STRUCTURAL EFFECTS ON CHARGE DELOCALIZATION IN BENZYLIC ANIONS

The pK(a)'s of the benzylic ketones 2-indanone (1), 2-tetralone (2), and 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 species 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 carbons of the anions. These correlations indicate that the order of negative charge delocalization into the ring for the anions is 1 > 2 > 3. This conclusion agrees with calculations of increments of negative charge distribution in the anions. It appears that the anions of 2-tetralone and 2-benzosuberone prefer to bend out of the plane of the aromatic ring at the expense of orbital overlap rather than remain planar at the expense of increased internal strain energy.