Alkenes preparation from vicinal dihalides

General Preparations.—Alkenes. Dehalogenation of vicinal dihalides is easily carried out using sodium in liquid ammonia, e.g. 1,2-dichlorocyclo-octane gave ds-cyclo-octene in 95% yield.The scope of the synthesis of ap-unsaturated carbonyl compounds by syn-elimination from 2-phenylselenoxides has been studied only low yields of the desired ketone were obtained from 2-phenylseleno-cycloheptanone and cyclo-octanone because of the competing Pummerer rearrangements, but improved yields could be obtained by prior conversion of the ketones into ketals. The reaction worked well for 2-methoxycarbonyl cyclic ketones, e.g. 2-methoxycarbonylcyclo-oct-2-en-l-one was prepared in 93% yield. ... 

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. 

A simple method for introducing a triple bond into an organic compound is to treat an appropriate dihalide with a strong base. Since vicinal dihalides (usually the bromide) are readily formed by reaction of bromine with an alkene, and geminal dihalides from aldehydes or ketones with phosphorus pentachloride, the method is a useful general procedure for the preparation of terminal and non-terminal alkynes from readily available starting materials. 

Dehaiogenation of vicinal (Latin vicinalis, neighboring) dihalides is severely limited by the fact that these dihalides are themselves generally prepared from the alkenes. However, it is sometimes useful to convert an alkene to a dihalide while we perform some operation on another part of the molecule, and then to regenerate the alkene by treatment with zinc this procedure is referred to as protecting the double bond. 

At this point, you know only one way to synthesize a ketone—the addition of water to an alkyne (Section 6.6). The alkyne can be prepared from two successive E2 reactions of a vicinal dihalide, which in turn can be synthesized from an alkene. The desired alkene can be prepared from the given starting material by an elimination reaction. 

---

See also in sourсe #XX -- [ , , , , , , , , , , , , , , , , , , , ]

---