Acid-catalyzed alpha halogenation

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... 

Based on the energy diagram in CTQ 11, which is rate-limiting in acid-catalyzed alpha-halogenation, keto-enol tautomerization or halogenation (circle one)... 

Acid-catalyzed alpha halogenation does not work for esters, amides, or carboxylic acids, but it does work for acid halides. This fact can be exploited to generate a-brominated carboxylic acid derivatives as shown in Synthetic Transformation 25.5. 

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... 

Q Give mechanisms for the acid-catalyzed and base-promoted alpha halogenations Problems 22-68 and 69 of ketones. Explain why multiple halogenations are common with basic catalysis, and give a mechanism for the haloform reaction. 

We have seen that ketones will undergo alpha halogenation in acid-catalyzed conditions. A... 

Sulfur tetrafluoride [7783-60-0] SF, replaces halogen in haloalkanes, haloalkenes, and aryl chlorides, but is only effective (even at elevated temperatures) in the presence of a Lewis acid catalyst. The reagent is most often used in the replacement of carbonyl oxygen with fluorine (15,16). Aldehydes and ketones react readily, particularly if no alpha-hydrogen atoms are present (eg, benzal fluoride [455-31-2] from benzaldehyde), but acids, esters, acid chlorides, and anhydrides are very sluggish. However, these reactions can be catalyzed by Lewis acids (HP, BF, etc). 

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