The Acidity of Chloro-Substituted Benzenes: A Comparison of Gas Phase, Ab Initio, and Kinetic Data

✍ Scribed by Manfred Schlosser; Elena Marzi; Fabrice Cottet; Heinz H. Büker; Nico M. M. Nibbering

• Publisher: John Wiley and Sons
• Year: 2001
• Tongue: English
• Weight: 103 KB
• Volume: 7
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/1521-3765(20010817)7:16<3511::aid-chem3511>3.0.co;2-u

✦ Synopsis

The deprotonation energies of benzene, chlorobenzene, all di-, tri-, tetrachlorobenzenes, and pentachlorobenzene have been determined in the gas phase using a Fourier transform ion cyclotron resonance mass spectrometer. The values measured differ only slightly, though significantly, from the corresponding data for oligofluorobenzenes. The heavier halogen acidifies orthopositions slightly less and meta-positions slightly more than fluorine does. More-over, the contributions of three or more chloro substituents are not perfectly additive. In fact the accumulation attenuates the contributions somewhat. Quantum chemical calculations at the MP2/6-311G* level reproduce the gas-phase acidities fairly well, but reveal special effects when extended to experimentally not observable benzenides carrying the halogens at anion-remote positions. Competition experiments have been performed to assess the relative reactivity of nine oligochlorobenzenes towards sec-butyllithium in tetrahydrofuran at À 100 8C. An almost exact linear correlation between logarithmic rates and gas-phase acidities has been found.