Fluorine effect

Fluorinated poly(arylene edier)s are of special interest because of their low surface energy, remarkably low water absorption, and low dielectric constants. The bulk—CF3 group also serves to increase the free volume of the polymer, thereby improving various properties of polymers, including gas permeabilities and electrical insulating properties. The 6F group in the polymer backbone enhances polymer solubility (commonly referred to as the fluorine effect ) without forfeiture of die thermal stability. It also increases die glass transition temperature with concomitant decrease of crystallinity. 

Fluorinated polyethers, synthesis of, 363 Fluorinated polymers, 361-363 Fluorine effect, 361 Fluorine-ended PEEK, 360 Foams... 

A well recognized effect, often referred to as the fluorine effect, is related to bond shortening upon progressive substitution of one center by electronegative groups such as fluorine316). The LCFC approach leads to a satisfactory understanding of this trend. 

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

Robin, M. B., Ishii, I., McLaren, R., and Hitchcock, A. P. (1988). Fluorination effects on the inner shell spectra of unsaturated molecules. J. Electron Spectrosc. Relat. Phen. 47, 53-92. 

On the basis of eqn. (45) it would be expected that nitryl fluoride in sulphuric acid is a nitrating agent (besides also possessing fluorinating and sulpho-fluorinating effects). 

Siddiqui, M. A., Driscoll, J. S., Abushanab, E., Kelley, J. A., Batch , J. J., Jr., Marquez, V. E. The" -fluorine effect" in the non-metal hydride radical deoxygenation of fluorine-containing nucleoside xanthates. Nocleosides, Nocleotides Nocleic Acids 2000, 19, 1-12. 

One clear tendency is apparent from the equilibrium product distribution of Letvis acid-induced transfluorination and halogen scrambling if fluorine can migrate it vill al vays tend to concentrate at one carbon atom. Most preferred reaction products are trifluoromethyl derivatives, followed by geminal difluoromethy] derivatives. This thermodynamic product control is often referred to as the general fluorine effect (Scheme 2.19). 

Scheme 2.19 The general" fluorine effect in Lewis acid-induced halogen scrambling.

The repulsive destabilization of sp -bound fluorine results in peculiar reactivity which has no real counterpart in hydrocarbon chemistry. This quite unique behavior of fluorinated olefins and arenes is summarized under the term special fluorine effect (as opposed to the previously discussed general fluorine effect ). The photochemically induced (Scheme 2.36) or fluoride ion-induced (Scheme 2.37) rearrangements observed can be rationalized as the system trying to reduce the number of energetically unfavorable fluorine atoms bound to sp -hybridized carbon. 

Figure 4.13 The key to an understanding the fluorine effect observed in liquid crystals with perfluorinated bridges in their mesogenic core structure lies in the steric repulsion between fluorine and hydrogen in neighboring rings.

The enol form (8) of the highly fluorinated cyclopentanone (7) is more stable than the keto form (Scheme 1.15), demonstrating the powerful fluorine effect on enolization [6]. It is well known that the transformation of the enol form (3) to the corresponding keto form was found not to be easy [2]. We all know that hemiacetals of ketones and aldehydes and enols of common ketones can exist only as intermediates and cannot be isolated as their stable form. However, the chemical nature of the highly fluorinated aliphatic carbonyl compounds is totally different from that of the nonfluorinated ones. 

By contrast, perf luoro/ir-f luoro effect reasoning correctly suggests B-fluorination of ethyl radical will increase the ionization potential because the species is nonplanar. For perfluoroethyl radical, the two fluorination effects on ionization potentials run counter to each other. Experiment does not help us decide which effect dominates because the heat of formation of C2F5+ is known no better than 15 kcal mol"1. That is, from appearance potentials of this ion from C2F5Z with Z = F, I, CF3 and C2F5 we obtain ionization potentials of 228, 218, 208 and 199 kcal mol"1 (ca. 9.88, 9.45, 9.02 and 8 63 eV). This is clearly too wide of a spread of values to be of thermochemical use to derive meaningful substituent effects. 

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