Ketones alpha halogenation

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... 

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... 

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. 

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... 

Alpha halogenation, as described in the previous section, occurs readily with ketones and aldehydes, but not with carboxylic acids, esters, or amides. This is likely due to the fact that these functional groups are not readily converted to their corresponding enols. Nevertheless, carboxylic acids do undergo alpha halogenation when treated with bromine in the presence of PBt3. 

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

Ketones and aldehydes will undergo alpha halogenation in acidic or basic conditions. 

This small amount of enol is able to react as a nucleophile and attack some electrophile. After the enol attacks the electrophile, the keto-enol equilibrium is re-established by producing some more enol (to account for the enol that disappeared as a result of the reaction). Slowly but surely, most of the ketone molecules end up converting into enols and reacting with the electrophile. The most common example is alpha-halogenation, which can occur when a ketone is treated with Br2 in aqueous acid (HsO ) to generate an a-halo ketone ... 

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. 

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). 

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

Halogens add to ketones at the alpha carbon in the presence of a base or an arid-When a base is used, it is dffieult to prevent haJogdnation at more than one of the alpha positions. The base is also consumed by the reaction with water as a by-product, whereas the acid acts as a true catalyst and is not consumed. 

When a base is used with a methy] ketone, the alpha carbon will become completely halogenated. This trihalo product reacts further with the base to produce a carboxylic acid and a haloform (chloroform, CHCI j bromoform, CHBr, or iodoform, CHIj). This is called the Haloform Reaction. 

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