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Lecture23 - alkynes are readily deprotonated with a strong...

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Lecture Summary 23 October 18, 2006 Chapter 8 - Alkynes: An Introduction to Organic Synthesis Hydrogenation of Alkynes Catalytic hydrogenation with hydrogen gas using Lindlar’s catalyst is specific for generating the cis alkene. Trans alkenes can be prepared by reduction of alkynes using lithium metal in ammonia. This reaction proceeds via two successive one-electron reductions and proton transfers from the ammonia. H H Li trans alkene NH 3 Li NH 3 H Li H NH 3 Oxidation of Alkynes Because of the additional pi-bond, ozone will cleave an alkyne to provide a carboxylic acid. The same product is produced using potassium permanganate. O 3 then Zn/H + or KMnO 4 O OH HO O + Acidity of Alkynes Due to the nature of the sp-orbital on the end of an alkyne (it has 50% S character) it can accommodate a lone pair of electrons more readily than an sp 2 or sp 3 orbital. Thus, terminal
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Unformatted text preview: alkynes are readily deprotonated with a strong base like sodium amide. NaNH 2 H + pK a ~ 25 pK a ~ 35 + Na NH 3 © 2006 Gregory R . Cook page 1 Chem 341 North Dakota State University Alkylation of Alkynes Alkyne anions will participate in substitution reactions with carbon-based electrophiles like alkyl halides. Thus, it is relatively straightforward to build up carbon chains by alkylating alkynkes. This works best with primary alkyl halides and methyl halides. While secondary halides may work, they do suffer from side reactions (elimination reactions) and are not ideal. Tertiary halides will not participate in this reaction as they are too hindered for the acetylide to attack it. NaNH 2 H Na H 3 C Br δ + δ-CH 3 © 2006 Gregory R . Cook page 2 Chem 341 North Dakota State University...
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