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Mark Brandon Printup

Why is ethanol more acidic than acetone?

Christopher Burgess  Follow

Taking the data from this answer by ron, the equilibrium constant for the keto-enol tautomerism seen in acetone is

$$K_\mathrm{eq} = \frac{\text{[enol]}}{\text{[carbonyl]}}$$

$$\begin{array}{lc}\hline\text{compound} & K_\mathrm{eq} \\\hline\text{acetaldehyde} & 6 \times 10^{-7} \\\text{acetone} & 5 \times 10^{-9} \\\hline\end{array}$$

This means that acetone exists predominantly (if not wholly) in its keto form. For every one molecule of the enol form observed, we can expect $\pu{2E8}$ molecules of the keto form.

Because of this, any deprotonation that takes place in acetone will have to be via breaking a $\ce{C-H}$ bond, and that is not a simple thing to do when you compare it to deprotonating an $\ce{O-H}$ bond in ethanol. Thus ethanol is more acidic than acetone.

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