Physical Properties of Carboxylic Acids

A carboxylic acid's $\text{-COOH}$ group is unusual among common functional groups in being able to act as both a hydrogen-bond donor (through its acidic $\text{-OH}$ hydrogen) and a hydrogen-bond acceptor (through the lone pairs on its carbonyl oxygen) at the same time. This dual capability lets two carboxylic acid molecules pair up so that each simultaneously donates a hydrogen bond to the other and accepts one back, forming a closed, ring-shaped cyclic dimer held together by two hydrogen bonds at once rather than just one.

Why this matters for boiling point. This dimerisation is strong enough that it persists not just in the pure liquid, but substantially even in the vapour phase and in non-polar solvents -- carboxylic acids exist as dimers under a very wide range of conditions, not only in the solid or liquid state the way an ordinary hydrogen-bonded liquid (such as an alcohol) might. Because each dimer behaves, for boiling-point purposes, as though it were a single particle of roughly double the individual molecule's mass, and because two hydrogen bonds must be broken (not just one, as for a simple alcohol-alcohol hydrogen bond) before the pair separates, carboxylic acids have distinctly higher boiling points than an alcohol of comparable molar mass -- often higher, in fact, than an alcohol with a considerably greater molar mass. Formic acid, $\text{HCOOH}$ (molar mass $46$), for example, boils at $100.5\,^\circ\text{C}$, higher than ethanol ($46$ g/mol, boiling point $78\,^\circ\text{C}$) despite formic acid actually having a smaller molecular formula weight when unassociated -- the difference is entirely attributable to the extra strength of the doubly hydrogen-bonded cyclic dimer that formic acid, but not ethanol, is able to form. …