Physical state, boiling points and solubilities of carboxylic acids

Lower aliphatic carboxylic acids, up to about nine carbon atoms, are colourless liquids with distinctly irritating odours; the higher homologues beyond that are instead colourless, odourless, wax-like solids of low volatility. Table 12.5 tabulates the boiling points of the lower acids, which -- like the aldehydes and ketones of section 12.6.2 -- rise steadily with each added -CH2- unit. Crucially, though, carboxylic acids have consistently HIGHER boiling points than alkanes, ethers, alcohols, aldehydes OR ketones of genuinely comparable molecular mass (worked out carefully in Table 12.6). The reason: in the liquid phase, carboxylic-acid molecules characteristically pair up into DIMERS, each pair held together by TWO separate hydrogen bonds simultaneously -- the acidic O-H hydrogen of one molecule hydrogen-bonds to the carbonyl oxygen of its partner molecule, and, symmetrically, the partner's own acidic hydrogen bonds back to the first molecule's carbonyl oxygen (Fig. 12.3). Because this dimer pairing effectively DOUBLES the size of the unit that has to be separated for boiling to occur, the resulting van der Waals contribution to the boiling point rises sharply too -- and, remarkably, acetic acid dimers persist even in the GAS phase, not just in the liquid. Taken together with 12.6.1 and 12.6.3, this gives the chapter's overall ranking of intermolecular-force strength: hydrogen bond > dipole-dipole attraction > van der Waals force -- and so the matching boiling-point order: carboxylic acids > alcohols > ketones > aldehydes > ethers > alkanes. On SOLUBILITY: lower aliphatic carboxylic acids, up to four carbon atoms, are fully miscible with water, thanks to hydrogen bonding forming directly between the acid molecules and the surrounding water molecules; as molecular mass rises further, though, the growing hydrophobic hydrocarbon portion of the molecule steadily reduces water solubility, so higher carboxylic acids -- and aromatic acids such as benzoic acid too -- end up practically insoluble in water at room temperature (though they remain soluble in less-polar organic solvents such as ether, alcohol or benzene). A closing 'Do you know?' box notes some commercially convenient solid forms of the two smallest aldehydes: formaldehyde is …