Alkanes, fluorinated/chlorinated

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]... 

Methane, ethane, and other alkanes react with fluorine, chlorine, and bromine. 

CFCs gained a lot of attention in the 1980s for their role in damaging the ozone layer. CFCs are synthetic alkanes with chlorine and fluorine attached. They were widely used as refrigerants and propellants (in spray cans) for many decades until scientists realized they were harming the planets atmosphere and possibly changing its climate. 

Until recently, halogen substituted alkanes were thought to be inert to xenon difluoride, but detailed examination has shown that common solvents such as dichloromethane (3) and chloroform (4) undergo fluorine-chlorine and fluorine-hydrogen exchange at room temperature due to hydrogen fluoride catalysis.15... 

Halogenation. Fluorination, chlorination, and bromination of alkanes catalyzed by superacids have been reported.1,2 Reactions may be carried out in the liquid phase, or in the gas phase over solid superacids or supported noble metal catalysts. High selectivity and relatively mild reaction conditions are the main features of these transformations. 

Table 3. Fluorination of Alkanes with Chlorine Trifluoride [37]...

Heat or light is usually needed to initiate this halogenation. Reactions of alkanes with chlorine and bromine proceed at moderate rates and are easily controlled. Reactions with fluorine are often too fast to control, however. Iodine reacts very slowly or not at all. We will discuss the halogenation of alkanes in Chapter 4. 

Of course, the overall AH for the 1 reaction of an alkane with chlorine, must also take into account the AH of this sea Step whfeh is-3 49 +243= lO kJ mol-1, making chterirtetoon mudi more exothermic than brommation. u naf(On continues the trend, and methane-fluorine mixtures are. explosive. For iodine, on the other hand, the first step becomes so endothermic, even for formation of atertiary redical, thatthe second step (AH= -234 +151=-83 kj mol-1) is not exothermic enough to make reaction favourable overall. Radical lodinatlons therefore do not take place. 

The selective monofluorination of difluorinated alkanes with cobalt(III) fluoride at 25 C to give trifluoroalkanes 10 has been reported. Fluorinations with cobalt(lll) fluoride are considered to proceed via carbocations and the selectivity of the fluorination is explained by the relative stabilities of the intermediate carbocations. Although treatment of alkanes with chlorine trifluoride results in fluorination. it is of little practical use due to over-fluorinalion. chlorination and in some cases isomeri7ation of the starting alkane. ... 

In this reaction, X can be fluorine, chlorine, or bromine, but not iodine. Iodine does not react well with alkanes. A common use of a halogented hydrocarbon is shown in Figure 23-3. 

The alkane is said to undergo fluorination, chlorination, bromination, or iodination according to whether X2 is F2, CI2, Br2, or I2, respectively. The general term is halogenation. Chlorination and bromination are the most widely used. 

By comparing the H° values for the sum of the two propagating steps for the monohalogenation of methane, we can understand why alkanes undergo chlorination and bromination but not iodination and why fluorination is too violent a reaction to be useful. 

In this substitution reaction, the halogen (fluorine, chlorine, bromine, iodine) substitutes one hydrogen atom in the alkane, forming hydrogen halide and alkyl halide as the products. The reactivity of halogens in the halogenation reactions is as follows ... 

Alkyl halides are compounds in which a halogen atom (fluorine, chlorine, bromine, or iodine) replaces a hydrogen atom of an alkane. For example, CH3CI and CH3CH2Br are alkyl halides. Alkyl halides are also called haloalkanes. The generic formula for an alkyl halide is R—X where X = fluorine, chlorine, bromine, or iodine. 

Halogens are not functional groups. Halogen atoms can be attached to the carbon of virtually any alkane structure. When a fluorine, chlorine, bromine, or iodine is attached to a carbon chain, the molecule is named to show that a halogen is present however, there is no unique suffix for the halogen. 

In these reactions a halogen atom replaces one or more of the hydrogen atoms of the alkane and the corresponding hydrogen halide is formed as a by-product. Only fluorine, chlorine, and bromine react this way with alkanes. Iodine is essentially unreactive, for a reason that we shall explain later. 

As in the radical halogenation of alkanes (Section 3-8), the exothermic nature of aromatic halogenation decreases down the periodic table. Huorination is so exothermic that direct reaction of fluorine with benzene is explosive. Chlorination, on the other hand, is controllable but requires the presence of an activating catalyst, such as aluminum chloride or ferric chloride. The mechanism of this reaction is identical with that of bromination. Finally, electrophilic iodination with iodine is endothermic and thus not normally possible. Much like the radical halogenation of alkanes, electrophilic chlorination and bromination of benzene (and substituted benzenes. Chapter 16) introduces functionality that can be utilized in further reactions, in particular C-C bond formations through organometallic reagents (see Problem 54, Section 13-9, and Real Life 13-1). 

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