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CHM247H1 (26)
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Chapter 9

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University of Toronto St. George
Barb Morra

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 9.1 NamingAlkynes • 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 2 • 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
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