9_Enol_Enolate

9_Enol_Enolate - Chem 267 Basic Organic Chemistry II Prof....

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Prof. Dr. Xiangdong Fang University of Waterloo Chem 267 Basic Organic Chemistry II 1
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2 O H keto form K OH enol form Enols and Enolization Enols and Enolization “aldehyde/ketone, and other carbonyl compounds having hydrogen atom(s) on the α -carbon , exist in solution as equilibrium mixtures of two or more isomeric forms” 1) the keto form represents a normal carbonyl compound; 2) the enol form gets its name from the combination of C=C bond and alcohol; 3) the interconversion between keto and enol forms is termed “ enolization ”, or keto-enol tautomerism ”; 4) keto and enol forms are NOT resonance forms (why?), but are called tautomers ;
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3 keto tautomer enol tautomer K = [ enol ] [ keto ] % enol acetaldehyde name H O OH H 2x10 -5 (in water) 2x10 -3 acetone O OH 2x10 -6 (in water) 2x10 -4 cyclohexanone O OH 2x10 -4 (in water) 0.02 ethyl acetoacetate O O OEt OH O OEt 4x10 -3 (in water) 0.4 8.7x10 -2 (liquid) 46 0.85 (in hexane) 8 acetylacetone O O OH O 0.25 (in water) 20 3.2 (liquid) 76 11.5 (in hexane) 92 phenol OH O >10 13 (in water) 100 keto-enol equilibria in various solvents Keto-Enol Equilibria 1) normally, the keto form is very much favoured over the enol form; 2) phenol and dicarbonyl compounds are extreme cases; 1) forming conjugate enones; 2) favourable 6- membered ring via hydrogen bonding; stable aromatic ring
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4 Hydrogen bonding leads to favourable 6-membered rings O O H H OO H hydrogen bonding O O H hydrogen bonding 6-membered ring conjugate enone H 6-membered ring non-conjuagte enone hydrogen bonding O OEt O H H H OEt hydrogen bonding O O H OEt hydrogen bonding more electron-withdrawing less electron-withdrawing
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5 Acid- or base-catalyzed keto/enol interconversion O OH - slow O slow OH OH - enol keto base-catalyzed keto-enol tautomerism H CH 2 O HOH major minor enolate
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6 Tautomerism is not resonance form 1) α -hydrogen atom to a carbonyl group is unusually acidic ; 2) α -hydrogen can be removed by a base to form an enolate anion, which can be stabilized by resonance delocalization;
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7 When more carbonyl groups attached to the same carbon. ..
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This note was uploaded on 07/07/2010 for the course CHEM 267 taught by Professor Fang during the Winter '09 term at Waterloo.

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9_Enol_Enolate - Chem 267 Basic Organic Chemistry II Prof....

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