Addition fluorine

Fluorine addition to alkenes is a violent reaction difficult to control and accompa med by substitution of hydrogens by fluorine Vicinal diicxlides on the other hand tend to lose I2 and revert to alkenes making them an infrequently encountered class of compounds... 

Fluorination of thiophene-2,5-dicarboxylic acitl with a sulfur tetrafluoritie-hydrogen fluoride mixture provides 2,5-bis(trifluoromethyl)thiophene in a 69% yield no fluorine addition to the thiophene ring occurs [229J. Also, imidazole mono- and dicarboxylic acids yield only the respective trifluo-romethyliinidazoles [230],... 

For toluene fluorination, the impact of micro-reactor processing on the ratio of ortho-, meta- and para-isomers for monofluorinated toluene could be deduced and explained by a change in the type of reaction mechanism. The ortho-, meta- and para-isomer ratio was 5 1 3 for fluorination in a falling film micro reactor and a micro bubble column at a temperature of-16 °C [164,167]. This ratio is in accordance with an electrophilic substitution pathway. In contrast, radical mechanisms are strongly favored for conventional laboratory-scale processing, resulting in much more meta-substitution accompanied by imcontroUed multi-fluorination, addition and polymerization reactions. 

Figure 13. Cyclic voltammogram of graphite (Lonza KS6, synthetic graphite) in 1 MLiCl04 in PC/novel fluorinated additive (90 10, v v) scan rate 30 pV s 1, Potentials vs. Li/Li+.

Copaux-Kawecki An improved version of the Copaux process for extracting beryllium from beryl, which permits recovery of the fluorine. Addition of ferric sulfate to the dilute sodium fluoride solution remaining after the separation of the beryllium hydroxide precipitates sodium tetrafluoroferrate, which is then used in place of sodium fluorosilicate. 

In contrast to saturated hydrocarbons, the unsaturated hydrocarbons react with atomic fluorine by two pathways, i.e. (atomic fluorine addition at >C=C< double bond and hydrogen substitution by fluorine atoms. The reaction of fluorine with aromatic hydrocarbons proceeds with the formation of F-derivatives and hydrogen atoms break off ... 

The C=C double bond in [XCF=CFBF3] is more easily accessible to oxidation (fluorine addition) than that of XCF=CFBF2 (28). The oxidation is initiated by polarized XeF2, formed in a channel parallel to the xenodeborylation, by the interaction of XeF2 and the strong Lewis acid, XCF=CFBF2 (eq 8). 

Consequently, the relative contribution of xenodeborylation to fluorine addition across the C=C double bond and the ratio of the [BF4] to [XCF=CFBF3] anions depends on the electron-withdrawing character of X (acidity of the borane) and the actual ratio of XeF2 (fluoride donor) to XCF=CFBF2 (fluoride acceptor). 

Fluorine is the most electrophilic halogen and only few examples of controlled addition of fluorine to carbon-carbon double bonds have been reported56 Milder reagents, such as XeF2, are generally used to form fluorine addition products. 

It is noteworthy that, at variance with bromination and chlorination which generally occur without isomerization of the disubstituted double bond, fluorine addition to the 1,2-bond of cis- and trans-1,3-pentadienes gives mainly the trans-adduct 13, besides smaller amounts of compounds 14-16 (equation 24). 

Most reactions of iodine heptafluoride are similar to those of iodine pentafluoride, except that it does not undergo any further fluorine addition reactions (See Iodine Pentafluoride). The compound reacts with a number of inorganic substances, forming their fluorides, and forms fluoro-derivatives with organics when in diluted form. 

Perfluorination of unsaturated hydrocarbons such as alkenes, allenes (Fig. 7) [53] and aromatics (Fig. 8) [54,55] is also possible since the total energy released upon fluorine addition to a carbon-carbon double bond (typically between 251.4-292.9 kjmol-1) is not sufficient to break carbon-carbon single bonds. 

Unsaturated carbon structures usually undergo addition of fluorine, but are vulnerable to fragmentation unless fluorine addition is carefully controlled. Aromatic rings saturate, but polymerization may be a serious side reaction. Many heteroaromatic molecules polymerize giving principally tars and oils. In contrast to the cobalt(III) fluoride process, benzene produces a significantly lower yield of perfluorocyclohexanc than does cyclohexane. 

