Keto-enol tautomerization reactions kinetics

We shall take up first the behavior of ketones toward the halogens, and see evidence that carbanions do indeed exist at the same time, we shall see an elegant example of the application of kinetics, stereochemistry, and isotopic tracers to the understanding of reaction mechanisms. And while we are at it, we shall see something of the role that keto-enol tautomerism plays in the chemistry of carbonyl compounds. 

The next stage of oxidation for organic compounds is to aldehydes and ketones. In acid solutions, aldehydes and ketones are often hydrolyzed to produce ketohydrates (two OH groups attached to the same carbon atom). They also undergo keto-enol tautomerism to convert into more acidic species which are then more capable of forming ionic bonds with metal ions. The principal kinetic question is whether the keto, enol, or ketohydrate form is the one involved in the oxidation reaction. The results of the most complete investigations of ceric oxidation of ketones and aldehydes are contained in table 4. 

The only isotope effects which are usually of significance in electroorganic mechanism considerations are those involving H and D in (a) primary kinetic isotope effects, (b) secondary solvent isotope effects where reactions are compared in pure H2O and D2O or pure h- and rf-alcohols, and (c) in prior protonation equilibria, e.g., with ketone reduction. Primary kinetic isotope effects having a magnitude of > 2.5 may be expected in reactions that involve a rearrangement with proton participation, e.g., keto-enol tautomerism prior to an electron transfer step. Most other H/D isotope effects arise from protonation equilibria prior to the rate-controlling electron transfer step (e.g., in ketone reduction RR CO + RR COH ) and the isotope effect is... 

We must now consider the kinetic analysis of catalysed reactions involving two proton transfers. These include the so-called prototropic isomerizations, notably keto-enol tautomerism. The simplest mechanisms for catalysis by acids and bases are as follows ... 

The study of prototropic tautomerization is intimately related to the study of proton transfer reactions. The study of the dynamics of proton transfer is as old as the study of reaction kinetics itself Indeed, the first reactions studied, that is, the inversion of sugar by Wilhehny in 1850 [65], involves a proton transfer as the elementary step in the reaction. In the first studies on the dynamics of tautomerization, primarily keto-enol tautomerization in acetone-like compounds were studied, which is a slow process involving a number of reaction steps of which the acid catalyzed keto-enol conversion was taken as the rate determining one [66]. In the past century, since 1910, nearly 2000 papers have been published on the kinetics of tautomerization, and in the first 60 years most of those were devoted to the ground-state reactions of the keto-enol type involving a C atom. Until the mid-1950s, only a handful of papers can be found this was obviously due to experimental hmitations. Two things are needed a method to start the reaction, and a method to follow it. In Dawson s experiments [66], the rate could be influenced by the amount of acid present, and the reaction could be followed because the enol produced... 

The review starts with a discussion of the mechanism of keto-enol tautomerisation and with kinetic data. Included in this section are results on stereochemical aspects of enolisation (or enolate formation) and on regioselec-tivity when two enolisation sites are in competition. The next section is devoted to thermodynamic data (keto-enol equilibrium constants and acidity constants of the two tautomeric forms) which have greatly improved in quality over the last decade. The last two sections concern two processes closely related to enolisation, namely the formation of enol ethers in alcohols and that of enamines in the presence of primary and secondary amines. Indeed, over the last fifteen years, data have shown that enol-ether formation and enamine formation are two competitive and often more favourable routes for reactions which usually occur via enol or enolate. 

Kinetics and spectroscopy of substituted phenylnitrenes, 36, 255 Kinetics, of organic reactions in water and aqueous mixtures, 14, 203 Kinetics, reaction, polarography and, 5, 1 Kinetic studies of keto—enol and other tautomeric equilibria by flash photolysis, 44, 325... 

The constant for reaction (31) for /3-diketones in tautomeric keto (HK) and enol (HE) forms, can be partitioned between Kk and KE (reactions 32 and 33). At equilibrium, equations (34) and (35) hold, and values obtained are given in Table 19. Kinetics showed that the reaction occurs through the enol form by parallel acid-independent and inverse-acid paths (Scheme 15).514 K... 

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