α-Stabilization of carbanions: Fluorine outperforms the heavier halogens

ORGN 38

A. Patrícia Bento, ap.bento@few.vu.nl1, Gernot Boche, boche@chemie.uni-marburg.de2, and Holger L. Hermann2. (1) Department of Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, NL-1081 HV Amsterdam, Netherlands, (2) Fachbereich Chemie, Philipps-Universität, Hans-Meerwein-Straße, D-35032 Marburg, Germany
"Carbanions", more precisely organometallic compounds, are among the most important reagents in organic synthesis. Yet, essential aspects of their nature are still incompletely or even incorrectly understood. A point in case is the basicity of the carbanions CH2X-. This basicity decreases continuously in the series X = F, Cl, Br, I. However, contrary to the common assumption, this occurs not because of the increasing α-stabilization of the carbanion CH2X- by the substituent X along this series. In fact, the opposite happens: F stabilizes CH2X- more effectively than Cl, Br and I (see illustration). Here we show that this apparent contradiction results from the usually ignored stabilization of the corresponding acids CH3X by X.

 

Physical Organic Chemistry: Calculations, Mechanisms, Photochemistry, and High-Energy Species
8:00 AM-11:40 AM, Sunday, 10 September 2006 Moscone Center -- Room 133, Oral

Division of Organic Chemistry

The 232nd ACS National Meeting, San Francisco, CA, September 10-14, 2006