The hydrogen fluoride catalyzed fluorine addition to acenaphthylene and 1,2-dihydronaph-thalene by xenon difluoride results (predominantly via ami attack) in the formation of vicinal difluorides ( 70% yield).37 Phenyl-substituted cycloalkenes 5 (n = 1-3) form the appropriate difluorides the configuration being dependent on ring magnitude.38... 

Fluorine addition with trifluoromethyl hypofluoritc to various uracil and cytosine bases and nucleosides has been extensively studied. Vicinal fluoro trifluoromethoxy or fluoro methoxy adducts, formed mainly by syn addition, are usually hydrolyzed thus resulting in fluoro-sub-stituted derivatives,42 48 e.g. formation of 24 and 25. 

Substituted phenylacetylenes react with F-Teda BF4 (6) in acetonitrile in the presence of water to give a,a-difluoro ketones 29 by regioselective fluorine addition.103,105 The phenylacetylene substituent affects the relative rate of fluorine addition. 

Reaction of pentafluoro-substituted benzene derivatives with xenon difluoride/boron trifluoride results in 1,4- and 1,2-fluorine addition with regiospecificity depending on the substituent.41 For example, the heptafluorocyclohexa-1,4-dienes 1 can be obtained in this way in high yield. 

Tetrachloroethene over potassium tetrafluorocobaltate(III) at 180°C gives112 mainly the fluorine addition compound l,l,2,2-tetrachloro-l,2-difluoroethane (41 %). 

The behaviour of elemental fluorine differs from that observed for chlorine or bromine, mainly because fluorine addition proceeds in a stereoselective syn manner (Scheme 22). The syn addition is explained by the formation of tight ion pairs forming the addition products or carbocation rearrangement products107. 

This article is focused on HDN, the removal of nitrogen from compounds in oil fractions. Hydrodemetallization, the removal of nickel and vanadium, is not discussed, and HDS is discussed only as it is relevant to HDN. Section II is a discussion of HDN on sulfidic catalysts the emphasis is on the mechanisms of HDN and how nitrogen can be removed from specific molecules with the aid of sulfidic catalysts. Before the discussion of these mechanisms, Section II.A provides a brief description of the synthesis of the catalyst from the oxidic to the sulfidic form, followed by current ideas about the structure of the final, sulfidic catalyst and the catalytic sites. All this information is presented with the aim of improving our understanding of the catalytic mechanisms. Section II.B includes a discussion of HDN mechanisms on sulfidic catalysts to explain the reactions that take place in today s industrial HDN processes. Section II.C is a review of the role of phosphate and fluorine additives and current thinking about how they improve catalytic activity. Section II.D presents other possibilities for increasing the activity of the catalyst, such as by means of other transition-metal sulfides and the use of supports other than alumina. 

Roma, P. Camino, G. Luda, M. P. Mechanistic studies on fire retardant action of fluorinated additives in ABS, Fire and Material, 1997, 21(4), 199-204. 

SCHEME 16. Stereochemistry of fluorine addition syn / anti ratio... 

The cycloalkene structure also influenced the rate of fluorine addition, while a remarkable difference was observed between 1-phenylcyclohexene and 4-phenyl-1,2 dihydronaphthalene66 (Scheme 17). 

It has been demonstrated that 1 -phenyl-4- J-butyIcyclohexene is a very appropriate olefin for studying the stereochemistry of fluorination or halofluorination, where isomerization in the primarily formed cyclohexyl fluoro carbenium ion could be avoided by introduction of a J-butyl group67. Acid-catalyzed liquid-phase fluorine addition resulted in equal amount of cis and trans adducts, with the nature of the catalyst little influencing the stereochemistry, while introduction of a methoxy group into the phenyl ring also had no significant effect on the stereochemical course of addition (Scheme 18). 

Acid-catalyzed liquid-phase fluorine addition with xenon difluoride to cis and trans 1-phenylpropene resulted in the formation of vicinal difluoride in high yield63. Photo-initiated reaction in methylene chloride at 0 °C also gave vicinal difluorides in a nonstereospecific manner, and a free-radical mechanism was suggested68. Unfortunately, this radical reaction does not appear to represent a general synthesis since reaction with 1-hexene, cyclohexene and 2,3-dimethyl-2-butene does not give vicinal difluoride products68. 

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