My PhD dissertation - Institut Fresnel

3.2 Alkyl Carbanions Stabilities in Solution
hydrocarbons
21
A few pioneering studies first carried out acidity measurements of
[109, , ] 122 123 such as indene, or phenylfluorene where the carbanions enjoyed
conjugative stabilization. The equilibrium studied were the exchange reactions of the type:
where M was sodium or potassium. Because of the non-polar media and of probable
covalent character in the RM bonds, it was not possible to derive Ka's which could be
compared quantitatively to those of stronger acids in aqueous media. Furthermore, none of
the techniques thus far described have been generally applicable to the less acidic
hydrogens found in simple hydrocarbons.
In order to evaluate the relative stabilities of carbanions derived from weakly acid
hydrocarbons, D.E. Applequist and D.F. O'Brien
[110] later studied equilibria by
permutational halogen/metal interconversion between cyclic (cyclobutyl, cyclopentyl) or
linear alkanes (ethyl, propyl, isobutyl, neopentyl). To determine equilibrium constants the
metalated substrates were added to halogenated ones and samples regularly withdrawn
were treated with water to replace lithium by hydrogen. Intending to avoid possible
complications due to the Wurtz coupling, and the dehydrohalogenation side reactions,
R.E. Dessy et al.
RH + R'M RM + R'H
[111] rather studied metal-metal exchange reactions between
organomagnesium and organomercuric species, relying on the fact that the most stable
carbanions would prefer to be paired to the most electropositive element. Although
meritorious, these earlier endeavors suffered a major problem. As the authors
admitted [110] , an ambiguity remained whether the results obtained were indeed reflecting
relative stabilities of carbanions in a quantitative way or whether they were partially
influenced by aggregation of the metallic species [47] . In conclusion, none of the
equilibrium techniques devised to sort carbanionic stabilities
[110, 111, , ] 124 125 could afford
accurate data concerning simple alkyl groups, devoid of any other factors' influence.