An alcohol is an organic compound characterized by a hydroxyl functional group ($\text{—OH}$) attached to a saturated carbon atom. The general chemical structure is $\text{R—OH}$, where “R” represents the rest of the carbon-containing molecule. This $\text{—OH}$ group allows alcohols to act as both an acid and a base. Acids donate a proton ($\text{H}^+$), while bases accept a proton. Alcohols are therefore known as amphoteric compounds, exhibiting both behaviors depending on the chemical environment, much like water.
Understanding Alcohol as a Weak Acid
Alcohols exhibit weak acidic behavior by donating their hydroxyl proton ($\text{H}^+$). This is possible because the oxygen atom in the $\text{O—H}$ bond is highly electronegative, pulling electron density away from the hydrogen. This electron-pulling effect makes the hydrogen susceptible to removal by a strong base.
When an alcohol loses its proton, the resulting negatively charged species is called an alkoxide ion ($\text{RO}^-$). The stability of this alkoxide ion determines the alcohol’s acid strength. Alkoxides are strong bases, meaning they readily reclaim a proton, which confirms the parent alcohol is a relatively weak acid.
Simple alcohols have acidity slightly lower than water, with typical $\text{pK}_\text{a}$ values ranging from 16 to 18. Since a higher $\text{pK}_\text{a}$ indicates a weaker acid, alcohols require a very strong base, such as sodium hydride ($\text{NaH}$) or sodium metal, to form the alkoxide ion.
Understanding Alcohol as a Weak Base
Alcohols function as weak bases because the oxygen atom in the hydroxyl group possesses two lone pairs of electrons. These available lone pairs allow the oxygen atom to accept a proton ($\text{H}^+$) from a stronger acid.
Accepting a proton forms a positively charged alkyloxonium ion ($\text{ROH}_2^+$). This reaction occurs when alcohols are treated with strong mineral acids. The formation of the oxonium ion is crucial in organic reactions because it converts the $\text{—OH}$ group (a poor leaving group) into water (a much better leaving group).
Alcohols are very weak bases, comparable in strength to water. The resulting oxonium ion is highly reactive and temporary. This ion has a $\text{pK}_\text{a}$ value of about $\text{-2}$, confirming it is a very strong acid that readily gives up its proton.
Relative Strength: Which Property Dominates
Alcohols are amphoteric, meaning they can act as a proton donor (acid) or a proton acceptor (base). The dominant property depends entirely on the reaction environment and the other substances present. In the presence of a very strong base, the alcohol acts as a weak acid, forming the alkoxide ion. Conversely, with a very strong acid, the alcohol acts as a weak base, forming the oxonium ion.
The basic character of an alcohol is generally the more chemically relevant property in synthetic organic chemistry. This is because the resulting oxonium ion is a common and highly reactive intermediate used to drive many reactions forward. While alcohols are weak acids with high $\text{pK}_\text{a}$ values, they only react as acids with unusually strong bases. Thus, in most practical scenarios, alcohols are best described as weak acids that form reactive intermediates under highly acidic conditions.
Structural Features That Change Reactivity
The structure of the alkyl group ($\text{R}$) attached to the hydroxyl group significantly influences both the acidic and basic properties of the alcohol. This influence is primarily due to the inductive effect, which describes how substituent groups affect electron density. Alkyl groups, such as methyl or ethyl, are considered electron-donating groups.
When the alkyl group donates electron density toward the oxygen, it destabilizes the resulting alkoxide ion, decreasing the alcohol’s acidity. This explains why acidity decreases from primary to secondary to tertiary alcohols as the number of electron-donating groups increases. The increased electron density also slightly enhances the alcohol’s basicity. Solvation is another factor, as larger alkoxide ions are less effectively stabilized by solvent molecules, further decreasing acidity.