10.2 : アルコールとフェノールの物理的特性

In alcohols and phenols, the high electronegativity of oxygen relative to carbon and hydrogen leads to a partial negative charge on oxygen and partial positive charges on hydrogen and carbon.

The opposite partial charges of oxygen–hydrogen bond dipoles in adjacent alcohol or phenol molecules attract each other in hydrogen-bonding interactions. In an aqueous solution, alcohols and phenols form large networks of hydrogen bonds with water molecules, which enhances their water solubility.

The nonpolar regions of phenol or alcohol molecules are attracted by dispersion forces, like the interactions observed between hydrocarbons.

The additional energy required to disrupt the hydrogen bonding in addition to the dispersion forces causes an increase in the boiling points of alcohols and phenols compared to hydrocarbons of similar molecular weights.

The boiling points of alcohols increase with the size of the alkyl region due to the greater surface area for interactions through dispersion forces.

However, the increased surface area of the nonpolar region, where solvation by water is unfavorable, results in a lower solubility of alcohols in water.

An alcohol with branching in the chain is more water-soluble than its linear equivalent, since branching reduces the contact surface of the nonpolar region. However, branched alcohols have lower boiling points than their linear analogs, which is consistent with weaker dispersion forces.

Additional hydrogen bonding sites, such as the second hydroxyl group in diols, increase the boiling point and water solubility of alcohols.

Cyclic alcohols exhibit higher boiling points than their linear analogs, which correlates with a tendency for denser packing in the liquid phase.

The boiling points of phenols are even higher due to the π–π stacking interactions between the planar aromatic rings. The comparatively large aromatic ring limits their solubility in water, but phenols exhibit better solubility than the corresponding alcohols.

In phenols, the hydrogen bonding is strengthened by the higher polarity of an oxygen–hydrogen bond dipole connected to an electron-withdrawing aromatic ring.