Physical Properties of Alcohols

The single most important structural fact about an alcohol's physical behaviour is that its

$-\text{OH}$ group can both donate a hydrogen bond (through its O--H hydrogen) and accept

one (through a lone pair on oxygen) -- so alcohol molecules hydrogen-bond extensively to one

another, and also to water.

Boiling point. Because breaking these intermolecular hydrogen bonds takes real energy on top of

the weaker dispersion forces present in every molecule, alcohols boil dramatically higher than

alkanes, haloalkanes or ethers of comparable molar mass, none of which can hydrogen-bond among

themselves the way an $-\text{OH}$-bearing molecule can. Ethanol ($M = 46$, b.p. $78\,°\text{C}$)

boils far above propane ($M = 44$, b.p. $-42\,°\text{C}$) or chloroethane ($M = 65$, b.p.

$12\,°\text{C}$), even though chloroethane is the heavier molecule -- the comparison shows the

effect is due to hydrogen bonding specifically, not simply molecular weight. Within the alcohols

themselves, boiling point still rises steadily with increasing chain length, exactly as for

alkanes, because the growing hydrocarbon part adds its own dispersion-force contribution on top of

the (roughly constant) hydrogen-bonding contribution from the single $-\text{OH}$ group; and

branching lowers the boiling point for the same steric/surface-area reason it does in alkanes, so

2-methylpropan-2-ol boils lower than its straight-chain isomer butan-1-ol.

Solubility in water. An alcohol dissolves in water by forming hydrogen bonds with water

molecules in place of the ones it would otherwise form with itself, so the lower alcohols

(methanol, ethanol, propan-1-ol) are completely miscible with water in all proportions. As the

hydrocarbon chain lengthens, however, the molecule's non-polar, hydrophobic part grows relative to

its single polar $-\text{OH}$ group, and that hydrocarbon 'tail' cannot hydrogen-bond with water at

all -- so water solubility falls steadily through the series (butan-1-ol is only partially miscible;

pentan-1-ol and higher members are only sparingly soluble, more like a hydrocarbon than like …