Collect. Czech. Chem. Commun. 2007, 72, 1158-1176

Revision of the Dual-Substituent-Parameter Treatment; Reaction Series with a Donor Reaction Centre

Stanislav Böhma and Otto Exnerb,*

a Department of Organic Chemistry, Institute of Chemical Technology, Prague, CZ-166 28 Prague 6, Czech Republic
b Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, CZ-166 10 Prague 6, Czech Republic


The dual-substituent-parameter (DSP) treatment was challenged previously as not generally valid. Just in the fundamental reaction, dissociation of 4-substituted benzoic acids and in similar reactions, DPS does not hold for acceptor substituents because the constant reaction centre is itself an acceptor. In this communication, the reverse case was examined, i.e., a reaction series with a donor reaction centre: basicity and acidity of 4-substituted anilines, and acidity of 4-substituted phenols. The reaction energies were calculated for 19 common substituents at the level B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p); the substituents effects were also estimated separately in uncharged molecules and in the ions in terms of isodesmic reaction. DSP is valid and its resonance term is highly significant for molecules with acceptor substituents; donor substituents behave differently and cannot be described by simple resonance constants. Basicity of substituted anilines is more complex since the substituent effects are quite different in the free base and in the protonated form: basicity is controlled by a combination of various effects. It is recommended to use DSP only for acceptor substituents with a donor reaction centre or vice versa; otherwise the accuracy is decreased. All results were obtained with isolated molecules and with resonance constants derived on isolated molecules; however, they retain their validity even with the common constants σR determined from the reactions in solution.

Keywords: Aniline derivatives; Isodesmic reactions; Phenol derivatives; Basicity; Acidity; Resonance energy; Substituent effect; DTF calculations.

References: 59 live references.