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)
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.