Hydrocarbons side-chain oxidation

Snyder and Rapoport photolysed phylloquinone (vitamin K-l, 82) in cyclohexane solution with the surface exposed to atmospheric oxygen and moisture. This system was adopted on the assumption that the in vivo photo-oxidation would occur with the hydrocarbon side-chain dissolved in a lipid layer, but the polar naphthoquinone moiety would be in contact with water. Under the... 

Thus we see that no matter what the nature of the side chain group in a benzene hydrocarbon, whether a single methyl group, a saturated poly-carbon chain or an unsaturated poly-carbon chain, each side chain always yields the carboxyl group as the final product. The intermediate products, however, and the ease with which the oxidation is effected varies with the character of the side chain so that in compounds containing two or more different hydrocarbon side chains one will be oxid-... 

Gasoline hydrocarbons volatilized to the atmosphere quickly undergo photochemical oxidation. The hydrocarbons are oxidized by reaction with molecular oxygen (which attacks the ring structure of aromatics), ozone (which reacts rapidly with alkenes but slowly with aromatics), and hydroxyl and nitrate radicals (which initiate side-chain oxidation reactions) (Stephens 1973). Alkanes, isoalkanes, and cycloalkanes have half-lives on the order of 1-10 days, whereas alkenes, cycloalkenes, and substituted benzenes have half- lives of less than 1 day (EPA 1979a). Photochemical oxidation products include aldehydes, hydroxy compounds, nitro compounds, and peroxyacyl nitrates (Cupitt 1980 EPA 1979a Stephens 1973). 

Fig. 4. The 26-hydroxylase and the 25-hydroxylase pathways for the oxidation of the hydrocarbon side chain of 5p-cholestane-3a,7a,12a-triol (V) to form cholic acid (X).

The oxidation of a hydrocarbon side chain attached to an aromatic ring is traditionally a difficult transformation to accomplish. The most common transformation is the conversion of toluenes to benzaldehydes and benzoic acids (Fig. 11). The most difficult stage is the introduction of oxygen as subsequent interconversion between oxidation levels is relatively easy as we have already seen. 

Fig. 13. Products available via Interox side chain oxidation technology (yields from hydrocarbon substrate in brackets)...

The presence of alcohols lias not been reported in the oxidation of these hydrocarbons, and although this fact does not preclude the intermediate formation of an alcohol as the first step in the oxidation, it does indicate that the mechanism of side chain oxidation is similar to that of aliphatic hydrocarbon oxidation since in this latter case even the proponents of the hydroxylation theory have had difficulty in isolating alcohols. 

Hydroxylation of benzene to phenol using hydrogen peroxide in the presence of heteropoly compounds was observed in this study. Other aromatic hydrocarbons that were used as substrates were toluene, ethylbenzene, o,p-xylene and isopropyl benzene under homogeneous conditions. Both side chain oxidation and ring hydroxylation were observed in presence of hydrogen peroxide. For example, toluene gave benzyl alcohol, benzaldehyde, o,p-cresols in presence of hydrogen peroxide, whereas benzaldehyde and benzyl alcohol were observed in presence of t-BuOOH. 

The best known of these is the ozonation of tertiary amines to amine oxides (II) (i). Henbest and Stratford (11) and Shulman (17) have shown that competing with this is an ozone attack on the alpha position of an alkyl side chain to produce various decomposition products of III. Henbest (11) showed that amine oxide formation is favored in chloroform and methanol, while side chain oxidation is predominant in hydrocarbon solvents. Also of considerable interest are the reported conversions, during ozonation, of phenylenediamines to Wursters salts (VII) (8, 14), of liquid ammonia to ammonium ozonate (VA) at a low temperature 18), and of amines to amine hydrochlorides (VB) in chlorinated hydrocarbon solvents 17, 19), Finally, an early report states that azobenzene and quinone are obtained upon ozonation of aniline (15). 

Acyclic or side-chain oxidations convert hydrocarbons and alcohols into their corresponding acids and their conjugates. These reactions have been observed in mammals, birds, frogs, turtles, and insects. Cooper and Brodie [26, 27] found that the alkyl side-chain of barbiturates and other drugs are oxidized to primary and secondary alcohols. Further oxidation of secondary alcohols to ketones is catalyzed by enzymes in the liver cell soluble fraction. 

