Collect. Czech. Chem. Commun. 2002, 67, 1505-1516

Comparing Isotope Effects and Rates for the Methanolic Sodium Methoxide Reactions of 9-R-Fluorene to Those for p-CF3C6H4CHClR (R = CH2Cl, CH2F and CF3)

Heinz F. Koch*, William C. Pomerantz, Erik L. Ruggles, Martijn van Laren and Anne-Marie van Roon

Department of Chemistry, Ithaca College, Ithaca, NY 14850, U.S.A.


Kinetic primary isotope effects and activation parameters associated with the methanolic sodium methoxide-promoted dehydrohalogenation reactions of 9-(chloromethyl)fluorene and 9-(fluoromethyl)fluorene are reported and compared to for p-CF3C6H4CHClCH2Cl and p-CF3C6H4CHClCH2F. The element effect, kHCl/kHF = 4 at 25 °C, for the fluorenyl compounds is only a tenth of the value, kHCl/kHF = 54, obtained for the benzylic compounds. Also reported are the activation parameters for the methanolic sodium methoxide reactions of 9-(trifluoro)methylfluorene for deuterium exchange and dehydrofluorination, and are compared to those for p-CF3C6H4CHClCF3. In both cases the exchange reaction is faster than the elimination; however, in the fluorenyl system there is a large difference between the entropies of activation for exchange, ∆S = -14 eu, and elimination, ∆S = +10 eu; however, the ∆S of 12-13 eu are the same for the benzylic systems. The eliminations for 9-(chloromethyl)fluorene and 9-(fluoromethyl)fluorene are 80 to 1,000 times faster than those for p-CF3C6H4CHClCH2X. The reactions of 9-(trifluoromethyl)fluorene are favored over p-CF3C6H4CHClCF3 by 200,000 for the dehydrofluorination and 1,000 for the exchange at 25 °C; however, due to the large differences in activation entropies, the exchange reaction is favored by 107 at -50 °C.

Keywords: Reaction kinetics; Isotope effects; Activation parameters; Mechanism; Dehydrohalogenations; Isotope exchange; Carbanions; Fluorinated compounds.

References: 23 live references.