Chain mechanism of alcohol oxidation

The kinetics of the initiated oxidation of alcohols is the same as for hydrocarbon oxidation [3,39 42], the rate equation being 

This expression is valid for the calculation of the ratio of rate coefficients. The general scheme given above may be considered as two alternnative mechanisms, viz. 

The reaction between two hydroxyperoxyradicals seems to be that of disproportionation 

The rate coefficient measured by the pulse radiolysis technique [51] is 2kt = 1.8 X 1071 mole-1 s-1, i.e. one order of magnitude higher than that measured by the sector technique [49]. The oxygen dissolved in cyclohexanol seems to be rapidly consumed on irradiation. Free hydroxyalkyl radicals R disappear partly by bimolecular interaction (R- + R ), the rate coefficient of which is high [51], 3.4 X 1081 mole-1 s-1. This is in agreement with the radiolysis yields obtained, viz. Gketone = 6.4 and GROOH = 0.34, whereas they should be the same if only reaction (3) occurs. 

Peroxy radicals of cyclohexene react with alcohols at 60° C with the following rate coefficients [270] (1 mole-1 s-1) 5.6 (C6H5CH2OH), 2.5 (cyclohexanol), 2.0 (i-PrOH), 1.9 (EtOH), 1.2 (rc-BuOH) and 0.3 (MeOH). 

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The traditional chain oxidation with chain propagation via the reaction RO/ + RH occurs at a sufficiently elevated temperature when chain propagation is more rapid than chain termination (see earlier discussion). The main molecular product of this reaction is hydroperoxide. When tertiary peroxyl radicals react more rapidly in the reaction R02 + R02 with formation of alkoxyl radicals than in the reaction R02 + RH, the mechanism of oxidation changes. Alkoxyl radicals are very reactive. They react with parent hydrocarbon and alcohols formed as primary products of hydrocarbon chain oxidation. As we see, alkoxyl radicals decompose with production of carbonyl compounds. The activation energy of their decomposition is higher than the reaction with hydrocarbons (see earlier discussion). As a result, heating of the system leads to conditions when the alkoxyl radical decomposition occurs more rapidly than the abstraction of the hydrogen atom from the hydrocarbon. The new chain mechanism of the hydrocarbon oxidation occurs under such conditions, with chain... 

Radical scavengers can be used in addition to the sequestrant approach, or as an alternative where sequestrants are not appropriate. Their use is based on conversion of very reactive radicals, produced during chain decomposition processes, to very stable radicals, hence stopping the chain mechanism. Simple alcohols can be effective stabilizer components, as are almost all aromatic compounds (commonly used as anti-oxidants in food or plastics), e.g. p-hydroxybenzoates, butylated hydroxytoluene, anisole, /7-butylcatechol, gallates. All of these materials give relatively stable radicals on one-electron oxidation. Sequestrant N-oxides act as scavengers through one-electron oxidation to stable nitroxides.166... 

A homogeneous catalytic solution to the alcohol inhibition problem (see the discussion under Uncatalyzed chain reactions of the oxidation of alcohol intermediates, above) does not appear to have been found. However, the presence of a heterogeneous oxidative dehydrogenation catalyst has been reported to be effective in the direct oxidation of alcohols to carbonyls and acids [109, 110]. The mechanism probably involves preliminaiy heterogeneous (oxidative) dehydrogenation of carbinols to carbonyls. If the carbonyl is an aldehyde, it is readily converted to the acid. Platinum, palladium, ruthenium, rhodium, and iridium catalysts, supported on carbon, are reported to be active and selective catalysts for the purpose [109]. Promoters such as cobalt and cadmium have been reported to be effective additives. 

The mechanism of alkylene oxide anionic polyaddition to hydroxyl groups, catalysed by alkali hydroxides, is discussed in chapters 4.1-4.1.5, the real active centre being the alkaline alcoholate, and the propagation reaction being the repeated SN-2 attack of the alcoholate anion on the a-carbon atom of the oxirane rings. The rapid equilibrium of the alcohol - alcoholate assures that each hydroxyl group from the reaction system is a chain initiator. 

It is seen fiem these data that the HO2 radicals are active as both oxidants and reducing agents. Peroxyl radicals formed in the oxidation of alcohols and aromatic amines possess the same reactivity, which forms a basis for the catalytic mechanism of chain termination in chain reactions of the oxidation of these compounds (see Chapter 11). 

