Nitration at the encounter rate in inert organic solvents

3 NITRATION AT THE ENCOUNTER RATE IN INERT ORGANIC SOLVENTS 

For nitrations in sulphuric and perchloric acids an increase in the reactivity of the aromatic compound being nitrated beyond the level of about 38 times the reactivity of benzene cannot be detected. At this level, and with compounds which might be expected to surpass it, a roughly constant value of the second-order rate constant is found (table 2.6) because aromatic molecules and nitronium ions are reacting upon encounter. The encounter rate is measurable, and recognisable, because the concentration of the effective electrophile is so small. 

A similar circumstance is detectable for nitrations in organic solvents, and has been established for sulpholan, nitromethane, 7-5 % aqueous sulpholan, and 15 % aqueous nitromethane. Nitrations in the two organic solvents are, in some instances, zeroth order in the concentration of the aromatic compound (table 3.2). In these circumstances comparisons with benzene can only be made by the competitive method. In the aqueous organic solvents the reactions are first order in the concentration of the aromatic ( 3.2.3) and comparisons could be made either competitively or by directly measuring the second-order rate constants. Data are given in table 3.6, and compared there with data for nitration in perchloric and sulphuric acids (see table 2.6). Nitration at the encounter rate has been demonstrated in carbon tetrachloride, but less fully explored.  

Compound nitrated t5% aqueous sulpholan (K) Sulpho- lan (C) 15% aqueous nitromethane (K) Nitromethane (C) 61-05 % perchloric acid (K) 68-3% sulphuric acid (K) 68-3 % sulphuric acid (C) Estimated relative rates  

Medard, L. Volkringer, M. (1933). C. r. hebd. Seanc. Acad. Sci., Paris 

Briner, E. Favarger, P. (1935). Helv. chim. Acta 18, 375. 

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