Physical Properties of Phenols

Phenol ($\text{C}_6\text{H}_5\text{OH}$) is, at room temperature, a colourless, low-melting

crystalline solid (melting point about $41\,°\text{C}$) with a characteristic sharp odour; on

prolonged exposure to air and light, however, samples slowly turn a pale pink to red colour,

because trace oxidation forms coloured, conjugated oxidation products -- a familiar practical sign

that a bottle of phenol is old rather than freshly prepared.

Hydrogen bonding and boiling point. Like an alcohol, phenol's $-\text{OH}$ group both donates

and accepts hydrogen bonds, so phenol molecules associate extensively with one another through

intermolecular hydrogen bonding, giving phenol a distinctly higher boiling point ($182\,° \text{C}$) than a hydrocarbon of comparable molar mass such as toluene (b.p. $111\,°\text{C}$,

$M = 92$, versus phenol's $M = 94$) -- exactly the same comparison, and for the same underlying

reason, made for alcohols against alkanes in Section 16.4.

Solubility. The same hydrogen-bonding $-\text{OH}$ group also lets phenol hydrogen-bond with

water, so phenol is moderately soluble in cold water (about $8$–$9\ \text{g}$ per $100\ \text{mL}$

at room temperature) and becomes fully miscible with water above about $66\,°\text{C}$; it is also

freely soluble in common organic solvents such as ethanol, ether and benzene. This moderate

water-solubility -- neither fully miscible like methanol nor essentially insoluble like a large

hydrocarbon -- reflects the same balance seen with alcohols, between the single polar,

hydrogen-bonding $-\text{OH}$ group and the comparatively large, non-polar aromatic ring it is

attached to.

Acidity as a physical clue. Phenol's aqueous solution is weakly acidic to litmus (turning blue …