Halogenation acid-catalyzed

As m the acid catalyzed halogenation of aldehydes and ketones the reaction rate is mde pendent of the concentration of the halogen chlorination brommation and lodmation all occur at the same rate Formation of the enolate is rate determining and once formed the enolate ion reacts rapidly with the halogen... 

Evidence for the mechanism shown in Figure 22.4 includes the observation that acid-catalyzed halogenations show second-order kinetics and follow the rate law... 

The mechanism of acid-catalyzed halogenation involves attack of the enol form on the electrophilic halogen molecule. Loss of a proton gives the a-haloketone and the hydrogen halide. 

Acid-catalyzed halogenation results when the enol form of the carbonyl compound serves as a nucleophile to attack the halogen (a strong electrophile). Deprotonation gives the a-haloketone. 

Acid-catalyzed halogenation is synthetically useful for converting ketones to a,/3-unsaturated ketones, which are useful in Michael reactions (Section 22-18). Propose a method for converting cyclohexanone to cyclohex-2-en-l-one, an important synthetic starting material. 

The mechanism of acid-catalyzed halogenation consists of two parts tautomerization of the carbonyl compound to the enol form, and reaction of the enol with halogen. Mechanism 23.3 illustrates the reaction of (CH3)2C=0 with Br2 in CH3COOH. 

Lewis acid-catalyzed halogen exchange (F. Swarts)... 

The main synthetic route to CFC, HCFC and Halons is the Swarts fluorination. Technically this is often achieved by reaction of a chlorinated or brominated precursor with anhydrous hydrofluoric acid in the presence of a solid Lewis acid catalyst, for example chromia. Other important reactions are Lewis acid-catalyzed halogen isomerization and hydrogenolysis of chlorine or bromine. 

The kinetics of acid-catalyzed halogenation show the rate of halogenation to be independent of halogen concentration, but dependent upon ketone concentration and acid concentration. 

In both types of halogenation the products are the same, except in acid-catalyzed halogenation racemization can occur i.e. isomers of the product are formed. 

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. 

Acid-Catalyzed Halogenation In the presence of adds, halogenation takes place through the slow formation of an enol followed by rapid reaction of the enol with the halogen. 

Part of the evidence that supports these mechanisms comes from studies of reaction kinetics. Both base-promoted and acid-catalyzed halogenations of ketones show initial rates that are independent of the halogen concentration. The mechanisms that we have written are in accord with this observation in both instances the slow step of the mechanism occurs before the intervention of the halogen. (The initial rates are also independent of the nature of the halogen see Practice Problem 18.5.)... 

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