Faraday, in 1834, was the first to encounter Kolbe-electrolysis, when he studied the electrolysis of an aqueous acetate solution [1], However, it was Kolbe, in 1849, who recognized the reaction and applied it to the synthesis of a number of hydrocarbons [2]. Thereby the name of the reaction originated. Later on Wurtz demonstrated that unsymmetrical coupling products could be prepared by coelectrolysis of two different alkanoates [3]. Difficulties in the coupling of dicarboxylic acids were overcome by Crum-Brown and Walker, when they electrolysed the half esters of the diacids instead [4]. This way a simple route to useful long chain l,n-dicarboxylic acids was developed. In some cases the Kolbe dimerization failed and alkenes, alcohols or esters became the main products. The formation of alcohols by anodic oxidation of carboxylates in water was called the Hofer-Moest reaction [5]. Further applications and limitations were afterwards foimd by Fichter [6]. Weedon extensively applied the Kolbe reaction to the synthesis of rare fatty acids and similar natural products [7]. Later on key features of the mechanism were worked out by Eberson [8] and Utley [9] from the point of view of organic chemists and by Conway [10] from the point of view of a physical chemist. In Germany [11], Russia [12], and Japan [13] Kolbe electrolysis of adipic halfesters has been scaled up to a technical process. 

Inspired by Gif or GoAgg type chemistry [77], iron carboxylates were investigated for the oxidation of cyclohexane, recently. For example, Schmid and coworkers showed that a hexanuclear iron /t-nitrobenzoate [Fe603(0H) (p-N02C6H4C00)n(dmf)4] with an unprecedented [Fe6 03(p3-0)(p2-0H)] " core is the most active catalyst [86]. In the oxidation of cyclohexane with only 0.3 mol% of the hexanuclear iron complex, total yields up to 30% of the corresponding alcohol and ketone were achieved with 50% H2O2 (5.5-8 equiv.) as terminal oxidant. The ratio of the obtained products was between 1 1 and 1 1.5 and suggests a Haber-Weiss radical chain mechanism [87, 88] or a cyclohexyl hydroperoxide as primary oxidation product. 

In the water-like solvent tert-butyl alcohol, a-tocopherol was found to prevent lipid oxidation, showing a distinct lag-phase for oxygen consumption. This was in contrast to quercetin or epicatechin, which were only weak retarders of lipid oxidation without any clear antioxidative effect. Quercetin or epicatechin, when combined with a-tocopherol, increased the lag-phase for oxygen consumption as seen for a-tocopherol alone. The stoichiometric factor for a-tocopherol, a-TOH, as chain-breaking antioxidant has the value n = 2 according to the well-established mechanism ... 

This oxidative process has been successful with ketones,244 esters,245 and lactones.246 Hydrogen peroxide can also be used as the oxidant, in which case the alcohol is formed directly.247 The mechanisms for the oxidation of enolates by oxygen is a radical chain autoxidation in which the propagation step involves electron transfer from the carbanion to a hydroperoxy radical.248... 

Alcohols, like hydrocarbons, are oxidized by the chain mechanism. The composition of the molecular products of oxidation indicates that oxidation involves first the alcohol group and the neighboring C—H bond. This bond is broken more readily than the C—H bond of the corresponding hydrocarbon, since the unpaired electron of the formed hydroxyalkyl radical interacts with the p electrons of the oxygen atom. 

The nonsaturated esters with tt-C=C bonds and without activated a-C—H bonds (esters of acrylic acid (CH2=CHCOOR) and esters of vinyl alcohols (RC(0)0CH=CH2)) are oxidized by the chain mechanism with chain propagation via the addition of peroxyl radicals to the double bond. Oligomeric peroxides are formed as primary products of this chain reaction. The kinetic scheme includes the following steps in the presence of initiator I and at p02 sufficient to support [02] > 10 4 mol L-1 in the liquid phase [49]. 

Organic acids retard the formation of nitroxyl radicals via the reaction of the peroxyl radical with the aminyl radical [10], Apparently, the formation of a hydrogen bond of the >N H0C(0)R type leads to the shielding of nitrogen, which precludes the addition of dioxygen to it, yielding the nitroxyl radical. Thus, the products of the oxidation of alcohols, namely, acids have an influence on the mechanism of the cyclic chain termination. 

As noted above, the duration of the retarding action of an inhibitor is directly proportional to the / value. In systems with a cyclic chain termination mechanism, the / coefficient depends on the ratio of the rate constants for two reactions, in which the inhibitor is regenerated and irreversibly consumed. In the oxidation of alcohols, aminyl radicals are consumed irreversibly via the reaction with nitroxyl radical formation (see earlier) and via the following reaction [11] ... 

Kinetic Characteristics of the Cyclic Mechanism of the Chain Termination on Nitroxyl Radicals in the Oxidation of Alcohols and Amines (Experimental Data)... 

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