Keto-enol tautomerism bromination

Carbonyl compounds are in a rapid equilibrium with called keto-enol tautomerism. Although enol tautomers to only a small extent at equilibrium and can t usually be they nevertheless contain a highly nucleophilic double electrophiles. For example, aldehydes and ketones are at the a position by reaction with Cl2, Br2, or I2 in Alpha bromination of carboxylic acids can be similarly... 

Hydroxypyridines react very much like phenols they are not subject to keto-enol tautomerism to a significant degree. In aqueous alkali the oxyanion is the reactive species with bromine entering positions ortho and para to that substituent. It has proved possible to achieve a high proportion of monobromination by the use of one molar equivalent of bromine. As the proportion of bromine to substrate was increased more... 

In an attempt to investigate the assumed keto-enol tautomerism of these compounds, a series of 2-oxoaIkylphosphonates were synthesized from triethyl phosphite and oc-bromoketones, and the amount of enol was determined by the bromine titration melhod. - These results, however, did not support the presence of the enol form because diethyl l,l-dimethyl-2-oxopropyIphosphonate, in which enolization is unlikely, had an enol content of 23% according to the consumption of bromine. 

It is reported in the literature (5-7) that m-bromo-phenolic compounds are more stable than their o- or p-brominated counterparts. An o- or p-Brominated phenol forms an unstable cyclohexadienone structure via keto-enol tautomerization. Upon heating, this unstable cyclohexadienone structure generates free radicals which, in turn, abstract a proton from a neighboring molecule to form HBr. 

Keto-enol tautomerism was discussed in Chapter 10 (Section 10.6) in connection with the hydration reaction of alkynes, including oxymercuration or hydroboration. The carbonyl form is more stable, and for most aldehydes and ketones only a tiny amount of enol is present. Acetone (2), for example, exists primarily as the ketone, and experiments show only 1.5 x 10 % enol. This experiment titrated acetone with diatomic bromine and measured the extent of reaction, which converted 2 to l-bromo-2-propanone (a-bromoacetone, 4). The second product in this reaction is HBr. By this experiment, the enol content of acetone—and presumably of other simple ketones—is remarkably small. 

Trifluoroacetylketene (91) has been generated in aqueous solution by flash photolysis. Rates of hydration to form the enol of 4,4,4-trifluoroacetoacetic acid (92e) have been measured, and also rates of the subsequent ketonization to the /3-keto acid (92k). Extensive rate and equilibrium constant data are reported for these reactions and for the ionizations of the tautomers. For example, the enol (92e) has acidity constants (in -logio form) of 1.85 and 9.95, for the acid and enol OH groups, respectively. Rates of enolization of (92k) have also been measured (by bromination) and, combined with an estimate of the hydration constant (K = 2900) of (92k), suggest that the keto-enol tautomeric constant is ca 0.5, about 100 times greater than that of its unfluorinated analogue. 

Thus it has been possible to show that in the bromination of acetone, a process which has been found to be unimolecular, not the normal keto-form, but the tautomeric enol-form reacts. The enol-form is present, in equilibrium with the keto-form, in amount too small to be measured. As soon as this amount has reacted a further quantity is formed and the process is repeated. That the reaction is unimolecular follows from the fact that it is the rate of rearrangement (I) which is measured, whilst the reaction of the enol with bromine (II) takes place with immeasurable rapidity (Lapworth). 

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