Kinetics of Aromatic Nitration

Heterogeneous nitration rates of the aromatic organic compound 1, 3-bis(trifluoromethyl)benzene in a well-agitated batch system have been measured in an extensive range of very concentrated mixed acids (aqueous HN0 3 and H 2 SO 4 ), with each run at essentially constant acid composition.The observed conversions correlated well with a firstorder dependence on the fraction of aromatic unconverted.Solubilities of the aromatic compound have been measured over the range of acid compositions.By eliminating solubility from the kinetic results, the apparent rate coefficient along the H 2 so 4 -H 2 0 base line is found to have a maximum value near 92 weight-percent H 2 SO 4 .The coefficient also exhibits a maximum on the HN0 3 -H 2 SO 4 base line at approximately 55 weight-percent H 2 so 4 .The nitration rates are correlated in a manner that indicates, at any given mixed-acid composition, (a) the nonideality of the reaction medium expressed by the activity -coefficient behavior; and (b) the ability of the medium to convert HN0 3 to N0 2 +, the attacking species in the nitration sequence.To apply this correlation to aromatics having varying resistance to nitratio:p., the correlation has been extended to the dilute mixed-acid region, using equilibrit1m equations based on the nitronium-ion mechanism.This correlation is generally applicable, except for effects that are not a direct result of the state of the acid medium--such as a change in aromatic solubility, a change in aromatic reactivity through complexing or protonation, or a change in the nitration mechanism characteristic of the particular aromatic.From the nitration kinetics, values of the Hammett acidity function at 25°C are estimated for mixed-acid compositions, as interpolations between previously determined experimental values along the HN0 3 -H 2 0 and H 2 so 4 -H 2 0 base lines.