Fluorine inductive effect

Substitution of fluorine into an organic molecule results in enhanced chemical stabiUty. The resulting chemical reactivity of adjacent functional groups is drastically altered due to the large inductive effect of fluorine. These effects become more pronounced as the degree of fluorine substitution is increased, especially on the same carbon atom. This effect demonstrates a maximum in fluorocarbons and their derivatives. 

Fluorinated Acids. This class of compounds is characterized by the strength of the fluorocarbon acids, eg, CF COOH, approaching that of mineral acids. This property results from the strong inductive effect of fluorine and is markedly less when the fluorocarbon group is moved away from the carbonyl group. Generally, their reactions are similar to organic acids and they find apphcations, particularly trifluoroacetic acid [76-05-1] and its anhydride [407-25-0] as promotors in the preparation of esters and ketones and in nitration reactions. 

Fluorine is, of course, the most electronegative atom on the periodic table. ap and F values (the pure field inductive effect) provide... 

If the competition data are compared with electronegativity values for the halogens 85>, then tetrafluorobenzyne is clearly in an anomalous position. The only reasonable explanation available at present is that tetrafluorobenzyne is so destabilized by the inductive effect of the fluorine atoms that it has lost a considerable amount of the selectivity which arynes normally show. Estimates for the heats of formation of the isomeric dichlorobenzynes and for tetrachlorobenzyne have recently been made from mass spectrometric studies and these do indicate a low stability for tetrachlorobenzyne 86>. Evidently more data are required for the tetrahalogenobenzynes. 

DFT study of the fluorinated DB[a,A]ACR-8,9-epoxides lead to the same conclusions regarding fluoronium ion character and stabilization at positively charged sites of the parent DB[a,/ ]ACR, and destabilization via inductive effect at positions bearing negative charge density. 

This enhanced reactivity of fluoromethyl cyanide is undoubtedly due to the inductive effect of the fluorine atom which produces an electron deficit on the carbon atom linked to the nitrogen, and presumably increases still further the polarity of the carbon-nitrogen bond, so that the electron displacements can be pictured as (IX). The increased polarity of the carbon-nitrogen bond will obviously facilitate polar addition of hydrogen chloride and alcohols (or phenols). 

The presence of fluorine strongly destabihzes a carbocation centered on the jS carbon because only the inductive effect takes place. " The effect on solvolysis or protonation reaction of double bonds can be very important. The destabilization of carbenium and alkoxycarbenium ions plays an importantrole in the design of enzyme inhibitors (cf Chapter 7) and in the hydrolytic metabolism of active molecules (cf. Chapter 3). 

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... 

On a thermodynamic level, the presence of fluorine atoms in the P position strongly stabilizes the anions (planar or not) either by inductive effect or by negative hyperconjugation (Figure 1.8). 

Free Radicals The inductive effect of fluorine atoms destabilizes radicals. For electronic reasons, fluorination has an important impact on... 

The )S-fluorinated radicals, such as the a-trifluoromethyl ones, are also destabilized due to the inductive effect. The dissociation energy grows from CH3CH2—H (97.7kcal/mol) to CF3CH2—H (102.0kcaFmol). ... 

For reasons of this sort Taft and Rakshys considered that the chemical shift of a Fluorine atom meta to a particular substituent is a measure of only the inductive effect of that substituent while of course the shift of an ortho or para i F nucleus is a measure of both inductive and conjugative effects. They proposed that the difference between the meta chemical shift and the para chemical shift be taken as a measure of the conjugative effect of a substituent. Their results for some of the systems of interest in the present context are summarised in Table 5. In both cases the second-row substituent appears to have a considerable electron accepting conjugative effect compared to its first-row analogue. This of course can be rationalised by partial double bonding. 

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