The reduction of alpha-X-acetophenones (X = PhO, Br, Cl) in hydrogen-donating solvents at elevated temperatures

The reduction of a-X-acetophenones (X = PhO, Br, Cl), as model compounds fo r lignin Liquefaction studies, has been investigated in the presence of a h ydrogen-donating solvent such as 9,10-dihydroanthracene (AnH(2)) or 2-propa nol, between 373 and 573 K. With alpha-phenoxyacetophenone (PAP) in AnH2, a cetophenone and phenol have been obtained with high selectivities. The mech anism involves the reverse radical disproportionation (RRD) with AnH(2). Hy drodebromination of alpha-bromoacetophenone (BrAP) is quantitative at 423 K using AnH(2) as a reducing agent. Now, the hydrogen transfer proceeds by a n uninhibited radical chain mechanism with anthracenyl radicals as the chai n carriers. For the kinetic analysis, the C-X (X = Br, Cl) bond dissociatio n enthalpies (BDEs) have been determined by means of very low pressure pyro lysis to give BDE(C-Br) = 271 kJ mol(-1) and, as a lower limit, BDE(C-Cl) a pproximate to 309 kJ mol(-1) at 298 K. The BDEs are quite at variance with recently published insights derived from an electrochemical study. For comp arison, density functional theory calculations (DFT) have been performed.