Chlorine, addition alkenes

Another common reaction is the chlorination of alkenes to give 1,2-dihaloalka-nes. Patell et al. reported that the addition of chlorine to ethene in acidic chloroalu-minate(III) ionic liquids gave 1,2-dichloroethane [68]. Under these conditions, the imidazole ring of imidazolium ionic liquid is chlorinated. Initially, the chlorination occurs at the 4- and 5-positions of the imidazole ring, and is followed by much slower chlorination at the 2-position. This does not affect the outcome of the alkene chlorination reaction and it was found that the chlorinated imidazolium ionic liquids are excellent catalysts for the reaction (Scheme 5.1-39). 

Attempts to control the stereochemistry of the addition of chlorine to alkenes using ephedrinium and cinchoninium salts give only low optical activity [31]. 

Michael Faraday reported in 1821 that chlorine addition to alkenes is Stimulated by sunlightand today this is taken to indicate the involvement of a free radical process (equation 26). Free radical chain mechanisms were proposed in 1927 by Berthoud and Beraneck for the isomerization of stilbene catalyzed by Br2 (equation 27), and by Wachholtz for bromine addition to ethyl maleate (equation 28).Later studies showed inhibition of halogen addition by reaction of the intermediate radicals with oxygen, and a free radical chain mechanism for solution and gas phase halogenations as in equation (26) was shown (equation 29). Kinetic and mechanistic... 

Room-temperature ionic liquids may be used as green recyclable alternatives to chlorinated solvents for stereoselective halogenation.577 The bromination of alkenes and alkynes in [bmim][Br] is a/m -stereospecific, whereas that of 1,3-dienes gives selectively the 1,4-addition products. The reactions of arylacetylenes, however, are not selective when carried out in [bmim][PF6]. Tetraethylammonium trichloride, a stable crystalline solid may be used in the chlorination of alkenes and alkynes to afford the products with exclusive anti stereoselectivity.578 It has... 

During slow thermal chlorination, elimination of HC1 from the monochloride with the resultant formation of an alkene followed by chlorine addition may be the dominant route to yield dichloroalkanes. This mechanism, however, is negligible in rapid thermal or photochemical reactions. 

Early attempts to add chlorine fluoride to alkenes led only to chlorine addition however, control of temperature and the correct choice of solvent allow addition of a positive chlorine to give a chloronium ion followed by fluoride ion attack, so that overall CIF addition takes place giving a-chloro-jS-fluoroalkanes. 

The environmentally important physical-chemical properties of PCAs (such as octanol-water partition coefficient (Kow), (water solubility WS), and vapor pressure (VP)) have been determined using the commercial products or with synthetic products resembling components of the commercial mixtures [13, 27]. Drouillard et al. [25,26] have reported physical-chemical properties of individual PCA congeners that were synthesized by chlorine additions to M-alkenes [28]. 

To do this we must draw the salt as a covalent compound or transfer one electron fr> chloride ion to the diazonium ion. The other product would be a chlorine radical. Addition alkene gives the more stable radical, which abstracts chlorine from the diazocompound and. the chain going. 

The idea that a cyclic bromonium ion was an intermediate was a novel concept at the time of its proposal in 1937. Much additional evidence, including the isolation of a stable cyclic bromonium ion, has been obtained since then to support it. Similarly, cyclic chloronium ions are believed to be involved in the addition of chlorine to alkenes. In the next section we shall see how cyclic chloronium and bromonium ions (halonium ions) are intermediates in a second reaction involving alkenes and halogens. 

Dichlorination of cyclopentane under free-radical conditions is not a realistic approach to the introduction of two chlorines in a trans-1,2 relationship without contamination by isomeric dichlorides. Vicinal dichlorides are prepared by electrophilic addition of chlorine to alkenes. The stereochemistry of addition is anti. 

The study of the addition of chlorine to alkenes is even older than the study of bromine addition. The synthesis of 1,2-dichloroethane was first reported by a group of Dutch chemists in 1795, and Michael Faraday investigated the addition of chlorine to ethene in 1821 In many ways the addition of chlorine to alkenes is similar to the addition of bromine to alkenes. For example, each additional alkyl group increases the relative reactivity of an olefin by a factor of 50 to 100 (Table 9.3). A Taft p a correlation is observed, and the p for chlorine addition is —2.9, which is similar to the value of —3.1 for addition of bromine. This result suggests that charges on the alkenyl carbon atoms in the transition structures for addition of chlorine are similar to those in the addition of bromine. ... 

There are significant differences between the bromine addition and chlorine addition reactions, however. The addition reaction of ethene and chlorine is exothermic by 44 kcal/mol, which is 15 kcal/mol more exothermic than the addition of ethene and bromine. Poutsma noted that addition of chlorine to alkenes in nonpolar solvents can occur by either radical or ionic pathways, but that oxygen inhibits the radical reaction. For example, addition of chlorine to neat (i.e., not diluted by solvent) cyclohexene (19) gave frans-l,2-dichlorocyclohexane (20), 3-chlorocyclohexene (21), and 4-chloro-cyclohexene (22) in a 1.95 1.00 0.60 ratio when the reaction was carried out imder a nitrogen atmosphere. In the presence of oxygen, the ratio was 3-4 1.00 0. ... 

Addition of chlorine to the isomers of 2-butene gave 97-98% anti addition product and 2-3% substitution product when the alkene was neat or was dissolved in a nonpolar solvent with oxygen present to suppress radicals. Under a nitrogen atmosphere, however, 18% of the product was substitution, and the 82% of the product that was derived from adding CI2 was not formed stereospecifically (Figure 9.23). Poutsma concluded that radical reactions are difficult to suppress in chlorine addition because they do not require an initiator and seem to begin spontaneously. ... 

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