The relative rates of hydrogen-abstraction by atomic bromine (from N-bromosuccinimide) at 80° have been determined for nuclear-substituted ethylbenzenes and benzhydryl methyl ethers, by means of intermolecular competitive reactions using, respectively, benzhydryl methyl ether and dibenzyl ether as reference standards. The results show, in the former series, a polar effect (ρ=–0·69) correlated by the σ+constants, and, in the latter series, the absence of such an effect.In attack by atomic bromine on the systems toluene, ethylbenzene, benzaldehyde dimethyl acetal, benzyl methyl ether, dibenzyl ether, and benzhydryl methyl ether, data are presented to show that the ρ-values bear a consistent inverse relationship to the relative reactivities of the systems.
The relative reactivities of nuclear-substituted ethylbenzenes, benzaldehyde dimethyl acetals, and diphenylmethyl methyl ethers toward the trichloromethyl radical at 80° have been determined by means of intermolecular competitive reactions. The results show a σ+ correlation by the Hammett equation with ρ-values of –0·53, –0·18, and –0·12, respectively, a decreasing polar effect consistent with the increasing reactivity of the three series.Relative reactivity data are presented to show that atomic bromine and the trichloromethyl radical exhibit similar selectivity in abstraction of benzylic hydrogen atoms. In contrast to atomic bromine, the ρ-values found for the trichloromethyl radical in a number of systems bear no consistent inverse relationship to the relative reactivities of these systems.
The e.s.r. spectrum of the radical PhCHCH2F has been studied for a variety of solvents and over a range of temperature. INDO calculations have been carried out for several conformations of the radical. The evidence suggests that rotation about the C–CH2F bond is hindered, with one of the rotamers strongly destabilized by steric interaction between the fluorine atom and the phenyl group. E.s.r. spectra of a number of related radicals have also been studied. Rotation of CH2F and CHF2 substituents is hindered, and some of the radicals appear to be significantly non-planar at the radical centre.
Enhancement of the reactivity of benzaldehyde relative to cyclohexane towards sulphuryl chloride at 40° in CCl4 was demonstrated by modifying the reactivity of the hydrogen-abstractor radical in the rate-determining step with added benzene and t-butylbenzene which are able to complex with atomic chlorine. Relative rates of chlorination of substituted benzaldehydes with SO2Cl2 have been determined in benzene, and in 2M-PhBut in CCl4 at 40°. The Hammett correlations with σ constants (ρ–0·68 and –0·86, respectively) are attributed to the selective benzene and t-butylbenzene π-complexed chlorine atom.
E.s.r. parameters are reported for 11 1,3-dioxolan-4-yl radicals each with two substituents at the 2-position. When the two substituents are different, the β-protons appear non-equivaient, and for some of the radicals selective broadening of the lines with M= 0 for the β-protons is observed. The implications of these observations are discussed. Some of the radicals show long-range splittings due to fluorine nuclei. These splittings are discussed in relation to splittings calculated for selected conformations by the INDO MO method.
Abstract Es wird die H‐Abspaltung durch atomares Brom, durch CCl 3 ‐Radikale und durch einen Chlor‐Benzol‐n‐Komplex untersucht, wobei Aryl substituiertes Phenyl ( ;z.B. mit tert.‐Butyl als 4‐ und C1, F, Br als 3‐ bzw. 4‐Substituenten) ist.
The relative rates of benzylic hydrogen abstraction from XC6H4CH2CH2Cl by atomic bromine, the trichloromethyl radical, and by the benzene-complexed chlorine atom have been measured by means of competitive reactions with N-bromosuccinimide, with bromotrichloromethane in carbon tetrachloride at 80°, and with sulphuryl chloride in benzene at 40°. The results, correlated by the Hammett equation, gave ρ values of –1·10, –0·93, and –0·76, respectively by use of σ+ constants for atomic bromine and σ constants in the other two cases.
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