Question on organic chemistry where we are required to find enol content
Thermodynamically, the greater enol content can be associated with greater acidity. Thus, the question can be rephrased as ‘which is the strongest acid of these four?’ We can ignore the interconversion of the methyl ester A into the ethyl ester B — it will happen because we are in ethanolic solution, but extremely slowly as we do not have any strong bases present. (And even if we had strong bases, these would deprotonate A more further reducing the susceptibility of nucleophilic attack on the carboxy group.)

For reference and posteriority, these are the compounds we are talking about:


methyl acetoacetate (A)
ethyl acetoacetate (B)
acetoacetone (C)
3-methylacetoacetone (D)


Simply by looking at the names we can see how highly related they are and how much subtle differences must matter. We can predict all four of them to be rather acidic since if the inter-carbonyl hydrogen is released and an enolate is formed, this enolate will always be in contact with an electron-withdrawing &#x3c0; system on the right-hand side: either another keto-group or an ester group.

That immediately gives us our first hint to compare the four: an ester group is less electron-withdrawing than a ketone, so we predict A and B to be less strong acids than C and D. This is convenient as it means we do not have to concern ourselves with the acidity difference of a methyl versus ethyl ester.

The difference in acidity between C and D is also subtle but more obvious. In both cases, the corresponding enolate anion would be planar. However, the methyl group is much larger than a hydrogen atom meaning that the enolate of D is subject to 1,3-allylic strain — which cannot be lessened without destroying planarity and thereby reducing conjugation. Thus, the enolate of C is more stable than the enolate of D.

This is backed by acidity data from the Bordwell tables — see table.

$$\textbf{Table 1: }\text{acidity data of the four compounds}\\
\begin{array}{lcccc}\hline
\text{solvent} &amp; \mathrm{p}K_\mathrm{a}(\textbf{A}) &amp; \mathrm{p}K_\mathrm{a}(\textbf{B}) &amp; \mathrm{p}K_\mathrm{a}(\textbf{C}) &amp; \mathrm{p}K_\mathrm{a}(\textbf{D}) \\ \hline
\ce{H2O} &amp; -/- &amp; 10.7 &amp; \phantom{0}9.0 &amp; -/-\\
\text{DMSO} &amp; -/- &amp; 14.2 &amp; 13.3 &amp; 15.1\\ \hline\end{array}$$