Alkynes An Introduction to Organic Synthesis

The rules for naming alkynes are like the rules for alkenes (Sec. 6.3), with a few exceptions. 

Alkynes can be prepared by elimination reactions of 1,2-dihalides, using a strong base. 

Vinylic halides give alkynes when treated with a strong base. 

Alkyne triple bonds result from the overlap of two sp-hybridized carbon atoms. 

The length (120 pm) and strength (835 kJ/mol) of a -OC- bond make it the strongest carbon-carbon bond. 

Online homework for this chapter may be assigned in Organic OWL. 

An alkyne is a hydrocarbon that contains a carbon-carbon triple bond. Acetylene, H—C=C—H, the simplest alkyne, was once widely used in industry as the starting material for the preparation of acetaldehyde, acetic acid, vinyl chloride, and other high-volume chemicals, but more efficient routes to these substances using ethylene as starting material are now available. Acetylene is still used in the preparation of acrylic polymers but is probably best known as the gas burned in high-temperature oxy-acetylene welding torches. 

Much current researcli is centering on polyynes—linear carbon chains of. s/ -hybridized carbon atoms, Polyynes with up to eight triple bonds have been detected in interstellar space, and evidence has been presented for the existence of carbyne, an allotrope of carbon consisting of repeating triple bonds in long chains of indefinite length. 

Alkyne nomenclature follows the general rules for hydrocarbons discussed in Sections 3.4 and 6.3. The suffix -yne is used, and the position of the triple bond is indicated by giving the number of the first alkyne carbon in the 

Compounds with more than one triple bond are called diynes, triynes, and so forth compounds containing both double and triple bonds are called enynes (not ynmes). Numbering of an enyne chain starts from the end nearer the first multiple bond, whether double or triple. When there is a choice in numbering, double bonds receive lower numbers than triple bonds. For example  

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As with alkyl and alkenyl substituents derived from alkanes and alkenes, respectively, alkynyl groups are also possible. 

Problem 8.2 There are seven isomeric alkynes with the formula C fljy. Draw and name them. 

Numbering the main chain begins at the end nearer the triple bond so that the triple bond receives as low a munber as possible. 

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