Fluorinated alkanes

Fluorination of Alkanes. Fluorination of alkanes is extremely difficult to control. The reaction usually results in substantial C—C bond rupture and can readily lead to explosion.136 However, several methods for controlled direct radical fluorination of hydrocarbons have been developed. The key and obvious observation was that the only reaction sufficiently exothermic to cause fragmentation is the termination step between a carbon radical and a fluorine atom. Consequently, if the atomic fluorine population and the mobility of hydrocarbon radicals are minimized, controlled fluorination becomes feasible. 

One way to obtain neutral or nearly neutral supercritical fluid/water systems is the use of gases other than CO2, such as lower alkanes, fluorinated hydrocarbons, or SFg. This may not always be applicable and these materials are likely to find less acceptance for a potential synthetic application. The inherent condition of low pH in the presence of compressed CO2 can be addressed by the addition of buffer... 

The lubricant properties of alkanethiols and fluorinated alkanes have been studied extensively by scanning probe techniques [163]. In agreement with experiments on LB monolayers it was found that the fluorocarbon monolayers show considerably higher friction than the corresponding hydrocarbon monolayers [164, 165 and 166] even though the fluorocarbons are known to have the lowest surface free energy of all organic materials. 

Chlorine Ammonia, acetylene, alcohols, alkanes, benzene, butadiene, carbon disulflde, dibutyl phthalate, ethers, fluorine, glycerol, hydrocarbons, hydrogen, sodium carbide, flnely divided metals, metal acetylides and carbides, nitrogen compounds, nonmetals, nonmetal hydrides, phosphorus compounds, polychlorobi-phenyl, silicones, steel, sulfldes, synthetic rubber, turpentine... 

Fluorinated Alkanes. As the fluorine content increases, the chemical reactivity decreases until complete fluorination is achieved, after which they are inert to most chemical attack, including the highly reactive element fluorine. Their lack of reactivity leads to their use in certain commercial apphcations where stabiUty is valued when in contact with highly reactive chemicals. 

Steric Factors. Initially, most of the coUisions of fluorine molecules with saturated or aromatic hydrocarbons occur at a hydrogen site or at a TT-bond (unsaturated) site. When coUision occurs at the TT-bond, the double bond disappears but the single bond remains because the energy released in initiation (eq. 4) is insufficient to fracture the carbon—carbon single bond. Once carbon—fluorine bonds have begun to form on the carbon skeleton of either an unsaturated or alkane system, the carbon skeleton is somewhat stericaUy protected by the sheath of fluorine atoms. Figure 2, which shows the crowded hehcal arrangement of fluorine around the carbon backbone of polytetrafluoroethylene (PTFE), is an example of an extreme case of steric protection of carbon—carbon bonds (29). 

Health and Safety Factors. Completely fluorinated alkanes are essentially nontoxic (16). Rats exposed for four hours to 80% perfluorocyclobutane and 20% oxygen showed only slight effects on respiration, but no pathological changes in organs. However, some fluorochemicals, especially functionalized derivatives and fluoroolefins, can be lethal. Monofluoroacetic acid and perfluoroisobutylene [382-21-8] are notoriously toxic (16). 

Mote stable catalysts ate obtained by using fluorinated graphite or fluorinated alumina as backbones, and Lewis acid halides, such as SbF, TaF, and NbF, which have a relatively low vapor pressure. These Lewis acids ate attached to the fluorinated soHd supports through fluorine bridging. They show high reactivity in Friedel-Crafts type reactions including the isomerization of straight-chain alkanes such as / -hexane. 

Deep fluorinalion of alkanes, ethers, acid fmlides, esters, alkyl chlorides, most ketones, ketals, orthoesters, and combinations of these functional groups produces principally the perfluonnated analogues (Table 2) Chlorine substituents (or chloro groups) usually survive fluorination... 

