Electron spin resonance studies of electron-transfer reactions with aromatic nitro-compounds in aqueous media

Abstract

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

One-electron oxidation of .alpha.-monoalkoxyalkyl radicals by tetranitromethane via an intermediate adduct. Influence of the radical structure on the rate of decomposition of the adduct

“…The methoxymethylene carbocation is suggested to react with water to yield a proton and a semiacetal which decomposes to formaldehyde and methanol (reactions 3 and 4). The N02 formed in reaction 2 disproportionates26 to CH3OCH2+ + H20 -CH3OCH2OH + H+ (3) CH3OCH2OH -CH3OH + CH20 (4) yield nitrite and nitrate. On the basis of the pulse conductivity experiments, the half-life for reaction 3 is <10 MS.…”

“…As shown in Table II and Figure With N20-saturated aqueous solution containing 0.1 M 1,4-dioxane and 0.5 mM TNM, pulse radiolysis experiments were additionally performed by using conductometric detection. At pH [4][5][6] the production of NF", as monitored optically at 350 nm, was found to be accompanied by quantitative formation of H+ which was complete within 10-20 ps after the pulse. Since the rate of formation of H+ was equal to that of NF", the lifetime of a carbocation produced by decomposition of the dioxanyl-TNM adduct is <1-2 ps.…”

“…At 5 ms after the pulse, the total yield of NF" was equal to that of OH + H. On the basis of the 1:1 transformation of the transient absorbing at 300 nm into NF", e(transient)300 nm = 10 800 M"1 cm"1. The buildup or decay kinetics and the yields of the fast and slow processes were independent of dose (0.2-2.0 krd) and independent of pH (pH [4][5][6][7][8][9][10][11][12][13].…”

Electron spin resonance studies of radiolytically produced radicals in aqueous nitroalkane solutions

“…(CH2OH ^CH20-+ H+) (9) ch2=no22-+ H20 -> CH3N02- (10) Reactions 9 and 10 account for the increase in the yield of the nitromethane anion radical in the presence of methanol. A similar electron-transfer reaction was observed in an alkaline nitrobenzene solution.…”

Optical spectra and reactivities of radical anions of 4-nitrobenzyl compounds produced by pulse radiolysis of acetonitrile solutions

“…This radical anion has also been observed in esr experiment~.28$~9931 Few other reports of spectra of anion radicals of 4-nitrobenzyl compounds exist in the literature. [29][30][31][32][33] We tentatively assign the initial spectra observed upon pulse radiolysis of acetonitrile solutions of 4-nitrobenzyl fluoride, 4-nitrobenzyl chloride, 4-nitrobenzyl bromide, 4-nitrobenzyl iodide, 4-nitrobenzyl alcohol, and 4-nitrobenzyl acetate to the corresponding radical anions on the basis of the known radiation chemistry of acetonitrile and the behavior observed with 4-nitrotoluene. With the alcohol, acetate, and fluoride similar spectra were observed with a progressive shift of the maxima to lower energies, possibly resulting from a (T--x interaction similar to the examples reported by Traylor, et ~1 .…”