Global and local analysis of the gas-phase acidity of haloacetic acids

The relative gas-phase acidity of halosubstituted acetic acids CH2XCO2H, CH X2CO2H and CX2CO2H (X = F, Cl, and Br) is analyzed in terms of global and l ocal descriptors of reactivity. The model is based on the analysis of proto n-transfer equilibria with reference to acetic acid CH3CO2H. The relative a cidity pattern displayed by this series is rationalized in terms of the har d and soft acids and bases principle. The relative stability between the ne utral species and the corresponding anions is in agreement with the maximum hardness principle. Charge transfer between the conjugated bases present i n the proton-transfer equilibria is correctly accounted for by using a clas sical ion transport model that introduces the electronic chemical potential of transfer. The local reactivity analysis based on regional Fukui functio ns and local softness displays a good correlation with the experimental gas -phase acidity within the series.