The high reactivity of N-H bonds has also been exploited to produce N-F denvatives without significant substitution on neighbonng C-H bonds, Diethyl-phosphoramidates of ammonia, alkylammes, and a,polar solvents to produce difluoroamine [57], N,N-difluoroalkylamines, and a,to-bis(At,7V-difluoroamino)alkanes [52] Acetamide undergoes fluonnation to give modest yields of N,N difluoroacetatnide and acetyl fluonde when fluorinated... 

Similar preference in replacement by fluorine of tertiary versus secondary and secondary versus primary hydrogens is observed in the fluorination of alkanes with chlorine trifluoride in 1,2-difluorotetrachloroethane at room temperature (Table 3). Skeletal rearrangements accompany the fluorination [31]... 

Polyfluoroalkyl- andperfluoroalkyl-substituted CO and CN multiple bonds as dipolarophiles. Dmzo alkanes are well known to react with carbonyl compounds, usually under very mild conditions, to give oxiranes and ketones The reaction has been interpreted as a nucleophilic attack of the diazo alkane on the carbonyl group to yield diazonium betaines or 1,2,3 oxadiazol 2 ines as reaction intermediates, which generally are too unstable to be isolated Aromatic diazo compounds react readily with partially fluorinated and perfluorinated ketones to give l,3,4-oxadiazol-3-ines m high yield At 25 °C and above, the aryloxa-diazolines lose nitrogen to give epoxides [111]... 

Alkanes spontaneously burst into flame in the presence of elemental fluorine. The reaction that takes place between pentane and F2 gives CF4 and HE as the only products. 

The reactivity of the halogens decreases in the order F2 > CI2 > Br2 > I2. Fluorine is an extremely aggressive oxidizing agent, and its reaction with alkanes is strongly exothennic and difficult to control. Direct fiuorination of alkanes requires special equipment and techniques, is not a reaction of general applicability, and will not be discussed further. 

Monooxygenation is distributed among a variety of bacteria and several have been examined for their potential to degrade fluorinated alkanes ... 

Substitution of the chlorines on the alkane bridge of DDT by fluorine gave a slower-acting knockdown, and the rate was not restored by the further substitution of fluorine for chlorine in the ring. 

The complete replacement of chlorine in the DDT molecule by fluorine gives (p-FCeH CHCFs, a compound with greatly decreased toxicity toward fruit flies and thrips and probably other species. This reduced activity is matched by the low activity of (p-ClCeH CHCFs which, when contrasted to the potency of DDT and DFDT, shows that the attenuation is caused by the replacement of the three chlorine atoms on the alkane bridge. The corresponding substitution of only one chlorine atom by fluorine gave a compound having as much activity as DDT against the chafer beetle. 

Propellants may be of a number of different types CFCs, hydrofluoroalkanes (HFAs), or alkanes. The composition impacts upon performance. A numerical system is employed to identify fluorinated propellants. The rules governing this numbering system allow the molecular structure to be derived from the numerical descriptor. The rules may be listed as follows ... 

There is a remarkable difference in the ab4 systems of SF5-acetylenes as compared to the respective SF5-alkenes or SF5-alkanes. Most noticeably, the order of appearance of the ab4 signals switches for SF5-acety-lenes, with the four fluorine signals due to the equatorial fluorines appearing downfield of the one fluorine signal due to the axial fluorine (Scheme 7.31). Relative to SF5-ethane, the SF5-acetylene equatorial fluorines have shifted 20ppm downfield, whereas its axial fluorine is shifted 10ppm upfield compared to those of SF5-ethane. 

The figures for fluorination reflect the weakness of the F—F [150kJ (36 kcal) mol ],and the strength of the H—F [560 kJ(134 kcal) mol" ], bonds. Fluorination normally requires no specific initiation (cf. p.324), and is explosive unless carried out at high dilution. That fluorination does proceed by a radical pathway, despite not requiring specific initiation, is demonstrated by the fact that chlorination may be initiated in the dark, and at room temperature, by the addition of small traces of F2. Bromination is a good deal slower than chlorination, under comparable conditions, as step (1)—H-abstraction by Br—is commonly endothermic. This step is usually so endothermic for I that direct iodination of alkanes does not normally take place. 

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