The electrolytic conductance of some tris( ortho-, meta- and para-substituted phenyl)arsine–halogen adducts in acetonitrile solution-A comparative study

Abstract

A conductometric titration study has been done of the tris( meta-substituted phenyl)arsine–halogen systems, ( m-YC 6H 4) 3As–X 2, and the tris( ortho-substituted phenyl)arsine–halogen systems, ( o-YC 6H 4) 3As–X 2 [Y=CH 3, F, and CF 3; X 2=Br 2, I 2, and IBr]. For every ( m-YC 6H 4) 3As–X 2 system studied the conductometric titration graph was similar in type to that of the corresponding ( p-YC 6H 4) 3As–X 2 system (reported earlier), whereas with the ( o-YC 6H 4) 3As–X 2 systems correspondence with the ( p-YC 6H 4) 3As–X 2 systems was only partial. In the ( m-YC 6H 4) 3As–X 2 systems examined, the titrations provided evidence for the formation of eight adducts in acetonitrile whilst in the corresponding ( o-YC 6H 4) 3As–X 2 systems only four adducts were indicated. All four of the 1:1 adducts [( m-YC 6H 4) 3As–Br 2 (Y=CH 3, F, CF 3) and ( o-CH 3C 6H 4) 3AsBr 2] and some of the 2:1 adducts [( m-CH 3C 6H 4) 3AsX 4 and ( o-CH 3C 6H 4) 3AsX 4 (X 2=Br 2, I 2, IBr)] which were shown by conductometric titration to form in acetonitrile have been isolated as crystalline solids. Measurement of the electrolytic conductance of the dichlorides, ( m-YC 6H 4) 3AsCl 2, and dibromides, ( m-YC 6H 4) 3AsBr 2, in acetonitrile gave molar conductivity values typical of weak electrolytes. The Hammett plots show that for each series there is a good correlation between molar conductivity and the electronic effect of substituent Y. Molar conductivity values for the available tris( ortho-substituted phenyl)arsine dihalides [( o-CH 3C 6H 4) 3AsX 2(X=C1, Br), ( o-FC 6H 4) 3AsC1 2, and ( o-CF 3C 6H 4) 3AsC1 2] are substantially higher than those of the corresponding meta- and para-substituted compounds; this enhancement of molar conductivity is attributed to the occurrence of a significant steric effect of the ortho substituent in these compounds.