/mol (200 kcal/mol). The principal side reactions produce acryUc acid, acetaldehyde, acetic acid, carbon monoxide, and carbon dioxide. A variety of other aldehydes and acids are also formed in small amounts. Proprietary processes for acrolein manufacture have been described (25,26). 

Tocotrienols differ from tocopherols by the presence of three isolated double bonds in the branched alkyl side chain. Oxidation of tocopherol leads to ring opening and the formation of tocoquinones that show an intense red color. This species is a significant contributor to color quaUty problems in oils that have been abused. Tocopherols function as natural antioxidants (qv). An important factor in their activity is their slow reaction rate with oxygen relative to combination with other free radicals (11). 

The calcium oxide product is supplemented with fresh limestone and returned to the fluidized bed. Two undesirable side reactions can occur in the regeneration of spent lime leading to the production of calcium sulfide ... 

The primary side reaction at the anode is the oxidation of hydroxyl ion to oxygen. In an undivided ceU, a side reaction takes place also at the cathode, ie, the unwanted reduction of MnO and MnO to lower valent manganese species. 

Propylene-Based Routes. The strong acid-catalyzed carbonylation of propylene [115-07-1] to isobutyric acid (Koch reaction) followed by oxidative dehydration to methacrylic acid has been extensively studied since the 1960s. The principal side reaction in the Koch reaction is the formation of oligomers of propylene. Increasing yields of methacrylic acid in the oxydehydration step is the current focus of research. Isobutyric acid may also be obtained via the oxidation of isobutyraldehyde, which is available from the hydroformylation of propylene. The -butyraldehyde isomer that is formed in the hydroformylation must be separated. 

An important side reaction in all free-radical nitrations is reaction 10, in which unstable alkyl nitrites are formed (eq. 10). They decompose to form nitric oxide and alkoxy radicals (eq. 11) which form oxygenated compounds and low molecular weight alkyl radicals which can form low molecular weight nitroparaffins by reactions 7 or 9. The oxygenated hydrocarbons often react further to produce even lighter oxygenated products, carbon oxides, and water. 

Manufacture and Processing. Mononitrotoluenes are produced by the nitration of toluene in a manner similar to that described for nitrobenzene. The presence of the methyl group on the aromatic ring faciUtates the nitration of toluene, as compared to that of benzene, and increases the ease of oxidation which results in undesirable by-products. Thus the nitration of toluene generally is carried out at lower temperatures than the nitration of benzene to minimize oxidative side reactions. Because toluene nitrates at a faster rate than benzene, the milder conditions also reduce the formation of dinitrotoluenes. Toluene is less soluble than benzene in the acid phase, thus vigorous agitation of the reaction mixture is necessary to maximize the interfacial area of the two phases and the mass transfer of the reactants. The rate of a typical industrial nitration can be modeled in terms of a fast reaction taking place in a zone in the aqueous phase adjacent to the interface where the reaction is diffusion controlled. 

Significant quantities of the diphenoquinone are also produced if the ortho substituents are methoxy groups (36). Phenols with less than two ortho substituents produce branched and colored products from the reactions that occur at the open ortho sites. It is possible to minimize such side reactions in the case of o-cresol oxidation by using a bulky ligand on the copper catalyst to block the open ortho position (38). 

Halophenols without 2,6-disubstitution do not polymerize under oxidative displacement conditions. Oxidative side reactions at the ortho position may consume the initiator or intermpt the propagation step of the chain process. To prepare poly(phenylene oxide)s from unsubstituted 4-halophenols, it is necessary to employ the more drastic conditions of the Ullmaim ether synthesis. A cuprous chloride—pyridine complex in 1,4-dimethoxybenzene at 200°C converts the sodium salt of 4-bromophenol to poly(phenylene oxide) (1) ... 

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