10.3
Acidity and Basicity of Alcohols and Phenols
Like water, alcohols are weak acids and bases. This is attributed to the polarization of the O–H bond making the hydrogen partially positive. Moreover…
While the neutral forms of alcohols and phenols are amphoteric in nature, their corresponding anions form weak bases.
The oxygen atom of an alcohol polarizes both its adjacent bonds. While the polarization of the O–H bond makes alcohol a weak acid, the lone pairs on oxygen make it both basic and nucleophilic.
The conjugate base of an aliphatic alcohol is an alkoxide ion, and that of a phenol is a phenoxide or phenolate ion. A negative charge is more stable on an oxygen atom than on nitrogen or carbon but most stable on a halogen atom.
Recall that conjugate base stability is a measure of acidity. Therefore, alcohols are more acidic than terminal alkynes, amines, and alkanes but less acidic than hydrogen halides.
Typical alcohols like methanol or ethanol are almost as acidic and basic as water. However, phenols are more acidic than alcohols but less acidic than carboxylic acids. Unlike alkoxide ions, the negative charge on the oxygen atom of phenoxide is stabilized by resonance and polarization via its aromatic ring.
Therefore, phenols can be deprotonated by a relatively weak base like hydroxide. However, alcohols can only be deprotonated by stronger bases like sodium metal or potassium metal or sodium hydride with hydrogen evolution.
The acidity of an alcohol is further influenced by the inductive effect and the positions of its substituents, as seen here with fluoride. Trifluoro- and trichloroethanol are stronger acids than ethanol, since their conjugate bases are stabilized by the electron-withdrawing effect of the adjacent halogen atoms.
In phenols, while electron-withdrawing nitro and chloro substituents enhance the acidity, methoxy and alkyl substituents only change it marginally. Naphthols are slightly more acidic than unsubstituted phenol.
The relative gas-phase acidity of alcohols increases from methyl to tertiary because of the polarization effect of α-alkyl substituents.
This trend is reversed in the solution phase due to the solvation effect, which makes a branched tert-butanol molecule less acidic than unhindered ethanol.
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Q1: Why are alcohols considered amphoteric compounds?
Alcohols are amphoteric because the oxygen atom polarizes both its adjacent bonds. The polarized O–H bond makes alcohol a weak acid, while lone pairs on oxygen make it both basic and nucleophilic. This dual nature allows alcohols to donate protons as acids or accept them as bases, similar to water.
Q2: How does phenol acidity compare to alcohol acidity?
Phenols are significantly more acidic than alcohols—approximately one million times stronger. The conjugate base of phenol, called a phenoxide or phenolate ion, is stabilized by resonance delocalization of the negative charge across the aromatic ring. This resonance stabilization, combined with the polar effect of benzene, enhances phenol acidity by eight orders of magnitude compared to cyclohexanol.
Q3: What bases are required to deprotonate alcohols versus phenols?
Alcohols require strong bases like sodium metal, potassium metal, or sodium hydride for deprotonation, which react violently with the alcohol. Phenols, however, can be deprotonated by weaker bases like hydroxide ion. This difference reflects the greater stability of the phenoxide ion compared to alkoxide ions due to resonance stabilization.
Q4: How do electron-withdrawing substituents affect phenol acidity?
Electron-withdrawing groups like nitro and chloro substituents significantly enhance phenol acidity by stabilizing the phenoxide ion through delocalization of the negative charge. Ortho- and para-positioned nitro groups are most effective because they are directly conjugated to the phenoxide oxygen. Meta-positioned nitro groups have less stabilizing effect, making m-nitrophenol less acidic than its ortho or para isomers.
Q5: Why does alcohol acidity trend differ between gas phase and solution phase?
In the gas phase, alcohol acidity increases from methyl to tertiary alcohols due to polarization effects of alpha-alkyl substituents. However, this trend reverses in solution because solvation effects stabilize branched molecules less effectively. Tert-butanol becomes less acidic than ethanol in solution due to steric hindrance reducing solvation of the bulky alkoxide ion.
Q6: How can alcohols and phenols be separated from a mixture using acid-base extraction?
Phenols can be selectively extracted from an ether solution using dilute sodium hydroxide, which converts phenol to its sodium salt in the aqueous phase while alcohol remains in the ether layer. Conversely, dilute sodium bicarbonate extracts carboxylic acids and phenols together, leaving alcohols behind. These separations exploit the different acid-base properties of each compound class.
Q7: How do halogen substituents influence alcohol acidity through inductive effects?
Halogen substituents enhance alcohol acidity by stabilizing the conjugate alkoxide ion through electron-withdrawing inductive effects. Trifluoroethanol and trichloroethanol are stronger acids than ethanol because their adjacent halogen atoms withdraw electron density, stabilizing the negative charge on the oxygen atom of the conjugate base.