Acidities of benzyl phenyl sulfones and of the corresponding radical cations. Homolytic bond dissociation energies (BDEs) of αCH bonds in benzyl phenylsulfones

The excellent linearity ( R 2 = 0·997) of a plot of p K a values for 17 m− and p ‐substituted benzyl phenyl sulfones, GC 6 H 4 CH 2 SO 2 Ph, vs. those for the corresponding arylacetonitrile, GC 6 H 4 CH 2 CN, demonstrates that substituent solvation and substituent solvation assisted resonance (SSAR) effects for p ‐CN, p ‐COPh, and p ‐SPh are nearly identical in these two substrates. The PhSO 2 group in PhCH 2 SO 2 Ph increases the BDE of the α‐CH bond by 2 kcal/mol, relative to toluene. The α‐CH bonds in GC 6 H 4 CH 2 SO 2 Ph sulfones are stabilized by 1–2 kcal/mol by acceptor G′s ( m ‐CN, p ‐CN, m ‐CF 3 , p ‐CF 3 ), but weakened by 1 and 5 kcal/mol, respectively, by donors ( p ‐OMe and p ‐NMe 2 ). The GC 6 H 4 CH 2 SO 2 Ph +. radical cation with G = H has a p K = −25. Its acidity is increased when G is an acceptor by as much as 9 to 10 kcal/mol (G = 3‐CN, 3‐CF 3 , 4‐CF 3 , 4‐NO 2 ), but is decreased when G is a donor by as much as 33 kcal/mol (G = NMe 2 ). When G = 4‐SPh the radical cation is stabilized, relative to G = H, by a larger amount (25 kcal/mol) than when G = 4‐OMe (18 kcal/mol). Structural changes along the series PhCH 2 SO 2 Ph, 2‐naphthyl‐CH 2 SO 2 Ph, 9‐anthryl CH 2 SO 2 Ph cause negligible changes in the acidities of these acids, but sizable decreases in the acidities of the corresponding radical cations. Introduction of a phenylsulfonyl group into the methyl group of 9‐methylanthracene or the 9‐position of fluorene or xanthene increase the BDEs by 3, 2, and 7 kcal/mol, respectively. These effects of PhSO 2 groups are compared and contrasted with those of CN groups.