Radical metal alkyl decomposition

Much of the early work on alkyl radicals of short life was, as we have seen (p. 301), carried out in the vapour phase through decomposition of metal alkyls, e.g. (23) ... 

The literature cited in this article covers references listed in chemical abstracts to the end of 1961 and in current chemical papers after that. A degree of selection has been exercised in omitting some references that are now of limited value. Although metal nitrosyls are included in the scope of this chapter, no kinetic data on their decomposition is available and they will not be considered further. The data on metal carbonyls is limited and will be dealt with in the first section. The decomposition of metal alkyls and aryls has been extensively investigated. These compounds will be discussed in groups based on the position of the central metal atom in the periodic table and, when warranted, a further subdivision will be made based on the attached organic radicals. 

The first step in the peroxide-induced reaction is the decomposition of the peroxide to form a free radical. The oxygen-induced reaction may involve the intermediate formation of a peroxide or a free radical olefin-oxygen addition product. (In the case of thermal and photochemical reactions, the free radical may be formed by the opening up of the double bond or, more probably, by dissociation of a carbon-hydrogen bond in metal alkyl-induced reactions, decomposition of the metal alkyl yields alkyl radicals.)... 

Competition between metal ion-induced and radical-induced decompositions of alkyl hydroperoxides is affected by several factors. First, the competition is influenced by the relative concentrations of the metal complex and the hydroperoxide. At high concentrations of the hydroperoxide relative to the metal complex, alkoxy radicals will compete effectively with the metal complex for the hydroperoxide. Competition is also influenced by the nature of the solvent (see above). Contribution from the metal-induced reaction is expected to predominate at low hydroperoxide concentrations and in reactive solvents. The contribution from the metal-induced decomposition to the overall reaction is readily determined by carrying out the reaction in the presence of free radical inhibitors, such as phenols, that trap the alkoxy radicals and, hence, prevent radical-induced decomposition.129,1303 Thus, Kamiya etal.129 showed that the initial rate of the cobalt-catalyzed decomposition of tetralin hydroperoxide, when corrected for the contribution from radical-induced decomposition by the... 

As indicated earlier, heat may be used to form radicals derived from the fission of weak bonds. A very common example of this is the thermal decomposition of metal alkyls. For example, tetraethyl lead, Pb(C2H5)4, readily gives ethyl... 

Taylor s experiments180 on the decomposition of metal alkyls led him to believe that the active alkyl groups, released by decomposition of the compounds, functioned in the same way as the active metal atoms. Experiments have shown that the presence of hydrogen atoms induces oxidation of ethylene at room temperature. The theory is advanced that the free alkyl radicals act in a manner similar to hydrogen atoms or metal fogs, i.e., as active oxidation centers producing a slow homogeneous combustion of fuel. This action of free radicals may accomit for the effects of non-metallic knock suppressors as aniline, toluidine, etc. 

Evidence for the above reactions comes from kinetic studies (ISO), and from the thermal decomposition of ethyltetracarbonylcobalt which gives ethylene and tetracarbonylcobalt hydride (130). Earlier studies have eliminated possible free radical mechanisms (4). Supporting evidence arises from the demonstration of the reversible addition of ethylene to a platinum hydride (see Eq. 27) and from the observation that a number of transition metal alkyls have been shown to lose readily a hydride ion from the 2-carbon of the alkyl group (101). 

On the other hand, the persistence of the dialkyl peroxide indicates that it must not derive from a mechanism involving such oxidants. Indeed such dialkyl peroxides are readily formed by metal-catalyzed decomposition of alkyl hydroperoxides and involve alkoxy and alkylperoxy radicals. The mechanism for dialkyl peroxide formation shown below is adapted for FeTPA from previously proposed schemes ... 

Nernst (1918) suggested that free radicals take part in chemical reactions and postulated a radical chain mechanism for the combination of H2 and CI2. Paneth and coworkers (1929) first demonstrated the existence of alkyl free radicals by decomposition of metal alkyls. Norrish (1931) suggested that free radicals could occur as intermediates in the photolysis of carbonyl compounds. Rice and Herzfield (1934) produced radicals from the dissociation of hydrocarbons. Up until relatively recently, radicals were regarded as highly reactive species, whose reactions were unselective and difficult to control (remember the radical chlorination of methane). The last 20 years have seen the field developed to such an extent that it is now recognized that radicals can take part in highly useful and selective reactions. 

On account of such rather discouraging results, there grew up a belief that transition metal alkyls and aryls were inherently unstable. It was suggested that transition metal—carbon cr-bonds must be very weak (thermodynamic instability). By implication an important mechanism for decomposition was thought to be homolysis to alkyl radicals, which led to mixtures of hydrocarbons (kinetic factors). 

