Disproportionation reactions inhibitors

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

The dinitrobenzenes display the characteristics of inhibitors for the more reactive vinyl acetate chain radicals. Two radicals are terminated during the induction period by each molecule of dinitrobenzene, indicating disappearance of inhibitor radicals by a disproportionation reaction. 

Vardanyan [65,66] discovered the phenomenon of CL in the reaction of peroxyl radicals with the aminyl radical. In the process of liquid-phase oxidation, CL results from the disproportionation reactions of primary and secondary peroxyl radicals, giving rise to trip-let-excited carbonyl compounds (see Chapter 2). The addition of an inhibitor reduces the concentration of peroxyl radicals and, hence, the rate of R02 disproportionation and the intensity of CL. As the inhibitor is consumed in the oxidized hydrocarbon the initial level of CL is recovered. On the other hand, the addition of primary and secondary aromatic amines to chlorobenzene containing some amounts of alcohols, esters, ethers, or water enhances the CL by 1.5 to 7 times [66]. This effect is probably due to the reaction of peroxyl radicals with the aminyl radical, since the addition of phenol to the reaction mixture under these conditions must extinguish CL. Indeed, the fast exchange reaction... 

As compared with the hydrolytic polymerization, the base catalysed polymerization is essentially more sensitive towards impurities. The inhibition or retardation by traces of various foreign substances follows from the extremely low concentrations of the imide formed by the disproportionation reaction ranging about in the order of magnitude 10 2mol-%, and from the high reactivity of the imide. The imides are the key intermediate in the polymerization mechanism and hence every substance which is able to react with imides acts either as an inhibitor or as a retarder of the base catalysed polymerization of caprolactam (67). 

The monomer will not autopolymerize at ambient temperatures therefore, it can be transported without an inhibitor. Like all fully or partially fluorinated ethylenes, CTFE can undergo a disproportionation reaction and thus must be handled properly. It forms high-molecular peroxides in reaction with oxygen and these can precipitate from solution. Thus, oxygen concentration of commercial CTFE is maintained below 50 ppm.47... 

Studies conducted in the presence of radical scavengers such as NO (refs. 383, 245, 409, 410), Oj (ref. 408) or H2S (ref. 246) have shown the importance of free-radical reactions in forming the products isobutane, 2,3-dimethylbutane, -butane, isopentane and others. The ethylene and propene yields are decreased by the presence of the scavengers owing to the disappearance of the fraction of these products that arises from disproportionation reactions. The products which are formed in the presence of inhibitors must arise from molecular or ion eliminations, ion-molecule reactions, excited molecule reactions or charge-neutralization reactions. Work on the inhibited radiolysis has led to a better understanding of the source of these products " . 

These catalysts are silica-aluminas whose cracking and disproportionation power has been altered by steam treatment, the use of an inhibitor, or of aluminas containing a halogenated compound or fluorine. They are very rugged, are employed without hydrogen and hence cannot isomerize ethylbenzene, which is therefore cracked or transformed by a disproportionation reaction into benzene and C10 aromatics. Consequently, they can only be used with feeds poor in ethylbenzene. However, no naphthenic hydrocarbons are formed. 

The basic features of the oscillatory mechanism of the BZ reaction are given by the Field-Koros-Noyes (FKN) model [14]. This involves three processes —A, B and C. Process A involves step (8) and step (9) from section A3.14.2.1, leading to removal of inhibitor bromide ion. Process B involves step (3) and step H) from Section A3.14.1.1 and gives the autocatalytic oxidation of the catalyst. This growth is limited partly by the disproportionation reaction... 

Any substance capable of reacting with free radicals to form products that do not reinitiate the oxidation reaction could be considered to function as free-radical traps. The quinones are known to scavenge alkyl free radicals. Many polynuclear hydrocarbons show activity as inhibitors of oxidation and are thought to function by trapping free radicals [25]. Addition of R to quinone or to a polynuclear compound on either the oxygen or nitrogen atoms produces adduct radicals that can undergo subsequent dimerization, disproportionation, or reaction with a second R to form stable products. 

In termination, unsaturated and saturated ends are formed when the propagating species undergo disproportionation, head-to-head linkages when they combine, and other functional groups may be introduced by reactions with inhibitors or transfer agents (Scheme 1.2). In-chain defect structures (within the polymer molecule) can also arise by copolymerization of the unsaturated byproducts of initiation or termination. 

