McKie, P. V. (1926)
The interaction between nitric acid and unsaturated compounds.
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The principal products of the interaction of fuming nitric acid with acetylene or ethylene are carbon dioxide and trinitromathane. As well, in the interaction with ethylene, and intermediate to the formation of trinitromethane, is formed the addition compound, nitroethyl alcohol. The course of reaction is fundamentally the same for both compounds, and takes place under varying conditions of temperature, concentration of acid and the presence or absence of metallic salts, though a marked effect - the simplification of the reaction and the reduction of by-products- is obtained by the addition of mercuric nitrate. Measurement of the quantities of carbon dioxide and of trinitromethane formed show that at a maximum only fifty per cent of the carbon of the hydrocarbon molecule is nitrated while fifty per cent is oxidised, and this is accompanied to a varying extent by a second reaction involving direct oxidation to carbon dioxide. The tetranitromethane which can be isolated by further nitration always represents considerably more than that which can be estimated as nitroform in the reaction mixture. Hence there are present in the product substances other than trinitronethane, but possibly intermediate to its formation, which are capable on further nitration of yielding tetranitromethane. Evidence is adduced that the reaction is in all cases one of simple addition to the unsaturated bond of the component parts of the nitric acid molecule, analogous to the addition to unsaturated compounds of the component parts of oxides of nitrogen. Hence, the nitric acid, through the medium of addition, is able to exercise its function both as a nitrating and as an oxidising agent, one carbon of the hydrocarbon molecule becoming nitrated, the other becoming oxidised, finally to carbon dioxide. The influence of mercuric nitrate is largely to increase the rate of absorption and hence of interaction. The mechanism of its effect is complex, but is largely to prevent the alternate reaction of simple oxidation, and hence to increase proportionally the addition reaction producing the nitro alcohol. A similar activating influence of the mercury salt is noticed in the nitration of aromatic compounds, and from this a theory of the mechanism of aromatic nitration is developed.
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Institution: University of London, Bedford College (United Kingdom).