TRANSMISSION OF ELECTRONIC EFFECTS - PROTON MAGNETIC RESONANCE CHEMICAL SHIFTS FOR BENZYL HALIDES IN SEVERAL SOLVENTS

Proton magnetic resonance (pmr) shifts are reported for substituted benzyl fluorides, chlorides, and bromides in several solvents. At finite concentrations, molecules of the same or different compounds interact specifically. Thus the apparent association constant for benzyl chloride and p-nitrobenzyl chloride in carbon tetrachloride at ca. 37° is K = 0.27 M-1 Obviously, comparisons of substituent effects should be made with chemical shifts at infinite dilution (V0). Even so, each V0 of the benzyl halides still reflects specific interactions with the solvent peculiar to itself. Attempted correlations of v° of the benzyl halides according to Hammett or Yukawa-Tsuno relations are poor. A literature survey of ca. 100 Hammett correlations of pmr data indicates that the majority of them have unsatisfactory correlation coefficients, e.g., ˂0.90-0.95. The p values appear to have no pattern. However, the use of a polar solvent with certain families, e.g., DMSO for phenols and amines, gives large p values and excellent correlations; presumably enhanced electronic effects typical of chemical reactions dominate possible anisotropy effects on V0. In general, p's cannot be used in the way that reactivity p's are used, namely as indices of transmission efficiency in aryl systems Ar-T-H. © 1969, American Chemical Society. All rights reserved.