17.2 Properties of Alcohols and Phenols

Alcohols and phenols both contain a hydroxyl ($-OH$) group, but the environment around that group gives them very different properties. Understanding their acidity, hydrogen bonding behavior, and how substituents tune these properties will help you predict reactivity throughout this unit.

Properties of Alcohols and Phenols

Acidity of alcohols vs phenols

Phenols are significantly more acidic than alcohols, and the reason comes down to what happens to the conjugate base after the proton leaves.

• Phenols have $pK_a$ values around 10, while alcohols fall in the $pK_a$ 16–18 range. That's a difference of roughly $10^6$ to $10^8$ in acid dissociation constant, so phenols are orders of magnitude more acidic.
• When phenol loses a proton, the resulting phenoxide anion is stabilized by resonance. The negative charge on oxygen delocalizes into the aromatic ring across four resonance structures (oxygen plus three ring carbons at the ortho and para positions). A more stable conjugate base means a stronger acid.
• When an alcohol loses a proton, the alkoxide anion has no aromatic ring to delocalize into. The negative charge sits localized on oxygen, making the alkoxide a stronger base and the alcohol a weaker acid.

Electron-withdrawing groups on the phenol ring stabilize the phenoxide anion even further, pushing acidity higher. For example, p-nitrophenol ($pK_a \approx 7.2$) is about 1,000 times more acidic than unsubstituted phenol because the $-NO_2$ group delocalizes the negative charge through both resonance and inductive withdrawal. Pentachlorophenol ($pK_a \approx 4.7$) takes this to an extreme with five electron-withdrawing chlorine atoms.

Alcohols can also become more acidic through inductive effects, though the changes are more modest. 2,2,2-Trifluoroethanol ($pK_a \approx 12.4$) is roughly 10,000 times more acidic than ethanol ($pK_a \approx 16$) because the three fluorine atoms pull electron density away from the oxygen, stabilizing the alkoxide.

Hydrogen bonding in alcohol properties

The $-OH$ group makes alcohols behave very differently from hydrocarbons of similar size, and hydrogen bonding is the reason.

• The partially positive hydrogen of one molecule's $-OH$ is attracted to the partially negative oxygen of another molecule's $-OH$. This intermolecular hydrogen bonding creates extra attractive forces that hydrocarbons lack.
• Boiling points are noticeably higher as a result. Ethanol (MW 46, bp 78.4 °C) boils much higher than pentane (MW 72, bp 36.1 °C) even though pentane is heavier. The extra energy needed to break hydrogen bonds before vaporization accounts for this.
• Water solubility also increases because alcohols can hydrogen-bond with water molecules. Short-chain alcohols (up to about 4 carbons) are fully miscible with water. As the hydrocarbon chain grows longer, the nonpolar portion dominates and solubility drops. By the time you reach 1-hexanol, solubility is quite limited.

Phenols also hydrogen-bond, giving them higher boiling points and moderate water solubility compared to similarly sized aromatic hydrocarbons.

Substituent effects on phenol acidity

Substituents on the aromatic ring can shift phenol acidity dramatically, and the position of the substituent matters just as much as its identity.

Electron-withdrawing groups (EWGs) increase acidity:

• EWGs stabilize the phenoxide anion by pulling electron density away from the ring, spreading out the negative charge.
• Resonance effects dominate over inductive effects. A substituent at the ortho or para position can directly participate in resonance with the ring, providing stronger stabilization. A meta substituent can only operate through the weaker inductive pathway.
• Common EWGs: $-NO_2$, $-CN$, $-CF_3$, $-C(O)R$, $-SO_3H$

Electron-donating groups (EDGs) decrease acidity:

• EDGs push electron density toward the ring, destabilizing the negative charge on the phenoxide anion.
• Again, resonance effects are stronger than inductive effects, so EDGs at ortho and para positions have the largest impact. At the meta position, only the weaker inductive donation operates.
• Common EDGs: $-R$, $-OR$, $-OH$, $-NH_2$, $-NR_2$

EDGs also tend to increase the nucleophilicity of phenols by raising electron density on the oxygen, which becomes relevant in substitution reactions.

Reactions of Alcohols and Phenols

• Dehydration of alcohols produces alkenes. Under acidic conditions (commonly $H_2SO_4$ with heat), the $-OH$ is protonated to become a good leaving group (water), and elimination follows. Tertiary alcohols react fastest because they form more stable carbocation intermediates.
• Electrophilic aromatic substitution (EAS) is characteristic of phenols. The $-OH$ group is a strong activating group and an ortho/para director, so phenols react more readily than benzene in bromination, nitration, and similar EAS reactions. Phenol, for instance, brominates with $Br_2$ in water without needing a Lewis acid catalyst.