057_Ho_11_RG_8_07 - Chem 057 J Walcott CENTER FOR LEARNING...

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Chem 057 CENTER FOR LEARNING AND TEACHING Handout #11 J. Walcott LEARNING STRATEGIES CENTER Fall 2007 Review Guide #8 Lectures: 10/12/07 – 10/19/07: Chapters 6 and 7 I. Alkyne Reactions Much of the chemistry of alkynes is similar to the alkene chemistry. Only the important reactions that are somewhat different are mentioned below. A. Acid catalyzed hydration. Initially, Markovnikov addition of H 2 O occurs to give an enol. Enols exist in equilibrium with carbonyl compounds, however, and the enol produced reacts in acid to give a ketone. The ketone and enol forms are tautomers - structural isomers. B. Hydroboration-oxidation. Initially, anti-Markovnikov addition of H 2 O occurs to give an enol, which rearranges to give a carbonyl compound. With a terminal alkyne, disiamylborane is used to prevent a second addition of BH 3 , and an enol is formed. The enol reacts in base so that an aldehyde is formed. Internal alkynes lead to formation of a ketone. C. Addition reactions with HBr HBr addition to the triple bond is regioselective. Following Markovnikov addition, the bromine adds to the carbon that bears the alkyl substituent. A second addition can occur resulting in two bromines adding to the same carbon. D. Catalytic hydrogenation. 1. An alkyne is reduced to an alkane if H 2 is used over the same kind of catalyst (Pd, Pt or Ni) used in the hydrogenation of an alkene. 2. When H 2 over a poison catalyst (NiB or Lindlar's catalyst = Pd/BaSO 4 ) is used, the result is a cis alkene. 3. When Na in NH 3 ( l ) is used as the reagent, the result is a trans alkene. E. Acidity of terminal alkynes. 1. Protons on sp hybridized carbon atoms are more acidic (lower pK a ) than protons on 2 hybridized carbons, which are more acidic than protons on 3 hybridized carbons. The hybridized carbon is electronegative and more compact so that the orbital containing the
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This note was uploaded on 09/12/2008 for the course CHEM 057 taught by Professor Walcott, j during the Fall '07 term at Cornell.

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057_Ho_11_RG_8_07 - Chem 057 J Walcott CENTER FOR LEARNING...

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