Gas phase acidity of substituted benzenes

Deprotonation thermochemistry of benzene derivatives C6H5X (X = H, F, Cl, OH, NH2, CN, CHO, NO2, CH3, C2H5, CHCH2, CCH) has been examined at the G3B3 level of theory. For X = F, Cl, CN, CHO and NO2, the most favorable deprotonation site is the ortho position of the phenyl ring. This regio-specificity is directly related to the field/inductive effect of the substituent. G3B3 gas phase acidities, ΔacidH° and ΔacidG°, compare within less than 4 kJ mol−1 with experimental data. A noticeable exception is nitrobenzene for which tabulated acidity appear to be underestimated by ca. 120 kJ mol−1.