STRUCTURAL EFFECTS ON CHARGE DELOCALIZATION IN BENZYLIC ANIONS

The pK(a)'s of the benzylic ketones 2-indanone (1), 2-tetralone (2), a nd 2-benzosuberone (3) were determined in DMSO (16.70,17.64, and 19.4, respectively), along with the C-13 chemical shifts of the neutral spe cies and the anions. Good correlations were observed between the pK(a) 's and the chemical shifts of the para carbons and one of the ortho ca rbons of the anions. These correlations indicate that the order of neg ative charge delocalization into the ring for the anions is 1 > 2 > 3. This conclusion agrees with calculations of increments of negative ch arge distribution in the anions. It appears that the anions of 2-tetra lone and 2-benzosuberone prefer to bend out of the plane of the aromat ic ring at the expense of orbital overlap rather than remain planar at the expense of increased internal strain energy.