Acetone acid-catalyzed bromination

For the acid-catalyzed ketone -> enol reaction of typical unhydrated ketones, AS appears to be close to the normal or collision theory value for a second-order reaction. For example, AS for the acid-catalyzed bromination of acetone is —12 e.u. (Rice and Kilpatrick, 1923). Since for normal keto-enol equilibria, AS0 is close to zero, the conclusion is that AS for the ketonization reaction would also be close to —12 e.u. 

Acid-Catalyzed Enolization of an Aldehyde or Ketone in Aqueous Solution 899 Acid-Catalyzed Bromination of Acetone 901 Cleavage of a Tribromomethyl Ketone 903... 

Acid-Catalyzed Bromination of Acetone THE OVERALL REACTION ... 

Propose a mechanism for the following reaction. (Hint Take note of aU of the products that are formed and base your answer on the mechanism for acid-catalyzed bromination of acetone shown in Section 18-3.)... 

It is now clear that an earlier ultraviolet study of acetone as an indicator (257) arrived at a misleadingly high basicity for acetone through failure to cover a wide enough range of acidities and to make proper allowance for solvent shifts. Unfortunately, this wrong value for the pKa was employed in a kinetic treatment of the acid-catalyzed bromination of acetone (4) which must now be reconsidered (61). 

Acetoxyandrost-5-en-17-one (59) is converted into the ethylene ketal (60) by treatment with ethylene glycol, triethylorthoformate and p-toluenesulfonic acid. The ketal is brominated with pyridinium bromide perbromide in THF and then treated with sodium iodide to remove bromine from the 5 and 6 positions. This gives the 16a-bromo compound (61) which is hydrolyzed in methanol to the free alcohol (62). Dehydrobromination is effected with potassium Fbutoxide in DMSO to give the -compound (63). Acid catalyzed hydrolysis of the ketal in aqueous acetone gives the title compound (64). ... 

Erlenmeyer was first to consider ends as hypothetical primary intermediates in a paper published in 1880 on the dehydration of glycols.1 Ketones are inert towards electrophilic reagents, in contrast to their highly reactive end tautomers. However, the equilibrium concentrations of simple ends are generally quite low. That of 2-propenol, for example, amounts to only a few parts per billion in aqueous solutions of acetone. Nevertheless, many important reactions of ketones proceed via the more reactive ends, and enolization is then generally rate-determining. Such a mechanism was put forth in 1905 by Lapworth,2 who showed that the bromination rate of acetone in aqueous acid was independent of bromine concentration and concluded that the reaction is initiated by acid-catalyzed enolization, followed by fast trapping of the end by bromine (Scheme 1). This was the first time that a mechanistic hypothesis was put forth on the basis of an observed rate law. More recent work... 

P cm 21.8 Show in detail how the enolization mechanism accounts for the following facts (a) the rate constants for acid-catalyzed hydrogen-deuterium exchange and bromination of acetone are identical (b) the rate constants for acid-catalyzed racemization and iodination of phenyl rec- tyl ketone are identical. 

This experiment shows that 4 is generated from 2, but how does this reaction occur A telling experiment shows that direct bromination of acetone is relatively slow, but in the presence of an acid catalyst, the reaction is much faster. This reaction is shown to be independent of the concentration of the halogen, except at very high concentrations of acid. It is known that the acid-catalyzed reaction begins with the acid-base reaction of acetone to give oxocar-benium ion 5, as seen in Chapter 18, Sections 18.5 and 18.6, but the presence of an acid catalyst also increases the percentage of enol 3. 

Homogenous catalysts may be further subdivided into specific-acid, specific-base, general-acid, general-base, nucleophilic, and electrophilic catalysts. A specific acid is merely a proton, a specific base is a hydroxyl ion. General-acids and -bases are any other acidic or basic species, respectively. The bromination of acetone illustrates these different catalysts. It may proceed by specific-acid catalyzed enolization (52) ... 

Cyanohydrins are formed selectively from saturated as against oc, 3-unsaturated ketones [113], and in the steroid field a further distinction can be made between Cs-ketones and C2o ketones by choice of suitable conditions [198]. Base-catalyzed exchange with acetone cyanohydrin is conveniently employed for their formation [198,199]. In the presence of this grouping which is quite stable to mildly acidic conditions, firee (x, unsaturated ketone groups can be converted into dienol ethers, benzyl thioenol ethers, and dioxolans by conventional methods [113], and saturated ketones can be brominated [200]. Cleavage of this group occurs under very mildly basic conditions such as in ethanol in the presence of pyridine [113]. An additional modification is provided by further conversion into cyanohydrin tetrahydropyranyl ethers which are more base-stable [199]. 

Homogeneous catalysts and the reaction components of these catalyst-catalyzed reactions remain in the same phase. Thus, the homogeneous catalysis requires both the catalyst and the reactants or the reaction components to be present in the same phase. For example, the net reaction between acetone and bromine in an acidic aqueous medium is expressed as the following ... 

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