Iso-propyl radical

CA, not found 7) J. Smid M. Szwarc, Kinetics of Decomposition of Iso-Butyryl Peroxide and Reactions of Iso-Propyl Radicals , Syracuse Univ, NY, contract DA-30-115-ORD-678 (1958) 8) H.T. Lee et al, Evaluation of... 

In our optical studies, two types of IR spectra have been obtained. The first one was measured for ARS with cations including the shortest alkyl radicals, namely, methyl-, ethyl- and iso-propyl-radicals (Fig.l, 2). For this... 

Chilton, H. T. J., and B. G. Gowenlock Reaction of nitric oxide with gaseous hydrocarbon free radicals. Part I. Iso-propyl radical. J. chem. Soc. [London] 1953, 3232. 

Ueda et al. reported a tandem radical addition-cycUzation reaction in aqueous media [184]. This reaction was initiated by single-electron transfer from indium to an alkyl iodide. Fragmentation of the iso-propyl iodide radical anion generated the iso-propyl radical, which triggered the addition/cyclization tandem. Final SET and in situ hydrolysis delivered cyclic sulfonamides in good yield but low stereoselectivity. 

The proofs for the above constitutions are as follows (i) Thymol yields cymene, i-methyl 4-iso-propyl benzene, by loss of the hydroxyl oxygen by means of phosphorus penta-sulphide. (2) Carvacrol may be synthesized from potassium cymene sulphonate by fusion with potassium hydroxide. Therefore both must be mono-hydroxy cymenes. (3) Thymol by means of phosphorus pentoxide splits off the iso-propyl radical yielding meta-cresol and propylene. (4) Carvacrol by the same reaction yields ortho-cresol. Therefore in thymol the hydroxy group is meta to the methyl group while in carvacrol it is ortho. The following relationships are thus established. 

In the above analysis of the iso-propyl radical decomposition the micro-canonical rate coefficients were obtained from an experimentally... 

Table V shows the relative rates for addition at 150° and most of the data comes from Tables III and IV. However two radicals (CH2C1 and CBr3. ) are included in this Table for which no accurate temperature variation is available. The general pattern of results confirms the picture built up from the tables of relative Arrhenius Parameters. The relative rates are the results of direct measurement and therefore probably represent a more accurate summary. A very noticeable feature of Table V is the low rates of addition of heptafluoro-iso-propyl radicals to CHF- and CF2- sites. This strongly suggests that classical "steric hindrance" plays a significant role in free radical addition.

The A-factor ratios for the reactions involving perfluoro-iso-propyl radicals are no greater than those for other radicals. 

Although the larger body of work in the area of conjugate additions involves carbanionic intermediates, some remarkable examples of enantioselective addition of radicals have also been documented. Stereocontrol is effected through the use of a chiral Lewis acid that activates the electrophilic acceptor [38, 47). Sibi has reported that acceptor 212 undergoes enantioselective addition of iso-propyl radical in the presence of Mgl and bisoxazoline 213 to give 214 in 97% ee and 91% yield (Equation 40) [160]. A clear correlation between the catalyst loading and the level of asymmetric induction was observed, with 30 mol % identified as optimal. 

Based on the analysis of the reactions in Scheme 3 and on previous studies (46, 47), a mechanism for the reaction was proposed in which the /x-peroxo complex, 16, may simultaneously abstract two hydrogen atoms from iso-propyl groups on the pyrazolyl ligands. Alternatively, because of the weak 0—0 bond, 16 may homolytically dissociate to form two Tp"Co(0-) oxo-radical moieties, and these species would then abstract hydrogen from the iso-propyl groups. In either case, the resulting carbon-centered radical can either react with solvent, as was observed for the Tp complex (46), or with another carbon-centered radical so as to regenerate the Tp"Co(OH) complex and produce a derivative of the Tp" complex with an iso-propenyl substituent, 18. Ultimately, either route would produce the (/x-OH)2 complex, 17. 

On the other hand, in a similar study recently carried out by Falconer et al. (41), no evidence was found for the presence of allyl radicals and the occurrence of reaction (23) appears unlikely. The products could be explained by assuming initial formation of hot iso- and n-propyl radicals. As will be discussed in the next section, 94% of iso- and 6% of propyl radicals are formed in this reaction. The further fate of both these radicals has to be considered, since hot n-propyl decomposes readily into C2H4 and CH3 but the same is not true for iso-propyl, where a simultaneous internal migration of an H atom must occur. 

