Fluorine effect elements electronegativity

Combining the highest electronegativity (4,0) with a rather small polarizability volume which amounts to not more than 0.5 makes fluorine a unique element. Its incorporation into hydrocarbon frameworks results in different electrostatic effects, which are sometimes rarely predictable. However, the influence of fluorine substituents on the acidity of nearby functional groups such as OH, NH,... 

Since fluorine is the most electronegative element, it should inductively destabilize carbocations. The stability of fluoromethyl cations in the gas phase decreases in the order CFH2+ > CF2H+ > CF3+ > CH3+. The trend in solution, however, could be different, due to solvent effects, ion pairing, and so on. Indeed, fluorine has been shown to provide stabilization for carbocations. The existence of CH3CF2+, in contrast to the elusive ethyl cation CH3CH2+, is a clear evidence that replacement of H atoms by F atoms provides stabilization for carbocations.524 Furthermore, it was found that in perfluorobenzyl cation C6F5CF2+ fluorine atoms in resonance positions (ortho and para) are more deshielded than those in meta positions.536 This indicates carbocation stabilization by back-donation. 

As with the other first-row elements, much of the chemistry of fluorine stands distinctly apart from that of its congeners. Anomalous effects are due partially to the extreme electronegativity of fluorine, partially to the inability for the fluorine octet to expand, and partially to the small size of the atom. The low F—F bond energy, coupled with the high energy of bonds formed between fluorine and other elements, makes many of the reactions of the F2 greatly exothermic, and a number take place with almost explosive violence. (Fluorination of organic compounds with undiluted fluorine, for instance, can be particularly hazardous.)... 

Electron withdrawal in these molecules is the result of a bond polarization from an inductive effect (Chapter 5). The electrons in a o bond between carbon and a more electronegative element such as N, O, or F will be unevenly distributed with a greater electron density towards the more electronegative atom. This polarization is passed on more and more weakly throughout the carbon skeleton. The three fluorine atoms in CF3H reduce the p K"a to 26 from the 48 of methane, while the nine fluorines in (CFa CH reduce the pKTa still further to 10. 

As the most electronegative element, fluorine would be expected to have the greatest effect on the stereochemical stability of the cyclopropyl radical and it does. When comparing ESR spectra of cyclopropyl radicals with 1-fluoro cyclopropyl radicals Kawamura and coworkers found the inversion frequencies of 1, 2 and 3 at — 99°C comparable to that found for the parent cyclopropyl radical, 10 s at — 175°C. In contrast, the inversion frequency of the a-fluorocyclopropyl radicals 4,5 and 6 is estimated to be lowered to 10 s at — 108°C. 

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