Keto/enol isomerization

The aldehyde or ketone is called the keto form and the keto enol equilibration referred to as keto-enol isomerism or keto-enol tautomerism Tautomers are constitu tional isomers that equilibrate by migration of an atom or group and their equilibration IS called tautomerism The mechanism of keto-enol isomerism involves the sequence of proton transfers shown m Figure 9 6... 

B. Keto-Enol Isomerization on Acidic Zeolite HZSM-5 Evidenced by H/D Exchange... 

The time scale of the classical temperatine-jnmp experiment ( 1 qs) as originally pioneered by Eigen has been shortened to nanoseconds and very recently to approximately 5 ps using lasers. The classical temperatnre-jump experiment has found only limited application to biological systems, in spite of its great success in determining, for example, proton transfer rates or keto-enol isomerizations. An important reason for its limited apphcation to enzyme research, apart from experimental difficulties such as optical artifacts as a result of the temperature-jump, is the relatively small deviation from equihbrium AG = AH —... 

STEPS 9-10 Hydrolysis and isomerization. Hydrolysis of the phosphate group at Cl of fructose 1,6-bisphosphate, followed by keto-enol isomerization of the carbonyl group from C2 to Cl, then completes gluconeogenesis. The isomerization is the exact reverse of step 2 in glycolysis (Figure 29.4). 

A large volume of work has been reported on rapid devolatilization of coal (heating rates approximating process conditions (21,22). Recently, the effects of coal minerals on the rapid pyrolysis of a bituminous coal were reported by Franklin, et al ( 23). They found that only the calcium minerals affected the pyrolysis products. Addition of CaCO3 reduced the tar, hydrocarbon gas and liquid yields by 20-30%. The calcium minerals also altered the oxygen release mechanism from the coal. Franklin, et al. attribute these effects to CaCOj reduction to CaO, which acts as a solid base catalyst for a keto-enol isomerization reaction that produces the observed CO and H2O. 

The process by which enols are converted to aldehydes or ketones is called keto-enol isomerism (or keto-enol tautomerism) and proceeds by the sequence of proton transfers shown in Figure 9.6. Proton transfer to the donble bond of an enol occurs readily because the carbocation that is produced is a very stable one. The positive charge on carbon is stabilized by electron release from oxygen and may be represented in resonance terms as shown on the following page. 

KETO-ENOL ISOMERISM ON TRANSITION METAL SURFACES, A DENSITY FUNCTIONAL THEORY STUDY... 

Keto-enol isomerism on transition metal surfaces... 

P-Keto acids, decarboxylation, 762—763, 768, 838, 840-841, 850 Keto-enol isomerism, 355, 705—707 Keto-enol tautomerism. See Keto-enol isomerism P-Keto esters acidity of, 831 alkylation of, 839-841, 850 Michael addition of, 846—847 nomenclature of, 832 preparation of... 

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