A range of aromatic oxidations involve direct SET from an organic substrate to the oxidant (catalyst, anode), leading to a radical cation [35]. Radical cations are much stronger acids than the parent hydrocarbon molecules [35a, b]. For example, the of toluene drops from 41 to ca. -13 with the removal of one electron, which makes deprotonation the predominant process in the transformation of the radical cation. Benzyl radicals formed in this way dimerize and participate in the side-chain oxidation (Scheme 14.5). On the other hand, radical cation can undergo attack by nucleophiles (H O, AcOH, etc.) followed by the second FT leading to the ring oxidation products [36]. 

In conclusion, our studies of the electrochemistry of some relevant antioxidants in different solvents and determination of their diffusion coefficients have shown that the electrochemical behaviour of the antioxidants is determined predominantly by the hydrophilic/hydrophobic interactions of the hydrocarbon side-chain and the electrostatic interactions of the antioxidant with the surfactant. The strong association of the antioxidant molecules with the SDS micelles results in the significant lowering of its diffusion coefficient and may even change the mechanism of oxidation reaction. 

Sterols are a class of lipids containing a common steroid core of a fused four-ring structure with a hydrocarbon side chain and an alcohol group. Cholesterol is the primary sterol lipid in mammals and is an important constituent of cellular membranes. Oxidization and/or metabolism of cholesterol yield numerous oxysterols, steroids, bile acids, etc., many of which are important signaling molecules in biological systems. Cholesteryl esters esterified with a variety of fatty acyls are enriched in... 

Oxidation of a side chain by alkaline permanganate. Aromatic hydrocarbons containing side chains may be oxidised to the corresponding acids the results are generally satisfactory for compounds with one side chain e.g., toluene or ethylbenzene -> benzoic acid nitrotoluene -> nitrobenzoic acid) or with two side chains e.g., o-xylene -> phthalic acid). 

Oxidation of side chains. The oxidation of halogenated toluenes and similar compounds and of compounds with side chains of the type —CHjCl and —CH OH proceeds comparatively smoothly with alkaline permanganate solution (for experimental details, see under AromcUic Hydrocarbons, Section IV.9,6 or under Aromatic Ethers, Section IV,106). The resulting acid may be identified by a m.p. determination and by other teats (see Section IV,175). 

The most abundant natural steroid is cholesterol. It can be obtained in large quantides from wool fat (15%) or from brain or spinal chord tissues of fat stock (2-4%) by extraction with chlorinated hydrocarbons. Its saturated side-chain can be removed by chromium trioxide oxidation, but the yield of such reactions could never be raised above 8% (see page 118f.). 

Oxidation of carbon side-chains has resulted in the synthesis of dithiazolyl ketone (82) and thiazolyl phenyl ketone (83). The hydrocarbon chain can also be dehydrogenated in acetic acid in the presence of... 

Oxidative Fluorination of Aromatic Hydrocarbons. The economically attractive oxidative fluorination of side chains in aromatic hydrocarbons with lead dioxide or nickel dioxide in Hquid HF stops at the ben2al fluoride stage (67% yield) (124). 

In the case of l,4-ben2oquinone, the product is steam-distilled, chilled, and obtained in high yield and purity. Direct oxidation of the appropriate unoxygenated hydrocarbon has been described for a large number of ring systems, but is generally utilized only for the polynuclear quinones without side chains. A representative sample of quinone uses is given in Table 5. 

This combination of monomers is unique in that the two are very different chemically, and in thek character in a polymer. Polybutadiene homopolymer has a low glass-transition temperature, remaining mbbery as low as —85° C, and is a very nonpolar substance with Htde resistance to hydrocarbon fluids such as oil or gasoline. Polyacrylonitrile, on the other hand, has a glass temperature of about 110°C, and is very polar and resistant to hydrocarbon fluids (see Acrylonitrile polymers). As a result, copolymerization of the two monomers at different ratios provides a wide choice of combinations of properties. In addition to providing the mbbery nature to the copolymer, butadiene also provides residual unsaturation, both in the main chain in the case of 1,4, or in a side chain in the case of 1,2 polymerization. This residual unsaturation is useful as a cure site for vulcanization by sulfur or by peroxides, but is also a weak point for chemical attack, such as oxidation, especially at elevated temperatures. As a result, all commercial NBR products contain small amounts ( 0.5-2.5%) of antioxidant to protect the polymer during its manufacture, storage, and use. 

Titov claims that the free radical mechanism applies for nitration of aliphatic hydrocarbons, of aromatic side chains, of olefins, and of aromatic ring carbons, if irf the latter case the nitrating agent is ca 60—70% nitric acid that is free of nitrous acid, or even more dil acid if oxides of nitrogen are present... 

---