Alpha halogenation of ketones

Mechanism 22-4 Base-Catalyzed Keto-EnolTautomerism 1047 Mechanism 22-5 Acid-Catalyzed Keto-EnolTautomerism 1047 22-3 Alkylation of Enolate Ions 1050 22-4 Formation and Alkylation of Enamines 1051 22-5 Alpha Halogenation of Ketones 1054... 

Give mechanisms for acid-catalyzed and base-promoted alpha halogenation of ketones and acid-catalyzed halogenation of acids (the HVZ reaction). Explain why multiple halogenation is common with basic catalysis, and give a mechanism for the haloform reaction. 

Carbonyl compounds are in a rapid equilibrium with their enols, a process called keto-enol tautomerism. Although enol tautomers are normally present to only a small extent at equilibrium and usually can t he isolated in pure form, they nevertheless contain a highly nucleophilic double bond and react with electrophiles in an a-substitution reaction. An example is the a halogenation of ketones on treatment with CI2 or Br2 in acid solution. Alpha hromination of carboxylic acids can be similarly accomplished by the Hell-Volhard-Zelinskii (HVZ) reaction, in which an acid is treated with Br2 and PBra. 

Alpha halogenation of aldehydes and ketones occurs readily under basic conditions and does not stop with the replacement of a single hydrogen atom. All a-hydrogen atoms are substituted. 

Halogenation of saturated aldehydes and ketones usually occurs exclusively by replacement of hydrogens alpha to the carbonyl group ... 

Mechanism 22-6 Base-Promoted Halogenation 1054 Mechanism 22-7 Final Steps of the Haloform Reaction 1056 Mechanism 22-8 Acid-Catalyzed Alpha Halogenation 1058 22-6 Alpha Bromination of Acids The HVZ Reaction 1059 22-7 The Aldol Condensation of Ketones and Aldehydes 1060... 

Addition of an Enolate to Ketones and Aldehydes (a Condensation) 1046 Substitution of an Enolate on an Ester (a Condensation) 1046 Base-Catalyzed Keto-EnolTautomerism 1047 Acid-Catalyzed Keto-EnolTautomerism 1047 Base-Promoted Halogenation 1054 Final Steps of the Haloform Reaction 1056 Acid-Catalyzed Alpha Halogenation 1058 Acid-Catalyzed Aldol Condensation 1063 1,2-Addition and 1,4-Addition (Conjugate Addition) 1085... 

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