The extent to which chain oxidation is inhibited depends on the activity and concentration of the antioxidant. A specific activity of an antioxidant as a retarding agent should be expressed per unit concentration of the inhibitor. If the antioxidant terminates chains, chain self-termination by the reaction of peroxyl radical disproportionation should be taken into account. As a result, one obtains the following expression for estimation of the activity F of the introduced amount of the antioxidant [18] ... 

The lower oxidation states are stabilized by soft ligands e.g. CO (Prob. 3). The aquated vanadium ions represent an interesting series of oxidation states. They are all stable with respect to disproportionation and labile towards substitution. They undergo a number of redox reactions with one another, all of which have been studied kinetically. Many of the reactions are [H ]-dependent. There has been recent interest in the biological aspects of vanadium since the discovery that vanadate can mimic phosphate and act as a potent inhibitor (Prob. 4). 

Chain termination occurs by combination or disproportionation of different polymer radicals. The termination rate, v is proportional to the polymer radical concentration, [ PJ, squared, with kt being the termination rate constant. Other possible chain termination processes are chain transfer and reaction of polymer radicals wifh inhibitors and radical trapping. ... 

The hexanes undergo side reactions even more readily than do the pentanes. Although disproportionation and cracking can be suppressed to some extent by the addition of cyclic hydrocarbons, this treatment is not effective enough to ensure satisfactory catalyst life, and hydrogen at relatively high pressure must be used as the inhibitor. 

Pentane Isomerization. Pentane isomerization, although carried out on a much smaller scale, increased the critical supply of aviation gasolines toward the end of the war. Two pentane processes—one developed by Shell and one by Standard (Indiana) —were commercialized before the end of the war. The principal differences between the butane and pentane processes are the use in pentane isomerization of somewhat milder conditions and the use of an inhibitor to suppress side reactions, principally disproportionation. In general, the problems of the butane processes are inherent also in pentane isomerization, but the quality of the feed stocks is less important. Catalyst life is much... 

IFF (molecular weight 100.02) is a colorless, tasteless, odorless nontoxic gas.11 It is stored as a liquid (its vapor pressure at -20°C is 1 MPa) and polymerized usually above its critical temperature of 33.3°C (91.9°F) and below its critical pressure 3.94 MPa (571 psi). The polymerization reaction is exothermic. In the absence of air it disproportionates violently to yield carbon and carbon tetrafluo-ride. This reaction generates the same amount of energy as an explosion of black powder. The decomposition is initiated thermally therefore, the equipment used in handling and polymerization of IFF has to be without hot spots. The flammability limits are 14 to 43% TFE bums when mixed with air and forms explosive mixtures with air and oxygen. The ignition temperature is 600 to 800°C (1112 to 1472°F).12 When stored in steel cylinder it has to be under controlled conditions and with a suitable inhibitor. 

Product formation in a radiolysis system is often complex as a result of the many different species present. There are, however, three main types of reactive species excited molecules, ions and free radicals. The excited molecules and ions are generated directly, while the free radicals are formed by dissociation of the excited molecules or ions. The reactions of these three species account for the products. Decomposition or combination and disproportionation are the main reactions of free radicals in the absence of inhibitors. Excited molecules can lead directly to molecular products by dissociation or to higher products by dimerization reactions. Ionic species can yield molecular products by dissociation (if excited) or by ion-molecule reactions with the formation of a new ion in each case. Neutralization of the positive ions by electrons or by negative ions produces additional molecular products. 

An added compound may react with the growing free radical to generate a new free radical that is not reactive enough to add to monomer a reaction chain is terminated but no new one is begun. Such a compound is, of course, an inhibitor (Sec. 2.14). Many amines, phenols, and quinones act as inhibitors. Although their exact mode of action is not understood, it seems clear that they are converted into free radicals that do not add to monomer instead, they may combine or disproportionate, or combine with another growing radical to halt a second reaction chain. 

The inhibitor radicals formed in the above reactions are stabilized by resonance to such an extent that they do not start chains and initiate polymerization. They disappear partly through disproportionation (forming quinone and hydroquinone) ... 

---