10.2: Physical Properties of Alcohols and Phenols

Alcohols are organic compounds in which a hydroxy group is attached to a saturated carbon. Phenols are a class of alcohols containing a hydroxy group attached to an aromatic ring. The physical properties of the alcohols and phenols are influenced by hydrogen bonding due to the oxygen–hydrogen dipole in the hydroxy functional group and dispersion forces between alkyl or aryl regions of alcohol and phenol molecules.

Alcohols possess a higher boiling point than aliphatic hydrocarbons of similar molecular weights due to intermolecular hydrogen bonding. As in hydrocarbons, the dispersion forces are the reason for the higher boiling point upon increasing carbon chain length.

Hydrogen bonding between the hydroxy group and water facilitates the solubility of alcohols in water. However, water solubility also depends on the length of the alkyl or nonpolar region of the molecule. Alcohols with an alkyl region of up to three carbon atoms are miscible with water. As the chain length increases, the increased surface area of the nonpolar region hinders solvation by the water molecules.

The solubility of branched alcohols is higher than that of linear alcohols of similar molecular weight. Branching reduces the surface area for intermolecular interactions between nonpolar regions; hence, the hydrophobic nonpolar region is smaller. Because of the weaker intermolecular interactions, the boiling points of branched alcohols are lower than the corresponding linear alcohols.

Multiple sites for hydrogen bonding in one molecule increase the boiling point; therefore, diols and amino alcohols possess higher boiling points and better water solubility than alcohols.

Compared to linear alcohols, cyclic alcohols can only exist in a limited number of conformations due to steric restrictions. The increased intermolecular interactions that arise from the close packing of cyclic alcohol in the liquid phase results in a higher boiling point as compared to that of a linear alcohol.

Intermolecular hydrogen bonds also play a role in defining phenols' high boiling point and solubility in water. The boiling point of phenol is higher than that of the corresponding aliphatic alcohol due to the close packing of phenol molecules, facilitated by π–π stacking interactions between the large, planar aromatic rings. Close-packed aromatic rings increase the surface area of the nonpolar region in the liquid phase and limit the solubility of phenol (9.3 g in 100 g H₂O). However, this solubility is higher than that of alcohols with a similar molecular weight due to the increased polarity of the oxygen–hydrogen bond dipole induced by adjacent electron-withdrawing aromatic rings.

Structure	Name	Molecular weight (g/mol)	Boiling point (oC)	Solubility (g/100 g H2O)
	1-Butanol	74	118	9.1
	Isobutanol	74	108	10
	tert-Butanol	74	83	miscible (∞)
	Pentane	72	36	insoluble
	Propane-1,2-diol	76	188	miscible (∞)
	1-Hexanol	102	156	0.6
	Cyclohexanol	100	162	3.6
	Phenol	94	182	9.3
	Toluene	92	110	insoluble

Alcohols are widely used as antiseptics due to their antibacterial properties. Isopropanol or ethanol is the major component of hand sanitizer. An ideal antibacterial agent should have a significant nonpolar region or alkyl region that can penetrate the cell membranes of microorganisms and destroy them. At the same time, it should have high solubility in the transport medium, which is water. In smaller alcohols, the optimal balance between these two conditions is fulfilled.

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.