Chapter 16-4

Chapter 16-4 - CHEM 262 Wei You Chapter 16. Aldehydes and...

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11 CHEM 262 Wei You Chapter 16. Aldehydes and Ketones (4)
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22 α -Halogenation α -Halogenation: aldehydes and ketones with at least one α - hydrogen react at an α -carbon with Br2 and Cl2 -Halogenation can be catalyzed by either acid or base O Br 2 CH 3 COOH O Br HBr Acetophenone + + α -Bromo- acetophenone
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33 Acid-Catalyzed α -Halogenation Step 1: acid-catalyzed enolization Step 2: nucleophilic attack of the enol on halogen H R R'-C-C-R O R' C C H-O R R slow C R R H-O C R' Br Br R' C C O Br R R H Br: - + fast +
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44 Base-Catalyzed α -Halogenation Step 1: formation of an enolate anion Step 2: nucleophilic attack of the enolate anion on halogen • • + - - Resonance-stabilized enolate anion + slow O H R O C C R' R' C C O: R'-C-C-R - : OH H 2 O R R R R + fast R' C C O Br R R Br Br Br + - R' C C O: R R
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55 α -Halogenation Acid-catalyzed halogenation: introduction of the electronegative halogen on the α -carbon decreases the basicity of the carbonyl oxygen toward protonation, thereby slowing down introduction of a second halogen. Base-promoted
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This note was uploaded on 06/12/2010 for the course CHEM 261 taught by Professor Austell during the Spring '08 term at UNC.

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Chapter 16-4 - CHEM 262 Wei You Chapter 16. Aldehydes and...

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