Alkenes halohydrins

Addition of Hypohalous Acids to Alkenes Halohydrin Formation... 

FIGURE 10.17 A test of the regiochemistry of the addition of Brg/HgO to alkenes. Halohydrin A would result from water attacking the more substituted carbon of the bromonium ion and halohydrin B would result from attack at the less substituted carbon. 

The stereochemistry of the alkene halohydrin epoxide sequence is the same as that observed in peroxy acid oxidation of alkenes. Substituents that are cis in the alkene remain cis in the epoxide. The combination of anti addition in forming the bromohydrin, followed inversion of configuration in conversion of the bromohydrin to the epoxide yields the same stereochemical result as syn epoxidation of an alkene. 

When treated with bromine or chlorine in aqueous solution alkenes are con verted to vicinal halohydrins A haloni um ion IS an intermediate The halogen adds to the carbon that has the greater number of hydrogens Addition is anti... 

In aqueous solution chlorine and bromine react with alkenes to fonn vicinal halohydrins, compounds that have a halogen and a hydroxyl group on adjacent carbons. 

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

The following section describes the preparation of epoxides by the base-promoted ring closure of vicinal halohydrins. Because vicinal halohydrins are customarily prepared from alkenes (Section 6.17), both methods—epoxidation using peroxy acids and ring closure of halohydrins—are based on alkenes as the starting materials for preparing epoxides. 

The formation of vicinal halohydrins from alkenes was described in Section 6.17. Halohydrins are readily converted to epoxides on treatment with base ... 

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.

Another method for the synthesis of epoxides is through the use of halo-hydrins, prepared by electrophilic addition of HO-X to alkenes (Section 7.3). When a halohydrin is treated with base, HX is eliminated and an epoxide is produced. 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

Halohydrin (Section 7.3) A 1,2-disubstituted haloalcohol, such as that obtained on addition of HOBr to an alkene. 

See Ref. 150, p. 136, for reagents that produce alkenes from p-halo ethers and esters, and from halohydrins. 

When an alkene is reacted bromine in aqueous solution (rather than CCI4), the major product is a halohydrin (halo alcohol). 

Problem 6.33 Alkenes react with aqueous Clj or Br to yield v/V-halohydrins, —CXCOH. Give a mechanism for this reaction that also explains how Br, and (CH,)jC=CH, give (CH,),C(OH)CH2Br. M... 

Halohydrins, formed by electrophilic addition of HO—Cl(Br) to alkenes (Problem 6.33), are treated with base to give epoxides. 

Alkenes are converted to halohydrins by the treatment of halides and water. When halohydrins are treated with a strong base (NaOH), an intramolecular cyclization occurs and epoxides are formed. For example, 1-butene can be converted to butylene oxide via butylene chlorohydrin. 

Addition of halides and water to alkenes preparation of halohydrins... 

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