A kinetic study of chlorine-isotopic exchange between lithium chloride and nitro-derivatives of chlorobenzene

Abstract

The kinetics of exchange between nitro-derivatives of chlorobenzene and lithium chloride labelled with ³⁶Cl have been measured. The second-order rate constants for 1-chloro-2,4,6-trinitrobenzene increase in the solvent sequence: methanol < ethanol < propan-2-ol–20% water < n-propanol < propan-2-ol–10% water < propan-2-ol < t-butyl alcohol acetone. 1-Chloro-2-nitrobenzene and 1-chloro-4-nitrobenzene were measured for sulpholan solutions in the temperature range 443–474 K. Other derivatives of chlorobenzene with two or three nitro-substituents gave convenient rate constants in acetone solution at normal temperatures. An ortho-nitro-substituent generally enhances the rate of chlorine isotopic exchange by ca. 4 × 10⁴, and a para-nitro-group by ca. 10⁶. The rate enhancement of nitro-groups in 1-chloro-2,4,6-trinitrobenzene is lowered by alkyl substitution in the 3- and 5-positions. A methyl or ethyl substituent reduces the rate of exchange by a factor of ca. 180, an isopropyl group by ca. 4·5 × 10³, a t-butyl group by ca. 8 × 10⁴, and methyl groups at both meta-positions by ca. 3 × 10⁵. The importance of steric factors in these reactions is discussed.