Electron spin resonance studies. Part XXXIII. Evidence for heterolytic and homolytic transformations of radicals from 1,2-diols and related compounds

Abstract

The reactions of the hydroxyl radical with a number of compounds containing the unit-CHX-CHY-, where X = OH or OMe and Y = OH, OAc, Halogen, or a protonated amino-group, have been studied by e.s.r. spectroscopy. The transient species involved have been identified on the basis of their hyperfine splittings and g-factors; in the case of the radical *CH(OH)*CH2NH3+ and its derivatives, clear trends in the proton and nitrogen splittings are observed as the NH hydrogen atoms are successively replaced by methyl groups, and these have been rationalised in terms of an interaction involving both hydroxy-and amino-substituents.The data not only support theview that radicalscontaining the fragment-C(0H)-C(OH)< undergo acid-catalysed elimination of water but also reveal further details of this reaction and information about related heterolytic processes; for example, there is evidence that the radical cation M e k H -t H, is formed transiently from the radical .CH(OMe)-CH,X (X = CI or OAc). Amines do not appear to fragment heterolytically from radicals of the type -CH(OH).CH2NR3+, but the formation of the species Me,"+ from the choline cation suggests that the C-N bond in the species *CH (OH)*CH2NMe3+ readily undergoes homolysis.previously been adduced, from e.s.r. generated by y-irradiation of an aqueous solution of the spectroscopic observations, that when ethylene glycol is oxidised with the hydroxyl radical [from the titanium-(111) ion-hydrogen peroxide system or photolysis of hydrogen peroxide 3], the first-formed radical (1) undergoes dehydration to give the radical (2) in an acidcatalysed reaction (i).2,3 A number of related transformations has been documented,2 and support for the occurrence of reaction (i) has subsequently been obtained from an investigation in which the radical (1) was

Redox reactions with ferrocene/ferricenium species of radicals derived by H atom abstraction from alcohols and diols

“…If only one radical species were produced and it were capable of reducing Fc+ but with a sufficiently low rate constant for the reaction equivalent to step [5] that other processes such as combination or disproportionation could compete, eq. [6] would still be applicable with f representing the fraction of all radicals that actually do react in this manner. However, it is not to be expected that f would be independent of RH concentration, in that a much greater efficiency of the redox reaction is to be expected at low RH concentrations, where far fewer radicals Re are produced than femcenium zwitterions, than at high RH concentrations, where the two species are generated in comparable amounts.…”

“…[6] and [7] to the value of AA found at the end of the flash. On this basis, the relative rate constant k3/k2 was determined for each alcohol, and using, as previously (I), the value (12) of 1.09 X 10'' dm3 mol-' s-' for k2, the rate constant for the reaction of the hydroxyl radical with the ferrocenyl anion, the rate constants for the reaction of OH with these alcohols were found to be, for tert-butanol, 7.6 x lo8; 2-methylbutan-2-01, 1.8 x lo9 and 2,3-dimethylbutan-2-01, 2.4 x lo9 dm3 mol-' s-I.…”

“…Furthermore, the correspondence between eq. [6] and these 1-propanol data was tested using the relevant reciprocal plot, pertaining to eq. [7].…”

“…Some (4)(5)(6) have used flow systems and ESR spectroscopy to identify the radicals or mixture of radicals formed. Others have been primarily concerned (7)(8)(9)(10)(11) with the rate constant for OH attack and these mostly used a pulse radiolysis technique, employing the effect of an added alcohol in reducing the fraction of OH radicals reacting with another solute, where such.…”

Elimination of Ammonium Ion from the a-Hydroxyalkyl Radicals of Serine and Threonine in Aqueous Solution and the Difference in the Reaction Mechanism

“…Unauthenticated Download Date | 6/7/19 3:38 AM determining C-N bond breakage and rather belongs into the first mentioned class of radicals (see reaction (1)). In view of the close similarity of radicals 4 a and 4b this result is quite unexpected.…”