25.3. Functional Groups http://www.ck12.org
Aliphatic alcohols can be classified according to the number of R groups attached to the carbon with the hydroxyl
group. If one R group is attached to that carbon, the alcohol is a primary alcohol. If two R groups are attached, the
alcohol is a secondary alcohol. If three R groups are attached, the alcohol is a tertiary alcohol. Shown below is an
example of each. The primary alcohol is 1-propanol, the secondary alcohol is 2-butanol, and the tertiary alcohol is
2-methyl-2-propanol.
The simplest aromatic alcohol consists of a benzene ring in which one of the hydrogen atoms has been replaced by
the hydroxyl group. The name of this compound is phenol.
Properties of Alcohols
The smallest and lightest alcohols (methanol, ethanol, and propanol) are completely soluble in water in all pro-
portions. Recall that water molecules interact with one another through hydrogen bonding, which is an attraction
between the lone pair of electrons on the oxygen of one water molecule and an electron-deficient hydrogen atom
on another molecule. The hydroxyl group of the alcohol also undergoes hydrogen bonding. In solution, the alcohol
molecules and the water molecules form hydrogen bonds with each other, resulting in complete miscibility. However,
as the length of the nonpolar hydrocarbon chain increases, the aqueous solubility decreases. The solubilities of 1-
butanol, 1-pentanol, and 1-octanol per 100 g of water are 7.4 g, 2.7 g, and 0.06 g, respectively.
Hydrogen bonding also explains the relatively high boiling points of alcohols compared to alkanes of similar molar
mass (Table25.2).