Alkyl radical decomposition

There are two categories of C2—C4 alkyl radical decomposition complexes (i) H split-off, and (ii) alkyl split-off. The first of the two complexes is the looser The activation energy for methyl or ethyl radical... 

Baldwin et al. qualified the absolute accuracy of their results by noting that the calculated values, as we have seen, are dependent on values of alkyl radical decomposition rates and the alkyl/alkylperoxy radical equilibrium constant. The work of Hughes et al. [122] now means that the data of Baldwin et al. can be appropriately scaled to give the results shown in Table 2.10. 

Alkyl radical decomposition produces a similar result ... 

Addition of radicals to unsaturated molecules and alkyl radical decomposition... 

Butyl radicals decompose quickly to form ethylene and propylene. At high temperatures, alkyl radical decomposition reactions constitute an important reaction class and the prevailing fate of alkyl radicals. Take 1-propyl and 1-butyl radicals, for example. These primary alkyl radicals give rise to the following / -decomposition reactions ... 

Alkyl radical decomposition reactions (to form primary radicals) ... 

As previously discussed, alkyl radicals decomposition reactions constitute an important fate and reaction path of alkyl radicals. Due to the very short lifetimes of alkyl radicals, Rice and Herzfeld (1933, 1934) suggested a complete decomposition mechanism where all the radicals larger than methyl were considered instantaneously decomposed into alkenes and H and CH3 radicals. In this mechanism, all the intermediate alkyl radicals decompose to directly form alkenes and smaller alkyl radicals. This would mean that the final ethylene production from a steam cracking process would be significantly overestimated when compared with the experimental measurements. For instance, the net and final result of the successive decomposition mechanism of 1-decyl radical would be 5 moles of ethylene and one H radical. 

At temperatures lower than approximately 900 K, high activation energies for alkyl radical decomposition make these processes relatively slow. Under such conditions, the most important reactions for alkyl radicals R consist of addition of... 

Rabinovitch B S and Setser D W 1964 Unimolecular decomposition and some isotope effects of simple alkanes and alkyl radicals Adv. Photochem. 3 1-82... 

As the temperature is increased through the NTC zone, the contribution of alkylperoxy radicals falls. Littie alkyl hydroperoxide is made and hydrogen peroxide decomposition makes a greater contribution to radical generation. Eventually the rate goes through a minimum. At this point, reaction 2 is highly displaced to the left and alkyl radicals are the dominant radical species. 

At the higher temperatures a decomposition of alkyl radicals, which is an olefin-producing variation of the -scission reaction, becomes competitive with reaction 23 (or sequence 2, 24) ... 

The extent of decarboxylation primarily depends on temperature, pressure, and the stabihty of the incipient R- radical. The more stable the R- radical, the faster and more extensive the decarboxylation. With many diacyl peroxides, decarboxylation and oxygen—oxygen bond scission occur simultaneously in the transition state. Acyloxy radicals are known to form initially only from diacetyl peroxide and from dibenzoyl peroxides (because of the relative instabihties of the corresponding methyl and phenyl radicals formed upon decarboxylation). Diacyl peroxides derived from non-a-branched carboxyhc acids, eg, dilauroyl peroxide, may also initially form acyloxy radical pairs however, these acyloxy radicals decarboxylate very rapidly and the initiating radicals are expected to be alkyl radicals. Diacyl peroxides are also susceptible to induced decompositions ... 

Diacyl peroxides are sources of alkyl radicals because the carboxyl radicals that are intitially formed lose CO2 very rapidly. In the case of aroyl peroxides, products may be derived from the carboxyl radical or the radical formed by decarboxylation. The decomposition of peroxides can also be accomplished by photochemical excitation. 

The thermal decomposition of diacyl peroxides has been the most frequently employed process for the generation of alkyl radicals. The rate and products of the unimolecular decomposition of acetyl peroxide have been the subject of many studies. Acetyl peroxide decomposes at a convenient rate at 70-80°C both in the solution and in the gas... 

The decomposition of diacyl peroxides provides a fairly clean method for the production of alkyl radicals and, therefore, it has been used in most quantitative and preparative studies of the alkylation of heterocyclic compounds,... 

