Alkynes from vicinal dihalides

In this example, the dihalide used is a geminal dihahde, which means that both halogens are connected to the same carbon atom. Alternatively, alkynes can also be prepared from vicinal dihalides, in which the two halogens are connected to adjacent carbon atoms ... 

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. 

An alkyne is a hydrocarbon that contains a carbon-carbon triple bond. Alkyne carbon atoms are sp-hybridized, and the triple bond consists of one sp-sp a bond and two p-p tt bonds. There are relatively few general methods of alkyne synthesis. Two good ones are the alkylation of an acetylide anion with a primary-alkyl halide and the twofold elimination of HX from a vicinal dihalide. 

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. 

Alkynes are prepared from alkyl dihalides via elimination of atoms or groups from adjacent carbons. Dehydrohalogenation of vicinal- or gemiwaZ-dihahdes is a particularly useful method for the preparation of alkynes (see Section 5.4.5). 

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. 

In some cases, we can generate a carbon-carbon triple bond by eliminating two molecules of HX from a dihalide. Dehydrohalogenation of a geminal or vicinal dihalide gives a vinyl halide. Under strongly basic conditions, a second dehydrohalogenation may occur to form an alkyne. 

Alkynes can be prepared from alkyl 1,2-dihalides (or vicinal dihalides) by the elimination of two molecules of HX using a strong base (e.g. NaNH2). 

Vicinal dihalides from alkenes and alkynes (Geigert et al. 1983b) ... 

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. 

Alkynes can be synthesized from alkenes via compounds called vicinal dihalides. 

If we have a terminal alkyne, such as could be made from an appropriate vicinal dihalide, we can use the alkynide anion derived from it to form carbon—carbon bonds by nucleophilic substitution. 

Alkynes can be prepared from either geminal or vicinal dihalides via two successive E2 reactions. 

Therefore, the last step of our synthesis must be formation of the alkyne from a vicinal dihalide ... 

Don t be confused by this retrosynthetic arrow. It indicates a hypothetical synthetic pathway thinking backward from the product (alkyne). In other words, the previous figure should be read as In the last step of our synthesis, the alkyne can be made from a vicinal dibromide. Now let s try to go backward one more step. We have learned only one way to make a vicinal dihalide, starting with an alkene ... 

The synthesis of an alkyne from either a geminal dihalide or a vicinal dihalide requires a strong base to eliminate two moles of hydrogen hahde. Two suitable reagents for this reaction are sodium amide in hquid ammonia as the solvent and potassium tert-butoxide in dimethyl sulfoxide. 

Alkynes are prepared by elimination reactions, as discussed in Section 8.10. A strong base removes two equivalents of HX from a vicinal or geminal dihalide to yield an alkyne by two... 

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