# Why isn't the hydroxyl group of citric acid deprotonated while the carboxyl groups are?

My AP chemistry book "Chemistry: the Central Science 12th edition" says that one molecule of citric acid generates 3 H+(aq) ions when dissolved in water. Looking at the structural formula:

There are three carboxyl groups. However, there are four instances of an -O-H link. Why don't all of these break? My guess is that the carboxyl groups can form resonance groups and thus obey the octet rule.

You are correct in saying that the carboxyl groups will have resonance structures after being deprotonated. Thus, the carboxyl groups are fairly acidic and will deprotonate in water. On the other hand, deprotonation of the hydroxyl group will not lead to any resonance stabilization, such that the hydroxyl group will be much less acidic than the carboxyl groups. In fact, the $\mathrm{p}K_\mathrm{a}$ of alcohols is usually in the range 16-20, which means that the alcoholic hydrogen will not deprotonate in water. It can deprotonate, however, in other solvents.