Isopropyl radical, disproportionation

While [Pt2(pop)4 ] is a strong reductant, it is not sufficiently reducing to transfer one electron to an alcohol (27). Extraction of a hydrogen in a primary photoprocess would produce an isopropyl radical that could undergo disproportionation to yield acetone (28) ... 

The l-ethyl-4-carbomethoxypyridinyl radical disproportionation products, aside from the pyridinium ion, probably included ethylamine, and a compound thought to be an ester dialdehyde (Eq. 31). The pH changes noted after radiolysis of buffer-free isopropyl alcohol-water solutions of the pyridinium ions revealed that the initial dihydropyridine 4 H) formed from 4 hydrolyzed and consumed another proton, presumably because of ethylamine. 

However, this is probably an oversimplification, as the disproportionation reaction is surface-catalyzed and the two reactions must have different energies of activation. This behavior of dimethylamino radicals is in marked contrast to the iso-electronic isopropyl radicals, the disproportionation-combination ratio of which is unaltered by temperature and siuface (2, 30). The surface-promoted radical decomposition is also in marked contrast to the absence of such effects in the isopropyl radical reactions (2) and must be attributed to the presence of the nitrogen atom. Amino radicals also undergo surface reactions (28). The occurrence of TMA suggests the participation of methyl radicals in the reactions, and a possible route is provided by Reactions 28 and 34. 

These observations may be interpreted as being derived from a pentacyanocobaltate( II)-catalyzed disproportionation of an isopropyl radical intermediate (Radical Mechanism I) (Reactions 9-11). 

In a bid to prove that the ligand is the source of the hydrogen atoms, the reaction was repeated with the analogous Tp -complexes. We reasoned that hydrogen atom abstraction from the isopropyl groups would generated isopropyl radicals, which should disproportionate into isopropyl- and isopropenyl-substituted ligands. Reaction of Tp "Pf Co-02 with half an equivalent of [(Tp -Pf MeCo)2( 4-N2)l produced a mixture of products, one of which was the dinuclear hydroxide complex [(Tp Co)2(ti-OH)2]. Hydrolysis of the whole product mixture, and examination of the recovered pyrazoles indeed showed that 10 % of the pyrazole fraction was 3-isopropenyl-5-methylpyrazole. 

Detailed quantitative results for studies of the disproportionation/ dimerization ratio of ethyl and isopropyl radicals are given in Table 9. These sodium flame results suggest that the disproportionation reactions... 

Results of disproportionation/dimerization ratios of ethyl and isopropyl radicals... 

The transition state for disproportionation requires overlap of the p C—H bond undergoing scission and the p-orbital containing the unpaired electron.18 This requirement rationalizes the specificity observed in disproportionation of radicals 29 (Section 1.4,2) and provides an explanation for the persistency of the triisopropylmcthyl radical (33) and related species (Section 1.4.3.2).166 In the case of 33, the P-bydrogens are constrained to lie in the nodal plane of the p-orbital due to stcric buttressing between the methyls of the adjacent isopropyls. 

CH3)2CDOH it is slowed by a factor of ca 5. This confirms that the primary reaction of 3BA with isopropyl alcohol is abstraction of the methine hydrogen to give BAH, which is observed spectroscopically, and the 2-hydroxy-2-propyl radical. These radicals combine and disproportionate to form the stable products. 

An even more exaggerated behavior is presented by the branched radicals isopropyl, sec-butyl, terf-butyl for which the ratio of rate constants for disproportionation to recombination are, respectively, 0.64, 2.3, and... 

Isotopic experiments show that ethane is formed by association of methyl radicals. Propylene results from disproportionation of isopropyl... 

An unusually stable alkyl radical 1 was discovered in 1985. The radical was isolated in the course of the reaction of fluorine gas with perfluoro 3-isopropyl-4-methyl-2-pentene and 3 - ethyl-2,4- dimethyl-2-pentene at ambient temperature [ 1 ]. The radical 1 is stable at room temperature to dimerization, disproportionation, treatments with 02, Cl2, Br2, I2, or aqueous acid and base, and even heating at 70°C in GC. It only decomposes by elimination of a trifluoromethyl radical through (3-scission on heating at 100°C, with a half-life of 60 min. 

An increase of disproportionation at the expense of recombination when ing from primary to secondary and tertiary alkyl radicals may be expressed by the ratio 1 5 25. An almost equal ratio for disproportionation and recombination has been found for reactions of the methyl radical with ethyl, isopropyl and rer/.butyl radicals [13]. 

Polyisoprene has also been pyrolysed in an inert atmosphere and here the main products are isoprene and l-methyl-4-isoprenylcyclohexene. The latter compound can disproportionate to l-methyl-4-isopropyIbenzene and methyl-l-iso-propylcyclohexenes and this reaction is catalysed by Ziegler-Natta catalyst residues or by carbon black. The dominant initiation process is j9-chain scission with the formation of two allylic radicals. The kinetics of thermal decomposition have been studied for cis- and rraiw-1,4-polyisoprene and the copolymer of isoprene with 4-isopropyl-methyl styrene and also for isoprene polymers containing 4-CjH4—Z—4-C(H4— and —CjH4—Z—C5H4N=N— units, where Z may be O, CHj, SOj or a single bond. ... 

The bulky ter/-butyl substituent loses the free rotation of methyl groups during aminyl radical recombination. This results in the loss of entropy during the formation of the transition state and a decrease in the rate constant. Two isopropyl groups in the corresponding aminyl radical sterically hinder recombination and, therefore, disproportionation occurs. The transition state is rather compact in this case, and its formation is also accompanied by the entropy loss, which explains the low value of kj. 

Hughes, GA., and L. Phillips (1967), The kinetics of disproportionation-combination reactions between the isopropoxyl radical and nitric oxide, and the pyrolysis of the O—O bond in di-isopropyl peroxide, J. Chem. Soc., 1967, 984-897. 

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