A Fluorine atoms attached to the carbanion centre

Superficially, we would expect the high electronegativity of fluorine to stabilise a carbanion centre, but measurements of acid strengths and exchange rates for a variety 

Base-catalysed H/D exchange experiments for a series of haloforms [80] (Table 4.10) demonstrate that carbanion formation is stabilised by halogen in the order 1 Br Cl F. When these results are combined with acidity measurements which show that CF3H (pKa 31) is little more acidic than methane (pKa 40) [81], we can conclude that, in these systems, fluorine attached to a carbanion centre is stabilising with respect to hydrogen but destabilising compared with the effects of other halogens. Similar conclusions can be drawn from pKa measurements of a number of halobis(trifluoromethyl) methanes [82]. 

On the other hand, the pKa values of a series of substituted nitromethanes [83] (Table 4.11) suggest that, whilst chlorine bonded directly to the carbanion centre increases acidity relative to hydrogen, fluorine decreases acidity and, therefore, decreases the stability of the corresponding carbanion. 

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