Influence of Aromatic Solvents on the Selectivity for Photochlorination of 2,3-Dimethylbutane with Molecular Chlorine

Article abstract of DOI:10.1021/ja00192a024

The effect of benzene and a wide variety of substituted benzenes upon the photochlorination of 2,3-dimethylbutane (DMB) has been investigated under standard conditions (0.15 M DMB, 2.0 M arene in CCl4 at room temperature).The standard selectivities, S_std^m, are given by the product ratios <2-ClDMB>/<1-ClDMB> under these conditions.For arenes which are less electron rich than benzene, log S_std^m values decrease monotonically with the increase in the arenes' ionization potentials and with the decrease in the arenes' ?-basicities.For arenes which are moreelectron rich than benzene, S_std^m values are greater than for benzene only for the mono- and dialkylated benzenes and 1,3,5-tri-tert-butylbenzene.Chlorination in the presence of trimethylbenzenes, more highly methylated benzenes, and anisole afforded less selective mixtures of DMB chlorides than benzene.The reduced selectivities of the most electron-rich arenes is attributed to the low reactivities of their Cl^./arene ?-complexes and the low yields of DMB chlorides, much of which are formed by hydrogen abstraction by the free Cl^. atom.Ipso substitution occurs with bromo- and iodobenzene and these two arenes are more selective than would be predicted.It has also been demonstrated by laser flash photolysis that Cl^./arene and Br^./arene ?-complexes will appear to react with O2 only if the arene contains substituents which can donate hydrogen to the halogen atom and/or complex, viz., CH3, C(CH3)3, and OCH3 substituents for chlorine but only CH3 and OCH3 for bromine.This apparent reaction is due to interference by O2 in the process which otherwise would very rapidly "regenerate" the halogen atom/arene complex followingsuch hydrogen abstraction.