Dihaloalkanes alkynes from

Alkynes have been prepared from 1,1- and 1,2-dihaloalkanes and from haloalkenes under the influence of a quaternary ammonium salt geminal- and vic-dibromoalkanes are converted into alkynes under liquid liquid [10, 11] and soliddiquid [12-14] conditions with Aliquat or tetra-n-octylammonium bromide... 

The other major dehalogenation pathway involves elimination of two halogens, leaving behind a pair of electrons that usually goes to form a carbon-carbon double bond. Where the pathway involves halogens on adjacent carbons, it is known as vicinal dehalogenation or reductive / -elimination. The fine dashed arrows in Fig. 4 illustrate this process for PCE. Note that this pathway can produce alkynes from vicinal dihaloalkenes [88,94,95], as well as producing alkenes from vicinal dihaloalkanes [96,97],... 

To prepare an alkyne from an alkene, the alkene is first treated with 1 mole of either bromine (Brj) or chlorine (CI2) to give a dihaloalkane (Section 6.3D). Treating the dihaloalkane with 2 moles of a strong base such as sodium amide (NaNH2) in liquid ammonia [NHj(/)] brings about two successive dehydrohalogenations. 

The two basic methods used to prepare alkynes are double elimination from 1,2-dihaloalkanes and alkylation of alkynyl anions. This section deals with the first method, which provides a synthetic route to alkynes from alkenes Section 13-5 addresses the second, which converts terminal alkynes into more complex, internal ones. 

Method A The dihaloalkane (O.l mol) and Aliquat (orTOA-Br) (l mmol) in petroleum ether (100 ml b.p. appropriate for separation from the alkyne by distillation) are added to powdered KOH (14 g, 0.25 mol) and the mixture is stirred for 1-8 h (Table 9.1). The mixture is then filtered and the alkyne is obtained by fractional distillation. 

Alkynes can be prepared from dihaloalkanes by elimination of two molecules of HX. This reaction requires very strongly basic conditions so potassium hydroxide at elevated temperatures or the stronger base sodium amide (NaNH2) is commonly employed. Examples are provided by the following equations ... 

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-dihaloalkane (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 elimination route to alkynes involves removal of two moles of hydrogen halide by strong base from a dihaloalkane. Thc most common kind of. sequence involves forming the dihaloalkane by addition of halogen to an alkene doitble bond. Because you already know how to make alkcnes. yoit now have access to alkynes in the following way ... 

Synthesis of an Aikyne from an Aikene (Section 7.5B) Treating an alkene with Bij or CI2 gives a dihaloalkane. Treating the dihaloalkane with NaNHj or another strong base results in two successive dehydrohalogenations to give an alkyne. 

Double Elimination from Dihaloalkanes to Give Alkynes X X... 

Because vicinal dihaloalkanes are readily available from alkenes by halogenation, this sequence, called halr enation-double dehydrohalogenation, is a ready means of converting alkenes into the corresponding alkynes. 

In Summary Alkynes are made from vicinal dihaloalkanes by double elimination. Alkenyl halides are intermediates, being formed stereospeciflcally in the first elimination. 

---