Variation of Hammett reaction constant in the accelerated formation of substituted 2,3-diarylquinoxalines in nebuliser microdroplets.

The accelerated condensation of a range of 4-substituted-1,2-phenylenediamines [YC6H3(NH2)2; Y = F, Cl, Br, CH3, CH3O, CN, CF3, CO2CH3 or NO2] and 4,5-disubstituted-1,2-phenylenediamines [Y2C6H2(NH2)2; Y = F, Cl, Br or CH3] with 3,3'- and 4,4'-disubstituted benzils [(ZC6H4)CO)2; Z = F, Cl, Br, CH3, or CH3O] to form the corresponding substituted 2,3-diarylquinoxalines has been investigated by competition experiments. Good Hammett correlations with a reaction constant, ρ, in the range 1.36 to 2.07 are found in each series of condensations with a given Y and a range of Z, thus indicating that the electron density distribution at the carbonyl carbon atom of the substituted benzil increases during the rate-limiting step, as would be expected if this component behaved as an electrophile. However, the relatively small changes in ρ as Y is varied suggest that the mechanism of the condensation is only marginally altered if this substituent is electron-donating (X = CH3O or CH3) rather than electron-withdrawing (X = CN or CO2CH3). Nevertheless, the overall rate of reaction is affected, as illustrated by the condensation of O2NC6H3(NH2)2 with [(ZC6H4)CO]2, which was too slow to permit the acquisition of reliable results.