Addition reactions 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. 

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... 

Mechanism of the o] mercuration of an alkene to yield an alcohol. TTiis electrophilic addition reaction involves a mercurinium ion intermediate, and its mechanism is similar to that of halohydrin formation. The product of the reaction is the more highly substituted alcohol, corresponding to Markovnikov regiochemistry. 

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

CH3CH=CH2, and allow halohydrin formation to occur with Br and NaOH at 25°. Addition of bromine (or chlorine) with water or sodium hydroxide results in compounds containing halogen and hydroxyl groups on adjacent carbon atoms. The reaction is ... 

PROBLEM 10.68 The Halohydrin formation animation was calculated using a symmetrical alkene. The symmetry of the alkene means that nucleophilic addition to the bromonium intermediate will not be regioselective. Select play for this reaction. Stop at the first intermediate and observe the calculated LUMO. Based on this data, where is the positive charge located Show four products that would be formed through nucleo-phihc attack at each of the places of LUMO density. 

There are a number of biological examples of halohydrin formation, particularly in marine organisms. As with halogenation (Section 8.2), halohydrin formation is carried out by haloperoxidases, which function by oxidizing Br or Cl ions to the corresponding HOBr or HOCl bonded to a metal atom in the enzyme. Electrophilic addition to the double bond of a substrate molecule then yields a bromonium or chloronium ion intermediate, and reaction with water gives the halohydrin. For example ... 

This chapter introduces several stereospecific addition reactions. Some of them occur exclusively via a syn addition (such as hydrogenation or hydroboration-oxidation), while others occur exclusively via an anti addition (such as bromination or halohydrin formation). When drawing the products of a stereoscpecific addition reaction, be careful to avoid drawing a wedge or a dash on a location that is not a chirality center. For example, consider the following syn dihydroxylation. In such a case, it is tempting for students to draw the products as if they have two chirality centers, like this ... 

Halohydrins are useful intermediates especially in the synthesis of epoxides. The main reaction is usually accompanied by the formation of a dihalide. When the reactions are performed in the presence of acetic acid, then acetates of the hydrins can be the predominant products. With several exceptions, alkenes with a nonfluorinated C = C bond have been subjected to halohydrinations. Halogen cations usually undergo addition to the substituted carbon of the C = C bond in (fluoroalkyl)ethenes. 

Electrophilic addition of the halogens and related X—Y reagents to alkenes and alkynes has been a standard procedure since the beginning of modem organic chemistry.1 Anti electrophilic bromination of such simple compounds as cyclohexene and ( )- and (Z)-2-butene, and variants of this reaction when water or methanol are solvents (formation of halohydrin or their methyl ethers, respectively), are frequently employed as prototype examples of stereospecific reactions in elementary courses in organic chemistry. A simple test for unsaturation involves addition of a dilute solution of bromine in CCU to the... 

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