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The Gas Phase Acidity of Oligofluorobenzenes and Oligochlorobenzenes: About the Additivity or Non‐Additivity of Substituent Effects
Author(s) -
HylaKryspin Isabella,
Grimme Stefan,
Büker Heinz H.,
Nibbering Nico M. M.,
Cottet Fabrice,
Schlosser Manfred
Publication year - 2005
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200400967
Subject(s) - fluorobenzene , chemistry , additive function , chlorobenzene , substituent , deprotonation , fluorine , halogen , computational chemistry , benzene , series (stratigraphy) , stereochemistry , organic chemistry , mathematics , ion , mathematical analysis , paleontology , alkyl , catalysis , biology
The deprotonation energies of benzene, fluorobenzene, all di‐, tri‐, and tetrafluorobenzenes, pentafluorobenzene, chlorobenzene, all di‐, tri‐, and tetrachlorobenzenes, and pentachlorobenzene have been calculated at various levels of second‐order Møller–Plesset and density functional theory. Taking the previously determined experimental data as a benchmark, good agreement was achieved in the chloro series even with moderate computational effort, whereas more extended basis sets have to be used to obtain meaningful numbers in the fluoro series. Apparently, most extensive electron correlation is required to avoid artifacts caused by the proximity of nonbonding lone pairs at the carbanionic center and at the fluorine atoms. When two or more fluorine substituents were introduced in the same aromatic ring, their individual effects (as defined by position‐dependent acidity increments) proved to be perfectly additive in the entire series. In contrast, the acidifying effect of chloro substituents was found to level off when the number of such halogens increases. Additivity or non‐additivity of element effects cannot be ascertained after having merely compared the acidity of mono‐ and disubstituted substrates, but only after having moved to higher degrees of substitution.