CHM 247 CH.9
Alkynes:An Introduction to Organic Synthesis
• Alkyne: A hydrocarbon that contains a carbon-carbon triple bond.
• Acetylene is a common name for the simplest alkyne H-C≡C-H
• The suffix –yne is used. Numbering begins at the end nearest the triple bond. More than
one triple bond are diynes, triynes, and so forth. If it contains a double and triple bond,
they are called enynes. Numbering of an enyne chain starts with the end nearer to the first
multiple bonds. If there is a choice, a double bond will receive the lower number.
9.2 Preparation ofAlkynes: Elimination Reactions of Dihalides
• Can be prepared by elimination of HX from alkyl halides.
• Treatment of a dihalide with a strong base such as KOH (in ethanol) or NaNH 2esults in
a twofold elimination of HX and formation of an alkyne.
• The twofold dehydrogalogenation takes place through a Vinylic halide intermediate.
9.3 Reactions of Alkynes:Addition of HX and X
• Acarbon-carbon triple bond results from the interaction of 2 sp-hybridized carbon atoms.
They lie at 180 to each other.
• Electrophiles undergo addition reactions with alkynes much as they do with alkenes.
Reactions with excess HX leads to a dihalide forming. It follows Markovnikov’s rule. •
• Avinylic carbocation has an sp-hybridized carbon and generally forms less readily than
an alkyl carbocation.
9.4 Hydration of Alkynes
• Alkynes can be hydrated with 1 of 2 methods. Direct addition of water catalyzed by
mercury(II) ion yields the Markovnikov product, and indirect addition of water by a
hydroboration-oxidation sequence yields the non-Markovnikov product.
Mercury(II)-catalyzed Hydration ofAlkynes
• Enol: A vinylic alcohol that is in equilibrium with a carbonyl compound.
• Aketone is what is formed with this method.
• Tautomers: Isomers that interconvert spontaneously, usually with the change in position
of a hydrogen. •
• Amixture of both possible ketones results when an unsymmetrical substituted internal
alkyne (RC≡CR’) is hydrated. The reaction i