The reactions of formic acid with and without addition of sodium formate with diphenylmethanol (4) and chlorodiphenylmethane (3) were compared to those with hydroxydiphenylacetic (benzilic) acid (12a) and chlorodiphenylacetic acid (14a). Formic acid did not favour any S(N)1-type reaction on 4, but a strong catalysis by iodide ion was observed. Sodium formate rapidly performed the substitution of the chlorine in 3. A similar outcome was obtained with chloro acid 14a, but the rationalization of the results is different. Chloro acid 14a and its methyl ester 14b were prompt to react, but the equilibria were shifted to alpha-formyloxy products 13 only by the addition of HCOONa. HCOOH was unable to perform any reduction on either 3 and 4 or 12a and 14a, a fact which was taken as evidence for concerted substitution mechanisms on ion pairs or betaine 15. Mechanistic implications are drawn.
