# Protonation of ketone and ester

If there is a ketone and ester functional group in the same compound and I want to protonate with $\ce{HCl}$, which one will protonate first? Is it the carbonyl group in the ketone or the carbonyl group in the ester and why? Is it related to the $\mathrm{p}K_\mathrm{a}$ of the functional group?

• The $\mathrm{p}K_\mathrm{a}$ values of protonated esters, protonated ketones and hydrochloric acid are close to each other (-7 to -6 according to www2.onu.edu/~b-myers/organic/2511_Files/pKa%20table.pdf), so $\ce{HCl}$ will protonate both groups, perhaps with a slight preference for the ketone. Jun 2 '15 at 17:36

As was mentioned in the comments, with reference to the Myers $\mathrm{p}K_{\mathrm{a}}$ table, the $\mathrm{p}K_{\mathrm{a}}$ values of a protonated ester and a protonated ketone are largely similar (in the region of $-7$ to $-6$).
All these effects will also slightly modify the $\mathrm{p}K_{\mathrm{a}}$ of the functional group you’re observing. The one with the higher $\mathrm{p}K_{\mathrm{a}}$ will be protonated first. Thus, it is impossible to give a general answer here, it depends on which molecule you have and which groups are where.
Side fact: $\ce{HCl}$ has almost the same $\mathrm{p}K_{\mathrm{a}}$, so the protonation will be very close to equilibrated between all three components.