Carbanion stabilization, fluorine

Explain the role of adjacent fluorine atoms in carbanion stabilization. Use the group orbitals of the trifluoromethyl group in your orbital analysis. What kind of group (X , C, or Z) is -CF3 (See Farnham, W. B. Dixon, D. A. Calabrese, J. C., J. 

The effect of fluorination on the reactivity of carbanions has been reviewed.65 66 Fluorination has the same influences on carbanion stability as does the effect of fluorination on acidity (see Section 1.2.). A lot of information has been derived from the effect of fluorination on CH acidity.43 The effect of fluorination on deuterium-hydrogen exchange processes in the case of highly fluorinated substances has been examined with estimates of pKa for highly fluorinaled substrates.67... 

Given that fluorine has a destabilizing influence upon planar carbanions, geometry considerations favor the formation of pyramidal anions.69 71 The influence of /(-fluorination on carbanion stability has been described in a variety of ways. In early work the role of negative hyperconjugation was proposed to account for these effects.72... 

The mechanism follows initial fluorine ion attack on the silicon atom yielding an ion which transfers the trifluoroviiiyl anion to the alkene, leading to formation of fluorotrimethylsilane and a carbanion. Stabilization of this species by fluoride ion elimination may give a mixture of isomeric dienes which isomerize in the presence of cesium fluoride under the reaction conditions to the end product 3. ... 

The formation of ethyl cyano(pentafluorophenyl)acetate illustrates the intermolecular nucleophilic displacement of fluoride ion from an aromatic ring by a stabilized carbanion. The reaction proceeds readily as a result of the activation imparted by the electron-withdrawing fluorine atoms. The selective hydrolysis of a cyano ester to a nitrile has been described. (Pentafluorophenyl)acetonitrile has also been prepared by cyanide displacement on (pentafluorophenyl)methyl halides. However, this direct displacement is always aecompanied by an undesirable side reaetion to yield 15-20% of 2,3-bis(pentafluoro-phenyl)propionitrile. 

Fluonnation of a sulfoxide-stabilized carbanion provided a route to fluorinated estrones after elimination of the sulfoxide [111] (equation 62). 

Fluorination Effects on the Stability of Reaction Intermediates (Carbocations, Carbanions, and Radicals)... 

Carbanions Although the presence of a fluorine atom in the a position has a stabilizing influence through the inductive effect, the repulsion induced by the... 

With regard to nonfluorinated olefins, the great reactivity of fluoroolefins toward nucleophiles comes from both a higher electrophilicity of the double bond, and from the stabilization of the carbanion, resulting from the addition of the nucleophile, by jS-fluorine atoms." °... 

The stabilization of a carbanion brought by a- or )S-fluorine atoms is thermodynamic. Indeed, because of the great reactivity of carbanions toward elimination of a fluoride ion, they may have short lifetimes a-fluorinated carbanions easily undergo a-elimination processes to carbenes, while )S-fluorinated carbanions undergo jS-elimination reactions. 

Answer. Two factors contribute to the stabilization of carbanions by an adjacent trifluoromethyl group or other perfluorinated groups. Most obviously, the strong inductive effect of the highly electronegative fluorine atoms will tend to increase the... 

In organo-fluorine chemistry many reactions are known where more or less stable ionic or radical intermediates are formed, for example, carbocations, carbanions. and partially fluorinated and perfluorinated radicals and carbenes. While some of these species are short lived, others are surprisingly stable and isolable. In the latter case electronic and steric arrangements often kinetically stabilize the intermediate. 

Dixon and Smart71 examined a series of fluorine-substituted phosphonium ylides. Fluorine stabilizes carbanions via an inductive effect, favouring pyramidal carbanions. Dixon and Smart argued that the first substitution of F on the ylidic carbon (H3P=CHF) causes the carbon to become very pyramidal (the sum of the angles at C is 338.5°), which reduces the potential overlap of the anionic orbital with any P orbitals. The leads to the long P—C distance of 1.723 A. The second F substitution (H3P=CF2) actually breaks the P—C bond and the system is best described as a weak interaction of phosphine with CF2. Trifluoromethyl groups act to stabilize the anions via hyperconjugation. This leads to a planar ylidic carbon in H3P=C(CF3)2. 

A fluorine substituent, however, has the opposite effect on geometry. Pyramidal ions are stabilized by fluorine and planar ions destabilized conjugation with the filled orbitals on fluorine is unfavorable. See A. Streitwieser, Jr., and F. Mares, J. Amer. Chem. Soc., 90, 2444 (1968). Chlorine, bromine, and iodine apparently stabilize an adjacent carbanion more than does fluorine, presumably because the destabilizing orbital overlap is less effective with the larger halogens (see Section 5.2, p. 227). J. Hine, N. W. Burske, M. Hine, and P. B. Langford, J. Amer. Chem. Soc., 79, 1406 (1957). 

The basicity of the anion CH2X , as a function of X, has been found to decrease in groups 16 (from OH to SH), 15 (from NH2 to PH2), and 14 (from CH3 to SiHs), because the a-stabilization of CH2X increases.5 In contrast, the basicity of CH2X along the series X = F, Cl, Br, I does not decrease, as commonly assumed. Fluorine has been found more effective than the heavier halogens for a-stabilization of carbanions. 

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