CHED 503 |
| The literature about carbanions makes use of carbonyl or nitro compounds with charge on hetero atoms due to pi-delocalization of the electrons. Carbanions with a beta-chlorine could stabilize the negative charge but are unstable and eliminate chloride ion with such ease that they are said to undergo concerted eliminations. A beta-F acts as the beta-Cl with “glue” to keep it in place, and allows the study of carbanions stabilized by halogens instead of pi-delocalization. Carbon-fluorine bonds also have different bond strengths depending on the environment and results in fluoride ions with different leaving abilities from -CH2F, -CHF2, -CF2CF3 or -CF3. CF3CHCl2 and C6H5CHClCF3 have similar gas phase acidities; however, their kinetic acidities measured by methanolic methoxide hydron-exchange kinetics differ by over 45,000. The {CF2CCl2}- is a localized carbanion with 45% of the charge on the negative carbon. Although {C6H5CClCF3}- is pi-delocalized, Gaussian calculations have 37% of the charge on the benzylic carbon. It is apparently not the amount of charge on the carbon, but whether the charge is in a localized or delocalized orbital. There is also a difference for the exchange reactions of 9-phenylfluorene compared to C6H5CH(CF3)2 or C6F5H when carried out in methanolic sodium methoxide. |
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Undergraduate Research Poster Session: Organic Chemistry
11:00 AM-1:00 PM, Monday, March 26, 2007 Hyatt Regency Chicago -- Riverside Center, Poster
Division of Chemical Education |