In both compounds there are type (I) azo functions surrounded by alkyl groups and one cyano group. Upon heating, tertiary alkyl radicals and cyano alkyl radicals are formed. These radicals are relatively stable due to hyper conjugation and, in the case of cyano substituted alkyl radicals, to resonance. Therefore, azo groups (I) have a high proneness to thermal decomposition. 

Dialkyldiazenes (15, R—alkyl) are sources of alkyl radicals. While there is dear evidence for the transient existence of diazcnyl radicals (17 Scheme 3.18) during the decomposition of certain unsymmetrieal diazenes49 51 and of cis-diazenes,54 all isolable products formed in thermolysis or photolysis of dialkyldiazenes (15) are attributable to the reactions of alkyl radicals. 

The decomposition of diacyl peroxides (36) is catalyzed by various transition metal salts,46,167 for example, Cu+ (Scheme 3.28).168,169 A side reaction is oxidation of alkyl radicals by the oxidized fonn of the metal salt e.g. Cu2+). 

The decomposition of the peroxyketals (53) follows a stepwise, rather than a concerted mechanism. Initial homolysis of one of the 0-0 bonds gives an aikoxy radical and an a-peroxyalkoxy radical (Scheme 3.36).306"08"210 This latter species decomposes by P-scission with loss of either a peroxy radical to form a ketone as byproduct or an alkyl radical to form a peroxyester intermediate. The peroxyester formed may also decompose to radicals under the reaction conditions. Thus, four radicals may be derived from the one initiator molecule. 

In the stepwise decomposition of azo-compounds such as 4, products can arise from reactions within the primary diazenyl-alkyl radical pair or from the secondary radical pair produced by loss of nitrogen from the... 

A general reaction mechanism for the grafting of MA onto EPM is given in Figure 13.3 [15,16]. Free-radical grafting of MA starts with the decomposition of the radical initiator, usually a peroxide [15,18]. The peroxide decomposes at elevated temperamres into the corresponding oxy radicals, which may further degrade to alkyl radicals and ketones. These oxy and alkyl radicals abstract... 

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

Although Ce(IV) oxidation of carboxylic acids is slow and incomplete under similar reaction conditions , the rate is greatly enhanced on addition of perchloric acid. No kinetics were obtained but product analysis of the oxidations of -butyric, isobutyric, pivalic and acetic acids indicates an identical oxidative decarboxylation to take place. Photochemical decomposition of Ce(IV) carbo-xylates is highly efficient unity) and Cu(ll) diverts the course of reaction in the same way as in the thermal oxidation by Co(IIl). Direct spectroscopic evidence for the intermediate formation of alkyl radicals was obtained by Greatorex and Kemp ° who photoirradiated several Ce(IV) carboxylates in a degassed perchloric acid glass at 77 °K in the cavity of an electron spin resonance spectro-... 

The mercuric hydride formed by reduction undergoes chain decomposition to generate alkyl radicals. 

Alkanes are formed when the radical intermediate abstracts hydrogen from solvent faster than it is oxidized to the carbocation. This reductive step is promoted by good hydrogen donor solvents. It is also more prevalent for primary alkyl radicals because of the higher activation energy associated with formation of primary carbocations. The most favorable conditions for alkane formation involve photochemical decomposition of the carboxylic acid in chloroform, which is a relatively good hydrogen donor. 

The mode of fission of some azo compounds into alkyl radicals and nitrogen has been studied by Pryor and Smith<8) using the following postulates (1) A molecule that decomposes by a concerted scission of both C—N bonds will not undergo cage return and will have a rate constant independent of viscosity (2) a molecule that decomposes by a stepwise scission of the C—N bonds can undergo cage recombination and the rate constant for decomposition will decrease with solvent viscosity increase provided that the lifetime of the radicals produced by the initial homolysis is of the same order... 

Simple alkyl radicals are very much more reactive, and were first studied systematically only in 1929. The radicals were generated by the thermal decomposition of organometallic compounds, such as PbMe4,... 

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) ... 

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

---