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Free radicals are produced by the reaction of hydroxyl radicats and amino-radicals with alkenes or halogenated alkenes. in each case the e.s.r. spectra are attributable to radicals formed by primary addition of an OH or NH, fragment to the olefinic double bond. chemical and Polymer Laboratory, Runcorn, Cheshire HAVING shown1 that the addition of *NH, or *OH radicals to ethylene, results, initially, in the formation of a free-radical species X-CH,cH, (X = NH, or OH), we now report an e.s.r. study of the analogous reactions with propene, but-2-ene, butadiene, crotyl alcohol, vinyl fluoride, and vinyl chloride, The addition of *NH, radicals to a range of vinyl compounds has been studied previously by Corvaja et aZ., and by F i ~c h e r . ~ EXPERIMENTAL An aqueous flow system of the type described by Dixon and Norman 4 was used. The e.s.r. spectra were recorded with a Varian V4500-10A e.s.r. spectrometer equipped with

The reduction of a number of substituted nitrobenzenes by electron-transfer from organic donors in aqueous media, in a flow apparatus, is described. The electron spin resonance spectra have been interpreted in terms of ion-pair formation between the electron donor and the nitro-compound.THE ease with which organic nitro-compounds can accept an electron has been demonstrated in a number of electron spin resonance studies. The electron donor may be an alkali metal,1-3 a cathodic surface during electrolysis,4 or a suitable donor anion such as hydroxy-methyleneJ6 alkoxide,' or the radical anion derived from alkaline solutions of sodium dithionite 8~~ among others.In the reduction of aromatic dinitro-compounds , radical anion spectra are often produced in which major coupling to only one nitrogen is seen, coupling to the other being extremely weak, in contrast to what might have been expected from the symmetry of the neutral nitro-aromatic compound. These results have been interpreted 2.13 in terms of ion-pair formation, although strong evidence for this only exists in the case of ionpairs formed between alkali metals (e.g., Na) and a dinitro-aromatic c o m p o ~n d . ~~~~Other systems also show such anomalous behaviour8~~Jl and it was in an attempt to understand these more fully that we have re-investigated the reduction of aromatic nitrocompounds using flow methods similar to those described previously.6The results provide, we suggest, evidence for the previously unobserved ion-pair formation between a nitro-aromatic compound and an organic electron donor molecule. EXPERIMENTALTitanous-Hydrogen Peroxide-Methanol Systems.-An aqueous flow system of the type described by Dixon and Norman 16 was used i n conjunction with a Varian V4500-10A e.s.r. spectrometer, equipped with 100 kc./sec. field modulation. The solution concentrations were as follows : A , Acidified (0. ~M-H,SO,) aqueous titanous chloride solution ( 0 . 0 2 ~) ; B, 4 ml. of 100 vol. hydrogen peroxide per 1. of 0.lM-aqueous sulphuric acid. Solution B contained 10-20~0 of methanol and sufficient nitro-compound was added to give a saturated solution. The reduction of nitrobenzene, m-and $-nitrobenzoic acid, p-nitrobenzyl

The e.s.r. spectra obtained by reduction of m-dinitrobenzene and 3.5-dinitrobenzoic acid with aqueous alkaline sodium dithionite have been re-investigated. In addition to the dominant interaction with one nitrogen nucleus which has been previously reported, a further splitting of 0.24 gauss due to the other nitrogen nucleus has been resolved in both spectra. The spectra exhibit line-width effects characteristic of the slow interconversion of two asymmetric conformers of the nitro-radical anion in which the environments of the nitro-groups are exchanged. A mean lifetime of 1 .I x 1 0-6 sec. has been estimated for the conformers of the 3,5-dinitrobentoate anion radical. The asymmetry of these conformers is attributed to strong interaction between the solvent and one nitro-group.

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