Falconer and Cvetanovic (40) attempted to obtain a more quantitative value for the fraction of nonterminal addition in the case of propylene. They produced hydrogen atoms by mercury photosensitized decomposition of H2, using at least 100 times as much H2 as C3H6 and total pressures of 40 and of 250 mm. Under these conditions the reactions of importance were the combination and disproportionation of the iso- and n-propyl radicals and their cross reactions, the combination of the two radicals with H atoms (assumed to be equally probable), and a very small amount of decomposition of hot n-propyl radicals. Disproportionation to combination ratios were taken as 1.64 for two iso-propyl, 1.14 for two w-propyl, and hence 1.39 was taken as the mean of the two values for one iso- and one n-propyl radical. Using these values and the analysis of the products, the nonterminal addition of H atoms to C3H6 and C3D6 was found to amount to 6 1%. 

Another family of inhibitors, hydrogen-donating agents such as iso-propyl and 1,4-di-ivo-propyl benzenes, was investigated by Lamouroux in order to reduce the formation of TBP-TBP dimers, which exhibit a very high plutonium retention of TBP (47). The presence of at least one mobile hydrogen on the iso-propyl group could produce a benzylic tertiary radical stabilized by resonance. The addition of such compounds reduced the concentration of the TBP-TBP dimers by about 50%. 

A group of perfluoroalkyl radicals which do exhibit marked increases in reactivity due to enhanced electrophilicity are the branched, 2° and 3° perfluoroalkyl radicals, specifically the perfluoro-iso-propyl and tert-butyl radicals [118]. 

Table 8 provides the absolute rates of addition of (CF3)3C, (CF3)2CF-, CF3CF2-, and CF3 to a group of alkenes of variable reactivity. It can be seen from the Table that both the perfluoro-iso-propyl and perfluoro-ferf-butyl radicals give evidence of much greater electrophilicity in their alkene addition reactions. For example, the latter radical reacts significantly (6.8 times) faster than CF3- with the nucleophilic a-methylstyrene (IP = 8.9 eV), while reacting somewhat (1.6 times) slower than CF3- with the more electrophilic pentafluorostyrene (IP = 9.2 eV). Comparative plots of all of the available rate data for alkene additions of CF3- and (CF3)3C- vs alkene IPs, as seen in Fig. 4, leave no doubt as to the relative electrophilicity of the two species. Moreover, the rates of addition of the very electrophilic, but non-o, planar perfluoro-ferf-butyl radical to the more nucleophilic... 

Table 8. Absolute rate constants for the addition of trifluoromethyl, pentafluoroethyl, heptafluoro-iso-propyl, and nonafluoro-ferf-butyl radicals to various olefins at 290 °K in FI 13 [117,118]...

Scheme 23 Hydroxyl radical-selective trapping reaction with y - locopheryl - iso-propyl ether (56) and y-tocopheryl ether derivatives (58, 59) showing different reactivity towards radicals...

To avoid repeating the argument on this point later we include here a reference to the iso-propyl and /-butyl radicals which we have already briefly discussed in connection with Fig. 4. It has been pointed out by Wheland that the number of resonating forms increases in the series ethyl, iso-propyl, /-butyl from three to six and to nine. This should cause a steady increase in resonance energy and may explain the observed steady fall in the bond energy. 

Propyl and Iso-propyl Benzenes.—In addition to the three trimethyl benzenes we still have three isomeric hydrocarbons of the composition C9H12. These compounds are isomeric, depending on the substitution in benzene of other radicals than methyl. Substitution of one propyl radical for one benzene hydrogen atom gives us a compound of the same composition as that obtained by substituting three methyl radicals for three hydrogen atoms. As the propyl radical has two isomeric forms, viz., that of normal propyl and that of isopropyl, so we have the two substitution products, propyl benzene, CeHs—CHa—... 

Fig. 12. Lifetimes of propyl radicals (n- and iso-) in the gas phase in reactions of Fl-atom and CH3-radical elimination.

The oxidative degradation of triazine herbicides like Atrazine [2-chloro-A4-ethyl-V6-iso-propyl-1,3,5-triazine-4,6-diamine (15)] can be effected by hydroxyl radicals generated from ozone, metal oxides or hydrogen peroxide/iron(II) salts (Fenton s reagent).22... 

Similar radicals from n-propyl, iso-propyl, and f-butyl acetates also studied. M Other adsorbed species also studied. 

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