One other item of historical interest may suffice. Paneth was an inorganic chemist who reported on simple free alkyl radicals, which he generated by the pyrolysis of alkylmetallic compounds of metals such as lead and whose half-life period he measured. About 3 years before, H. Taylor (a ph)rsical chemist) published in a less generally used periodical, his ideas and experimental observations on studies concerned with catalytic reduction processes of hydrogen-ethylene mixtures. He wrote that the simplest method of liberating free alkyl radicals into a mixture of these gases was by decomposition of metal alkyls such as diethylmercury and tetraethyllead. 

Transition metal-promoted hydroperoxide deconposition is inqiortant to the oxidative stability and quality of foods for several reasons. First, the abstraction of hydrogen from an unsaturated fatty acid results in the formation of a single alkyl radical. Followii hydrogen abstraction, oxygen adds to the alkyl radical to form a peroxyl radical and subsequent abstraction of a hydrogen from another fatty acid or antioxidant to form a lipid hydroperoxide (Figure 1). These reactions by themselves do not result in an increase in free radical numbers. If these reactions were tiie only steps in tiie lipid oxidation reactions, the rapid exponential increase in oxidation that is commonly observed in lipids would not occur. Transition metal-promoted decomposition of lipid hydroperoxides results in the formation of additional radicals (e.g. alkoxyl and peroxyl) which exponentially increase oxidation rates as they start to attack otiier unsaturated fatty acids. 

The selective ability of higher-valent metal ions to oxidize organic free radicals has been examined and two mechanisms have been proposed for the decomposition of cerium(iv) carboxylates in the presence of olefins and aromatic hydrocarbons (i) involving a non-decarboxylative route with the generation of carboxyalkyl radicals via thermal decomposition, and ( ) the formation of alkyl radicals and COg in the decarboxylation reaction. E.s.r. studies using cerium(iv) have been used to provide evidence for the formation of the sulphite radical anion SO3 in the oxidation of sulphite. ... 

A free radical procedure for preparing Pb(CH3)4 and mixed tetraalkyllead compounds by passing CH3CI and other alkyl chlorides over a heated mixture of lead and an initiator metal, like copper, is described in [277]. Reaction of lead deposited as a mirror on the inside of a quartz tube with CH3 radicals formed by decomposition of di-tert-butylperoxide is employed to prepare Pb(CH3)4 from small amounts of lead, e.g., from minerals for mass spectrometric analysis [278, 279]. Pb(CH3)4 is formed by reaction of CH3 radicals with recoil products of 224Ra [280, 281, 282] see also [283]. 

Among the early works on free radicals, the importance of experiments of F. Paneth and W. Hafeditz is often quoted. They supplied evidence that aliphatic free radicals occur in the decomposition of metallic alkyls, such as dimethyl-mercury (Hg(CH3)2), and tetramethyl- or tetraethyllead (Pb(CH3)4 or Pb(C2H5)4). Paneth saturated a stream of nitrogen with Pb(CH3)4 vapor. The vapors were then heated to 450 °C. Decomposition of Pb(CH3)4 deposited a lead mirror on the heated part of the tube. When the vapors from the decomposition passed over the deposited lead mirror at 100 °C, the mirror slowly disappeared. It was concluded that the following reaction had taken place ... 

Alkyl hydroperoxides are among the most thermally stable organic peroxides. However, hydroperoxides are sensitive to chain decomposition reactions initiated by radicals and/or transition-metal ions. Such decompositions, if not controlled, can be auto accelerating and sometimes can lead to violent decompositions when neat hydroperoxides or concentrated solutions of hydroperoxides are involved. 

Dialkyl peroxydicarboaates are used primarily as free-radical iaitiators for viayl monomer po1ymeri2ations (18,208). Dialkyl peroxydicarboaate decompositioas are accelerated by certaia metals, coaceatrated sulfuric acid, and amines (44). Violent decompositions can occur with neat or highly concentrated peroxides. As with most peroxides, they Hberate iodine from acidified iodides. In the presence of copper ions and suitable substrates, dialkyl peroxydicarbonates have been used to synthesi2e alkyl carbonates (44) ... 

Cesium forms simple alkyl and aryl compounds that are similar to those of the other alkah metals (6). They are colorless, sohd, amorphous, nonvolatile, and insoluble, except by decomposition, in most solvents except diethylzinc. As a result of exceptional reactivity, cesium aryls should be effective in alkylations wherever other alkaline alkyls or Grignard reagents have failed (see Grignard reactions). Cesium reacts with hydrocarbons in which the activity of a C—H link is increased by attachment to the carbon atom of doubly linked or aromatic radicals. A brown, sohd addition product is formed when cesium reacts with ethylene, and a very reactive dark red powder, triphenylmethylcesium [76-83-5] (C H )2CCs, is formed by the reaction of cesium amalgam and a solution of triphenylmethyl chloride in anhydrous ether. 

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