Halogenation, of alkenes

The electrophilic addition of HX or X2 to alkenes generates alkyl halides with one or two halogen atoms, respectively (see Section 6.2.2 for a detailed discussion of the mechanisms). 

The electron-rich alkene double bond repels the electrons in the bromine molecule to create a partial positive charge on the bromine atom near the double bond. An intermediate bromonium ion is formed, which reacts to give the /ram-dibromide derived from anti- addition (i.e. the two Br groups add to the alkene from the opposite sides). 

Overall anti- addition as the two bromine atoms land up on the opposite sides of the planar alkene 

As the product with the anti- stereochemistry is formed in excess over the syn- addition product (in which the two Br groups add to the alkene from the same side), the reaction is stereoselective (i.e. one particular stereoisomer of the product is formed in excess). 

The electron-rich alkene double bond reacts with a proton so as to make the most stable intermediate carbocation. The addition is regio-selective, and the so-called Markovnikov (also spelt Markownikoff) product is formed. If a peroxide is added, the reaction proceeds via radical intermediates to give the anti-Markovnikov product. 

Alkenes have an electron-rich C=C bond that can act as a nucleophile 

A tranr-l,2-dibromocycloalkane has the two Br atoms on opposite sides of the ring 

Overall and addition as the -q two bromine atoms land up i on opposite sides of the I planar alkene p 

See also page 410, Section 2 for the halogenation of vinylic organometallics. 

Bromine and chlorine add to alkenes to yield 1,2-dihalides, a process called halogenation. For example, each year approximately 6 million tons of 1,2-dichloroethane (ethylene dichloride) are synthesized industrially hy the addition of CI2 to ethylene. The product is used both as a solvent and as starting material for the manufacture of poly(vinyl chloride), PVC. Fluorine is too reactive and difficult to control for most applications, and iodine does not react with most alkenes. 

Interestingly, when the halogenation reaction is carried out on a cyclo-alkene, such as cyclopentene, only the trans stereoisomer of the dihalide product is formed rather than a mixture of cis and trans isomers. We therefore say that the reaction occurs with anti stereochemistry, meaning that the two halogen atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

FIGURES.) Formation of a bromonium ion intermediate by reaction of Br, witli an alkene. 

The reaction occurs in a single step and results in overall electrophilic addition of Br to the alkene. 

How does the formation of a bromonium ion account for the observed anti stereochemistry of addition to cyclopentene If a bromonium ion is formed as an intermediate, we can imagine that the large bromine atom might shield one side of the molecule. Reaction with Br ion in the second step can then occur only from the opposite, unshielded side to give the trans product. 

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In agreement with the involvement of ionic intermediates for electrophilic halogenation of alkenes, an important role is also exerted by the solvent. Not only the reaction rate is strongly solvent-dependent, but also the stereochemical course of the addition process may be affected by the polarity of the medium. Solvent properties determine the reaction rate the overall kinetic order the nature of the products the stereochemistry of the products... 

When halogenation of alkenes is carried out in aqueous solvent, a vicinal halohydrm is obtained. The reaction is regioselective, and follows the Markovnikov rule. The halide adds to the less substituted carbon atom via a bridged halonium ion intermediate, and the hydroxyl adds to the more substituted carbon atom. The reaction mechanism is similar to the halogenation of alkenes, except that instead of the halide nucleophile, the water attacks as a nucleophile. 

More than six decades after the postulation of the cyclic bromonium ion intermediate in electrophilic bromination of alkenes important, new findings are still emerging.567 Updated general treatments of the halogenation of alkenes became available.568,569... 

Table 6.1. Oxidative halogenation of alkenes and alkanes on polystyrene.

The obtained results are of great interest in fine chemistry and organic synthesis, since epoxides are very versatile building blocks and halogenation of alkenes is still carried out using hazardous reagents and drastic conditions. 

Selenium and tellurium reagents have been used for stereoselective halogenations of alkenes. For example, trans addition of benzeneselenenyl chloride to alkenes followed by the displacement of the seleno moie with chloride can lead to dr-1,2-dichlorides (equation 25). The addition of 2-naph-thyltellurium trichloride proceeds in an anti stereospecific manner (equation 26), whereas tellurium tetra-diloride gives a mixture of syn and anti adducts. The reaction of allyl esters with tellurium tetrachloride accompanies acyl migration to give the l-(trichloiottlluro)-3-chk>io adduct (54 equation 27). ... 

Semiconductor-mediated photoelectrochemical oxidation of halide salts provides a procedure for the halogenation of alkenes via excitated halide species. For example, bromination of cyclohexene has been performed in a Ti02/Bu4NBr (or PhaPMeBr) /O2 system. The reactive bromine species probably arise from a one-electron oxidation of adsorbed bromide ions on the semiconductor by photoin ation, which produces surface-bound bromine atoms (equation 40). 

The stereoselective tram addition of sulfenyl halides and other sulfenyl derivatives to alkenes can be rationalized by assuming a bridged thiiranium ion, by analogy with the mechanism proposed for stereoselective halogenation of alkenes. 

Beside the Grignard and other C-C bond-forming reactions, a number of functional group transformations may also serve as an entry into allylic systems. Some of them, namely the reduction of a, -unsaturated carbonyl compounds (products of crotonic condensation), halogenation of alkenes at the allylic position with Af-bromosuccinimide (NBS) and epoxide isomerization, are shown in Scheme 2.56. 

In summary, the mechanism for halogenation of alkenes occurs in two steps ... 

Because halogenation occurs exclusively in an anti fashion, cis and trans alkenes yield different stereoisomers. Halogenation of alkenes is a stereospecific reaction. 

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