Halogenation of acetone

Kinetic evidence in the hydroxide-catalyzed halogenation of acetone provides definitive mechanistic information. The overall reaction is... 

The oxidation of n-butanal by CUCI2 in dimethylformamide showed simple second-order kinetics in the presence of lithium chloride . At 83 °C, 2 s 2x10 1.mole". sec". a-Monohalogenation occurs in 97% yield. Cu(Il)-catalysed enolisation followed by ligand-transfer is proposed. a-Halogenation of acetone is accomplished by CUCI2, viz. 

In enzymes, the active site may possess acid and base groups intimately associated with the conjugate base and acid functions, respectively, of the complexed substrate the push-pull mechanism is possible but might not be a driving force. The halogenation of acetone in the presence of aqueous solutions of carboxylic acid buffers exhibits the rate law of Equation 11.2 where the third-order term, although small, has been shown to be significant and due to bifunctional concerted acid-base catalysis (Scheme 11.13) ... 

Scheme 11.13 Bifunctional acid-base catalysis in the halogenation of acetone via its enol.

The lowest value is based on the assumption that the halogenation of acetone occurs on encounter with the molecular halogens. 

One of the very first kinetic examples of this catalyzed prototropy was found in the halogenation of acetone. In polar solvents, it is found that the rate of halogenation of acetone is first-order in acetone, zero-order in halogen, X2, and subject to general acid-base catalysis ... 

The most numerous cases of homogeneous catalysis are by certain ions or metal coordination compounds in aqueous solution and in biochemistry, where enzymes function catalytically. Many ionic effects are known. The hydronium ion H3O and the hydroxyl ion OH catalyze hydrolyses such as those of esters ferrous ion catalyzes the decomposition of hydrogen peroxide decomposition of nitramide is catalyzed by acetate ion. Other instances are inversion of sucrose by HCl, halogenation of acetone by H and OH , hydration of isobutene by acids, hydrolysis of esters by acids, and others. 

The Bronsted plot for the general acid-catalysed halogenation of acetone is illustrated in Figure 10. Carboxylic acid catalysts exhibit... 

Figure 10 Halogenation of acetone catalysed by oxygen and ammonium ion acids...

Fig. 1. Brensted relationship for the base-catalysed halogenation of acetone [3] -I-, amines , pyridines O, carboxylate ions and water.

There is of course ample evidence that acid-base catalysis in solvents of low dielectric constant does not necessarily involve a concerted process. Such a process cannot operate when catalysis is effected by a single acid or base present in an aprotic solvent, and there are many examples of this, including typical prototropic reactions such as the halogenation of acetone, the racemization and inversion of optically active ketones, and the mutarotation of nitrocamphor. Moreover, in the isomerization of mesityl oxide oxalic ester in chlorobenzene, which depends kinetically on the interconversion of two isomeric enols, the velocity in a solution containing both an amine and an acid is no greater than the sum of the velocities for the two catalysts separately, in contrast to the behaviour found by Swain for the mutarotation reaction. 

It was first shown by Lapworth that in aqueous solutions of strong acids acetone reacts with bromine and chlorine at a rate which is indexable 22. HALOGENATION OF ACETONE IN AQUEOUS SOLUTION AT 25°C CATALYSED BY HYDROGEN IONS... 

Extensive work, especially by Dawson and his collaborators, has shown that in buffer solutions of carboxylic acids the halogenation of acetone is catalysed by both carboxylic acids and carboxylate ions, as well as by hydrogen and hydroxide ions. In this situation of general catalysis by both acids and bases the observed velocity constant will be given by an expression of the form... 

The halogenation of acetone is also catalysed by a number of alkyl-pyridines, but apparent acid catalysis by ammonium ions was traced to a specific mechanism,... 

The studies that led to this mechanism were carried out in London over a century ago by Arthur Lapworth, who found that the rate of halogenation of acetone is the same regardless of the halogen and its concentratioa... 

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