Halogen addition, alkene vicinal

The addition reaction of an alkene with a halogen like bromine or chlorine gives a vicinal dihalide. The halogen molecule is split and the halogens are added to each end of the double bond (Following fig.). Vicinal dibromides are quite useful in the purification or protection of alkenes since the bromine atoms can be removed under different reaction conditions to restore the alkene. Vicinal dibromides can also be converted to alk5mes. 

Addition of halogens (Sections 6 14-6 16) Bromine and chlorine add to alkenes to form vicinal dihalides A cy clic halonium ion is an intermediate Stereospecific anti addition is observed... 

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... 

The necessary vicinal dihalides are themselves readily available by addition of Br2 or Cl2 to alkenes. Thus, the overall halogenation/dehvdrohalogenation sequence makes it possible to go from an alkene to an alkyne. for example, diphenylethylene is converted into diphenylacetylene by reaction with Br2 and subsequent base treatment. 

Dimethyl sulfoxide and chlorine form highly reactive intermediates which are of some limited use as oxidants for alcohols. These intermediates are related to those derived from the reaction of the halogens with dimethyl sulfide and probably have a structure such as (27). When formed at -4S C they allow the oxidation of primary and secondary alcdiols to aldehydes and ketones when used in a two-fold excess. For very simple alcdiols the reaction proceeds in yields of greater than 90%, but there are considerable drawbacks if some types of additional functionality are present in the molecule, e.g. alkenes react very rapidly to form vicinal dichlorides. 

Halogenation is the addition of halogen X2 (X = Cl or Br) to an alkene, forming a vicinal dihalide. 

Dehydrohalogenation of vicinal dihalides is particularly useful since the dihalides themselves are readily obtained from the corresponding alkenes by addition of halogen. This amounts to conversion—by several steps—of a double bond into a triple bond. 

F2 and I2 are halogens, but they are not used as reagents in electrophilic addition reactions. Fluorine reacts explosively with alkenes, so the electrophilic addition of F2 is not a synthetically useful reaction. The addition of I2 to an alkene is a thermodynamically unfavorable reaction The vicinal diiodides are unstable at room temperature, decomposing back to the alkene and I2. 

A vicinal dihalide (abbreviated rTtc-dihalide) is a compound bearing the halogens on adjacent carbons vicinus, Latin adjacent). Vicinal dihalides are also called 1,2-dihalides. A vicinal dibromide, for example, can be synthesized by addition of bromine to an alkene (Section 8.1). The w c-dibromide can then be subjected to a double dehydrohalogenation reaction with a strong base to yield an alkyne. 

The addition of an acid such as HX to an alkene leads to formation of an alkyl halide. If the acid catalyzed addition of water or an alcohol is examined, the product is an alcohol or an ether. Oxymercuration also leads to addition of water to an alkene. The functional group exchange can be generalized as shown, where X = Cl, Br, I, OH, OR. When an alkene reacts with a halogen, the product is a vicinal dihalide. The functional group exchange is shown where X = Cl, Br, I. When bromine in water or chlorine in water is reacted with an... 

Halogens, although they do not appear to contain electrophilic atoms, add to the double bond of alkenes, giving vicinal dihalides. These compounds have uses as solvents for dry cleaning and degreasing and as antiknock additives for gasoline. 

In Summary Halogens add as electrophiles to alkenes, producing vicinal dihalides. The reaction begins with the formation of a bridged halonium ion. This intermediate is opened stereospecifically by the halide ion displaced in the initial step to give overall anti addition to the double bond. In subsequent sections, we shall see that other stereochemical outcomes are possible, depending on the electrophile. 

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