Evidence for equilibration of carbonyl compounds with enols

Evidence for equilibration of carbonyl compounds with enols 

If you run the NMR spectrum of a simple carbonyl compound (for example, 1-phenyl-propan-1-one, propiophenone ) in D2O, the signal for protons next to the carbonyl group very slowly disappears. If the compound is isolated from the solution afterwards, the mass spectrum shows that those hydrogen atoms have been replaced by deuterium atoms there is a peak at (M + 1) or (M +2) instead of at AT . To start with, the same keto-enol equilibrium is set up. 

when the enol form reverts to the keto form, it picks up a deuteron instead of a proton because the solution consists almost entirely of D2O and contains only a tiny amount of DOH (and no H2O at aU). 

Notice that the double bond in this enol could be either Eor Z. It is drawn as 2 here, but in reality is probably a mixture of both— though this is irrelevant to the reaction. We shall not be concerned with the geometry of enols in this chapter, but diere are some reactions that you will meet in later chapters where it is important, and you need to appreciate that the issue exists. 

The process can now be repeated with the other hydrogen atom on the same carbon atom. 

When we were looking at spectra of carbonyl compounds in Chapter 15 we saw no signs of enols in IR or NMR spectra. Dimedone is exceptional—although any carbonyl compound with protons adjacent to the carbonyl group can enolize, simpler carbonyl compounds like cyclohexanone or acetone have only a trace of enol present under ordinary conditions. The equilibrium lies well over towards the keto form (the equilibrium con- 

This is because the combination of a C-C double bond and an O-H single bond is (slightly) less stable than the combination of a C 0 double bond and a C-H single bond. The balance between the bond energies is quite fine. On the one hand, the O-H bond in the enol is a stronger bond than the C-H bond in the ketone but, on the other hand, the C=0 bond of the ketone is much more stable than the C=C bond of the enol. Here are some average values for these bonds.  

Typical amounts of enols in solution are about one part in 105 for normal ketones. So why do we think they are important Because enolization is just a proton transfer, it is occurring all the time even though we cannot detect the minute proportion of the enol Let us look at the evidence for this statement. 

Any reaction that simply involves the intramolecular transfer of a proton is called a tautomerism. Here are two other examples. 

This sort of chemistry was discussed in Chapter 8 where the acidity and the basicity of atoms were the prime considerations. In the first case the two tautomers are the same and so the equilibrium constant must be exactly 1 or, If you prefer, the mixture must be exactly 50 50. in the second case the equilibrium will lie on one side or the other depending on the nature of R. 

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EVIDENCE FOR THE EQUILIBRATION OF CARBONYL COMPOUNDS WITH ENOLS... 

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