Fluorine substitution effect hydrocarbons

Effects of Methyl and Fluorine Substitution on the Metabolic Activation and Tumorigenicity of Polycyclic Aromatic Hydrocarbons... 

The halogenated hydrocarbons are generally of low acute toxicity, but several are associated with anesthetic effects and cardiac sensitization. Cardiac sensitization to halogenated alkanes appears related to the number of chlorine or fluorine substitutions. Halogenated alkanes in which >75% of the... 

Studies on the organo-transition metal chemistry of OFCOT and other fluorocarbons indicate that organometallic molecules with perfluorinated carbon skeletons possess structural and chemical properties equally as rich as their hydrocarbon analogues. Their structures and chemistry have helped our understanding of the effects of fluorine substitution on the physical and chemical behavior of organometallic compounds. 

Because of the rate-enhancing effects of fluorine substitution on cyclization reactions of partially-fluorinated radical systems, it is likely that, unlike for pure hydrocarbon systems, it will be possible to utilize such processes to make six-, seven-, and even larger membered rings in radical systems which have appropriate fluorine substitution. 

We have also employed ETS to study the effect of fluorine substitution on the ir orbitals of benzene and ethylene (10). Here we briefly discuss the results for the fluoroethylenes. Fluorine substitution is known to cause only small shifts in tv ionization potentials (IP) of unsaturated hydrocarbons (1 1). For example, the vertical iv IP s of ethylene and perfluoroethylene agree to within 0.1 eV. The reason that has been most often forwarded to explain this is that the electron withdrawing inductive effect, which stabilizes the occupied orbitals, is nearly cancelled by the destabilizing resonance mixing of the fluorine p orbitals with the ir orbitals of the ethylenic double bond. 

Since fluoroalkyl groups are uniformly acid-strengthening and the order of magnitude of the effect, relative to other haloalkyl groups, is consistent with electronegativities F > Cl, etc., it may be expected that the same situation occurs when fluorine is attached to unsaturated carbon. Inspection of the data in Table 4.5 quickly indicates a more complicated situation because, while fluorine substitution increases the acidity relative to the hydrocarbon analogue, the acidities are lower than the corresponding chlorocarbon compounds in the cases of the acrylic acids and phenols. 

Hecht, S.S., Amin, S., Melkikian, A.A., LaVoie, E.J., and Hoffmann, D. (1985) Effects of methyl and fluorine substitution on the metabolic activation and tumorigenidty of polycyclic aromatic hydrocarbons, in Polycyclic Hydrocarbons and Carcinogenesis (ed. 

In yet another interesting system, Dolbier examined the 3,3-shift of 1,1,6,6-tetrafluoro-l,5-hexadiene to the 3,3,4,4-tetrafluoro isomer which has activation parameters somewhat lower than those of the parent system AH = 29.9 kcal/mol and A5 = —18.5 e.u. However, while the terminal geminal difluoro substitution on the threo- Shea compound (Scheme 7.89) lead to similar activation parameters as the hydrocarbon, the activation parameters for the me so isomer were extraordinary in the sense of a very high activation enthalpy and a positive entropy of activation, 49.5 kcal/mol and 8.1 e.u., respectively. The inhibition of the boat-Uke transition state by terminal fluorine substitution was argued to be a steric effect preventing the formation of a bond with eclipsed C-F interactions. 

Substitution of fluorine for hydrogen in an organic compound has a profound influence on the compound s chemical and physical properties. Several factors that are characteristic of fluorine and that underHe the observed effects are the large electronegativity of fluorine, its small size, the low degree of polarizabiHty of the carbon—fluorine bond and the weak intermolecular forces. These effects are illustrated by the comparisons of properties of fluorocarbons to chlorocarbons and hydrocarbons in Tables 1 and 2. 

The introduction of the halogens onto aromatic rings by electrophilic substitution is an important synthetic procedure. Chlorine and bromine are reactive toward aromatic hydrocarbons, but Lewis acid catalysts are normally needed to achieve desirable rates. Elemental fluorine reacts very exothermically and careful control of conditions is required. Molecular iodine can effect substitution only on very reactive aromatics, but a number of more reactive iodination reagents have been developed. 

Unsaturated fluorinated compounds are fundamentally different from those of hydrocarbon chemistry. Whereas conventional alkenes are electron rich at the double bond, fluoroal-kenes suffer from a deficiency of electrons due to the negative inductive effect. Therefore, fluoroalkenes react smoothly in a very typical way with oxygen, sulfur, nitrogen and carbon nucleophiles.31 Usually, the reaction path of the addition or addition-elimination reaction goes through an intermediate carbanion. The reaction conditions decide whether the product is saturated or unsaturated and if vinylic or allylic substitution is required. Highly branched fluoroalkenes, obtained from the fluoride-initiated ionic oligomerization of tetrafluoroethene or hexafluoropropene, are different and more complex in their reactions and reactivities. 

Freeman, Crosby, Lawson (13), Kropp, and Windsor 17, 18) had reported that hydrated europium chloride crystals and aqueous solutions of europium salts, respectively, showed considerable enhancement of ion fluorescence and lifetimes upon substitution of H2O by D2O, the latter workers have reported (37) that wet hydrocarbon solutions of the thenoyltrifluoroacetone chelate of europium showed no enhancement of quantum yield and lifetime upon replacing the H2O present by D2O. They suggested that whereas deuteration enhanced the fluorescent properties of the free ions by inhibiting nonradiative deactivation of the Do level, the quantum efficiency of Do luminescence obtained upon direct excitation of this level (JO, 11) is already so high in the fluorinated diketone chelates e.g. thenoyltrifluoroacetone) that deuteration of the environment should have little further enhancing effect. 

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