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 [YC 6 H 3 (NH 2 ) 2 ; Y = F, Cl, Br, CH 3 , CH 3 O, CN, CF 3 , CO 2 CH 3 or NO 2 ] and 4,5-disubstituted-1,2-phenylenediamines [Y 2 C 6 H 2 (NH 2 ) 2 ; Y = F, Cl, Br or CH 3 ] with 3,3’- and 4,4’-disubstituted benzils [(ZC 6 H 4 )CO) 2 ; Z = F, Cl, Br, CH 3 , or CH 3 O] 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 = CH 3 O or CH 3 ) rather than electron-withdrawing (X = CN or CO 2 CH 3 ). Nevertheless, the overall rate of reaction is affected, as illustrated by the condensation of O 2 NC 6 H 3 (NH 2 ) 2 with [(ZC 6 H 4 )CO] 2 , which was too slow to permit the acquisition of reliable results.