Alkenes halohydrin formation

Addition of Hypohalous Acids to Alkenes Halohydrin Formation... 

Yet another example of an electrophilic addition is the reaction of alkenes with the hypohalous acids HO—Cl or HO-Br to yield 1,2-halo alcohols, called halohydrins. Halohydrin formation doesn t take place by direct reaction of an alkene with HOBr or HOC1, however. Rather, the addition is done indirectly by reaction of the alkene with either Br2 or Cl2 in the presence of water. 

Figure 7.3 Mechanism of the oxymercuration of an alkene to yield an alcohol. The reaction involves a mercurinium ion intermediate and proceeds by a mechanism similar to that of halohydrin formation. The product of the reaction is the more highly substituted alcohol, corresponding to Markovnikov regiochemistry.

Halohydrin formation, as depicted in Figure 6.13, is mechanistically related to halogen addition to alkenes. A halonium ion intermediate is formed, which is attacked by water in aqueous solution. 

CHAPTER 8 Ionic Addition of HX to an Alkene 332 Free-Radical Addition of HBr to Alkenes 334 Acid-Catalyzed Hydration of an Alkene 338 Oxymercuration of an Alkene 340 Hydroboration of an Alkene 345 Addition of Halogens to Alkenes 350 Formation of Halohydrins 352 Epoxidation of Alkenes 360 Acid-Catalyzed Opening of Epoxides 362 Olefin Metathesis 376... 

Alkene oxymercuration is closely analogous to halohydrin formation. The reaction is initiated by electrophilic addition of (mercuric) ion to the alkene to give an intermediate mercurinium ion, whose structure resembles that of a bromonium ion (Figure 7.5). Nucleophilic attack of water, followed by loss of a proton, then yields a stable organomercury addition product. The final step, reaction of the organomercury compound with sodium boro-hydride, is not fully understood but appears to involve radicals. Note that... 

Preparation of Alkenes A Preview of Elimination Reactions Addition of Halogens to Alkenes 234 Halohydrin Formation 237... 

From the following alkene precursors, show products of halohydrin formation. 

Halohydrin formation (Section 6.17) When treated with bromine or chlorine in aqueous solution, alkenes are converted to vicinal halohydrins. A halonium ion is an intermediate. The halogen adds to the carbon that has the greater number of hydrogens. Addition is anti. 

In a reaction resembling halohydrin formation (Section 6.17), vicinal haloethers are prepared from alkenes by reaction with an alcohol in the presence of halogens— usually bromine or iodine. This haleotherification proceeds through a cyclic halonium ion, which reacts with the alcohol. 1-Methylcyclohexene undergoes iodoetherification with ethanol in the presence of iodine to give ran -l-ethoxy-2-iodo-l-methylcyclohexane. 

A MECHANISM FOR THE REACTION ] Halohydrin Formation from an Alkene 365... 

The more highly substituted carbon atom bears a greater positive charge because it resembles a more stable tertiary carbocation. [Notice how this reaction (and its explanation) resembles that given for halohydrin formation from unsymmetrical alkenes in Section 8.14 and attack on mercurinium ions.]... 

Alkenes can be converted into epoxides by treatment with peroxy acids or via halohydrin formation and subsequent epoxidation. Both procedures are stereospecific. 

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