Physical Properties of Carboxylic Acids

(i) Aliphatic carboxylic acids up to nine carbon atoms are colourless liquids with a pungent (often sharply sour) odour; the higher members are odourless, wax-like solids. (ii) Carboxylic acids have HIGHER boiling points than aldehydes, ketones, and even alcohols of comparable molecular mass (Question 24), because carboxylic acid molecules associate through genuine INTERMOLECULAR hydrogen bonding -- and, distinctively, each -COOH group can form TWO such hydrogen bonds simultaneously (donating one O-H...O and accepting one O...H-O), so that in the vapour phase most carboxylic acids actually exist as cyclic, hydrogen-bonded DIMERS rather than as free monomers, an association even stronger than the single hydrogen bond an alcohol can form. (iii) Lower aliphatic carboxylic acids (up to four carbons) are miscible with water in all proportions, owing to hydrogen-bond formation with water molecules; higher carboxylic acids are insoluble in water, owing to the increasingly dominant hydrophobic character of their growing hydrocarbon portion -- the simplest AROMATIC carboxylic acid, benzoic acid, is likewise insoluble in water despite having only seven carbons, since the bulky, non-polar benzene ring already outweighs the polar -COOH group's water-solubilising effect. (iv) Vinegar is a 6-8% aqueous solution of acetic acid. Pure, water-free acetic acid is called GLACIAL acetic acid, so named because it forms ice-like crystals on cooling: whe …