Alkynes halogenation-double dehydrohalogenation

A Halogenation-Double Dehydrohalogenation Used in Alkyne Synthesis... 

Illustrate the use of halogenation-double dehydrohalogenation in the synthesis of the alkynes... 

But-l-en-3-yne ( vinylacetylene ), F, is made industrially by the CuCl-catalyzed dimerization of ethyne. How would you make it We learned that alkenes can be converted to alkynes by halogenation-double dehydrohalogenation (Section 13-4). This approach suggests 1,3-butadiene as a starting material. We have seen, however, that halogenation of 1,3-butadiene, such as bromination, is not as straightforward as that of a monoalkene 1,2- and 1,4-addition occurs (Section 14-6). Is that a problem Answer No. The 1,2-dihalobutene will eliminate normally, first to 2-bromobutadiene (via deprotonation at the more acidic allylic position) and then to F ... 

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... 

Double dehydrohalogenation (Section 9 7) Reaction in which a geminal dihahde or vicinal dihahde on being treated with a very strong base such as sodium amide is converted to an alkyne by loss of two protons and the two halogen substituents... 

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. 

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. 

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