2-Butyl radical, decomposition

D. W. Placzek, B. S. Rabinovitch, and F. H. Dorer, Intramolecular energy relaxation. Butyl radical decomposition at high pressure, J. Chem. Phys. 44 279 (.1966). 

A singlet pair of <-butyl radicals produced by peroxide decomposition disproportionate to yield isobutane and isobutene [equation (41)]. Both products show E/A multiplet effects. 

This retardation of ethyl tert-butyl peroxide decomposition may possibly be caused by competition between the inhibitor and the peroxide for methyl radicals (Reactions 1-4). 

The para-nitro ester 71d generated only 4-nitroaniline (70%) and 4,4 -dinitroazoxybenzene (10%) when it underwent decomposition (Scheme 29). These products could have been derived from either a triplet nitrene or a triplet nitrenium ion precursor. Homolysis of the N—O bond to generate radical intermediates was ruled out because of the nearly quantitative yield of pivalic acid derived from 71d. The pivaloxy radical would have undergone rapid decarboxylation to generate CO2 and the rert-butyl radical under these conditions. Since no rearrangement product was observed, it was tentatively concluded that this ester underwent direct decomposition to 4-nitrophenyl-nitrene without the intermediacy of a nitrenium ion. ... 

Allowance must also be made for some oxidation of the 3-keto -2-butyl radicals since the yield of ethylene is less than expected if the thermal decomposition (Reaction 2) were the only fate of the 3-keto-2-butyl radicals. 

Rabinovitch et al. (85) studied the reaction of H atoms with trans-ethylene-d2 as a function of ethylene pressure in the temperature range — 78 to 160°C. They were able to account for all secondary reactions of the hot ethyl radicals and to determine the rates of their decomposition (relative to stablization). Simultaneously they calculated the theoretical rates on the basis of the Rice-Ramsperger-Kassel theory of uni-molecular reactions, using expressions derived by Marcus (71), and found a reasonable agreement with the experimental values. Similar satisfactory agreements had been found previously by Rabinovitch and Die-sen (84) for hot sec-butyl radicals. Extensive studies of hot radicals produced by H or D atom additions to various olefins have been carried... 

Comparison of the cyclic systems in Table 17 leads to the opposite conclusion, however the destabilization of the 1-norbornyl radical relative to the 1-adamantyl is less for the azo decompositions. Perhaps the mechanism of the azo decompositions of the more unreactive systems is different from that of, for example, the f-butyl azo compound (i.e. the rate determining step of the 1-norbomyl azo compound may be a one bond homolysis rather than the synchronous two bond fission of the f-butyl system312, 315)). Also, the smaller 1-norbornyl/1-adamantyl rate ratio for the f-butyl perester decompositions may be due to a greater influence of polar effects in these reactions 309a). This problem is under active investigation 309a). 

Thus the primary reaction in di-fe/f-butyl peroxide decomposition is a usual decomposition reaction in the scheme (1.10) the secondary reaction induced by the initial one is described by the scheme (1.11). It is worthy of note that both decomposition reactions are described by the same overall equation, and free radical is the general intermediate particle. 

Thermal decomposition of butane involves the unimolecular decomposition of sec- and n-butyl radicals as chain carrier steps, but little quantitative information can be obtained from the work. The best estimation of the A factor for the thermal decomposition of butyl radical is based on the high-temperature photolysis of 2-methylbutanal and has the value, log A — 15.32.64 Corresponding values for w-propyl radicals2 were 15.3655 and 13.9.62 In view of the complexity of these experimental systems, these compare reasonably with the value of 14.35 in Table XX. 

The photochemical dissociation of di-(-butyl peroxide appears to proceed in a very similar way to the thermal decomposition. Dorfman and Salsburg47 photolyzed di-(-butyl peroxide using 2537 and 2650 A. radiation between 25 and 75°C. They found that the main products were ethane and acetone in relative yields of 1 2. With low intensity radiation, some methane was obtained and, even though acetone was removed to prevent its reactions becoming appreciable, (-butanol was produced. The evidence is therefore very strong that di-f-butyl peroxide decomposes by the usual breaking of the peroxide 0—0 bond and the (-butyl radicals then rapidly decompose to give acetone and methyl radicals, i.e.,... 

A reported synthesis (13) of tert-biltyldifluoramine capitalized on the equilibrium dissociation of tetrafluorohydrazine into NF2 free radicals (9) by generating tert-butyl radicals via the decomposition of azoisobutane in the presence of tetrafluorohydrazine. Azoisobutane has been synthesized by two methods (4,16). Using the more efficient of these methods (16), which gave us a 30% yield of the intermediate, the overall yield of tert-butyldifluoramine obtained in the two-step reaction sequence was only 6% of theoretical. 

To provide a less ambiguous method for measuring an olefin scavenger/unimolecular decomposition method was devised (143). When a 1 10 mixture of H C0/cis-2-C4H8 is photolyzed at wavelengths near 300 nm, the photolytic H atoms are scavenged by cis-2-butenes to form vibrationally hot sec-butyl radicals. 

A practical examle of this kind of measurement is the study by Sato and Otsu of di-tert. -butyl peroxalate decomposition in p-xylene as solvent in the presence of vinyl monomer and 2-methyl-2-nitrosopropane (at room temperature the initiator is decomposed into Me3CO and C02). Due to repulsive forces, the spin-trapping agents reluctantly react with oxyl radicals. Therefore the following reactions mutually compete... 

The other chain is produced by the decomposition of the sec-butyl radicals (reaction 6 ) to give methyl radicals + propylene. The chain cycle consists of reactions 6 and 8 and would be represented by the over-all stoichiometry... 

Ethylene arises from the ethylidene radical. The decomposition of jec-propyl radical to give a hydrogen atom and propene is taken to be the major source of hydrogen atoms this process is not distinguishable from decomposition of i-butane to a hydrogen atom and a butyl radical followed by decomposition of the latter to a methyl radical and propene. 

Propene formed in the photolysis of neo-CjHij-neo-CjDij mixtures is C3H6 and C3D6, indicative of a molecular formation. The decomposition of the t-butyl radicals to form propene and a methyl radical seems the likely source the overall reaction being... 

Another minor reaction product is methane. It appears to be a secondary product, being negligible at low conversions and of increasing importance as the reaction proceeds - . Sworski and Burton have indentified methyl radicals in the system these may originate either from the chain initiation step or from the decomposition of -butyl radicals. Recent identification of C3H6 among the products strongly supports the validity of the latter assumption. 

AH = 34.7+0.1 kcal.mole and AS = 10.5 0.2 eu, which are similar to homolysis of the peroxide bond in di-t-butyl peroxide (see the section on peroxides). Furthermore, pyruvic acid (CH3COCO2H) is a product of the reaction, which is an expected product from homolysis of the peroxide bond. Apparently decomposition via (X) is of minor importance compared to free radical decomposition of the peroxide linkage. 

Stringfellow and coworkers showed that the Bu AsHj and BU2ASH pyrolyses were accelerated by an increase in the t-butyl radical concentration. The independent t-butyl radical source was azotertiary-butane (BujNj). The self-pyrolysis temperatures for 50% decomposition were reduced to 350 °C for BujAsH and 300 °C for Bu AsH2 under the gas-flow conditions of MOCVD reactors. On the basis of analyses of the products of the pyrolyses they proposed the plausible mechanisms shown in Schemes 4 and 5. 

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