Ketones tautomerization

In acid solution the half-wave potentials for these processes are pH dependent. The overall reaction involves two electrons and is irreversible. Bond cleavage is believed to lead to the enol as shown in Scheme 5.4. Where, as with acetophenone, the ketone product is electroactive at more negative potentials, the wave height for ketone reduction is less than expected and is limited by the rate of enol to ketone tautomerism. This is because the enol is not electroactive. 

C is correct. Tautomerization is an equilibrium represented by a proton shift. Ketones tautomerize to form enols where the carbonyl carbon from the ketone becoms part of an alkene by forming a double bond with a neighboring carbon. In choice C, the carbonyl carbon does not form part of the alkene. 

Compound B is the doubly ketonic tautomeric form of hydroquinone, compound C, to which it isomerizes on standing in water. 

When substituents are present in the a position, the enol ketone tautomerism creates an asymmetric carbon and the deconjugated derivative is obtained as a racemic mixture. In a chiral environment, protonation of one or the other enantioface of the dienol leads to either the R or the S enantiomer. Differences in the diastereoisomeric interactions developed between the chiral auxiliary and the two enantiofaces will induce enan-tioselective production of the deconjugated product. 

When pyruvate kinase transfers the phosphoryl group from phosphoenolpyruvate to ADP, the remaining enediol reminant, enolpyruvate, is much more unstable than its ketone tautomer, pyruvate. This enol-ketone tautomerization drives the overall reaction toward ATP formation by removing the enolpyruvate by converting it to pyruvate. 

The resulting ketone tautomerizes to an enol. In this case, the enol form predominates in the equilibrium, because of conjugation between the alkene and the lactone carbonyl. 

The ketone tautomerizes to die enol form, which in this case is more stable because of extended conjugation. 

The remaining ketone tautomerizes, producing a pyrone ring. 

The ketone tautomerizes to the enolic form, that in this molecule is the most stable tautomer because of having an alkene conjugated with an ester. 

Let s explore how this process occurs. The ketone tautomerizes to generate a small amount of enol. Then comes the critical step the enol functions as a nucleophile to attack Br2 (the electrophile) ... 

Benedict s solution also oxidizes ketoses. We certainly do not expect this because Benedict s solution does not oxidize ketones. However, a-hydroxy ketones tautomerize in basic solution,... 

Scheme 3.92 Triggering via acid-catalyzed enol-ketone tautomerization [348],...

Tautomerism may be of many kinds as illustrated in this book and in Ref. [1]. However, tautomerism may be less obvious depending on the rates of interconversion or the equilibrium constant may be very much in favor of one form. A typical example of the latter is acetone and other ketones. Nevertheless, tautomerism plays a major role in the chemistry of alkyl ketones. Tautomerism may occur both in the gas phase, namely the liquid and condensed phases, and in the solid state, namely in the ground state as well as in the excited state. As tautomerization often involves the movement of light atoms, proton transfer is the usual case and hydrogen bonding will often be involved. Tautomerization can of course be both intra- and intermolecular, and in the latter case a solvent molecule may also be involved. For some characteristic cases, see Ref. [1, Chapter 3]. 

The accepted reaction mechanism involves coordination/activation of the triple bond to the metal followed by water addition, enol-ketone tautomerization followed... 

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