DETERMINATION OF THE GAS-PHASE ACIDITIES OF HALOGEN-SUBSTITUTED AROMATIC-COMPOUNDS USING THE SILANE-CLEAVAGE METHOD

The gas-phase acidities of halogen-substituted aromatic compounds have been determined in a flowing afterglow-triple quadrupole apparatus wi th use of the silane cleavage method developed by DePuy and co-workers [C. H. DePuy, S. Gronert, S. E. Barlow, V. M. Bierbaum and R. Damraue r, J. Am. Chem. Sec., 111, 1968 (1989)]. In this method the relative y ields of siloxide ion products produced in reactions of OH- with trime thylsilyl- or phenyldimethylsilyl-substituted aromatic compounds are c orrelated with the difference in gas-phase acidity of the accompanying neutral products. Acidities are reported for different ring-positions in fluoro-, chloro- and bromobenzene, chloro- and bromonaphthalene an d benzyl chloride. Excellent precision is achieved in most cases, with assigned uncertainties less than 2-3 kcal/mol. Good agreement is obta ined between the acidities determined with use of two different types of silane precursor. Halogen-substitution increases the gas-phase acid ities of benzene and naphthalene by similar amounts (13-14 kcal/mol). The effects on different ring-positions in benzene and naphthalene are shown to be primarily inductive in nature, falling-off by a consisten t 2.5-3.5 kcal/mol per bond separating the acidic site from the haloge n-bearing carbon in the chlorine and bromine-substituted systems. Larg er effects are evident in the positional acidities of fluorobenzene. T he meta and para position acidities of halobenzenes are shown to be li nearly correlated with the acidities of the corresponding meta and par a halophenols, haloanilines and halotoluenes.