Alkenes => dihalides

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... 

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

Because vicinal dihalides are prepared by addition of chlorine or bromine to alkenes (Section 6 14) alkenes especially terminal alkenes can serve as starting mate rials for the preparation of alkynes as shown m the following example... 

Bromine and chlorine add rapidly to alkenes to yield 1,2-dihalides, a process called lialogenatioii. For example, approximately 6 million tons per year of 1,2-dichloroethane (ethylene dichloride) are synthesized industrially by addition... 

Alkynes can be prepared by the elimination of HX from alkyl halides in much the same manner as alkenes (Section 7.1). Treatment of a 1,2-dihaloaJkane (a vicinal dihalide) with excess strong base such as KOH or NaNH2 results in a twofold elimination of HX and formation of an alkyne. As with the elimination of HX to form an alkene, we ll defer a discussion of the mechanism until Chapter 11. 

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. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Alkenes react rapidly with chlorine and bromine in non-nucleophilic solvents to form vicinal dihalides. 

Monodehydrohalogenation of allylic halides is another classical method for diene synthesis24. This method is complementary to double dehydrohalogenation as both the 1,2-dihalides and allylic halides are readily accessed from alkenes. The commonly employed protocol for diene synthesis, particularly for cyclic 1,3-dienes, is through the allylic monobromination of the alkene with A-bromosuccinimide or related reagents followed by dehydrobromination with hindered bases such as DBN or DBU (equation l)25. 

From a mechanistic point of view, the titanocene(II)-promoted intramolecular cyclopro-panation of gem-dihalides possessing a terminal double bond is interesting. Although the products of ring-closing metathesis, i. e. cycloalkenes, are produced in certain cases, the treatment of 6,6- and 7,7-dihalo-l-alkenes (e. g. 39 and 40) with titanocene(II) species affords bicyclo[3.1. OJhexane and bicyclo[4.1.0]heptane derivatives 41 and 42, respectively (Scheme 14.19) [34],... 

Reaction with the appropriate a.to-dihalide gives rise to 4, 5- or 6-merabered rings. Three-membered rings are formed by a similar reaction between the alkene... 

Poly(alkene sulfide)s are prepared by allowing the dihalide to drip slowly under vigorous stirring into a moderately concentrated aqueous polysulfide solu-... 

Alkenes are obtained by the transformation of various functional groups, e.g. dehydration of alcohols (see Section 5.4.3), dehalogenation of alkyl halides (see Section 5.4.5) and dehalogenation or reduction of alkyl dihalides (see Section 5.4.5). These reactions are known as elimination reactions. An elimination reaction results when a proton and a leaving group are removed from adjacent carbon atoms, giving rise to a tt bond between the two carbon atoms. 

Electrophilic addition to terminal alkynes (unsymmetrical) is regioselective and follows Markovnikov s rule. Hydrogen halides can be added to alkynes just like alkenes, to form first the vinyl halide, and then the geminal alkyl dihalide. The addition of HX to an alkyne can be stopped after the first... 

Addition of halides to alkenes preparation of alkyl dihalides... 

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