How does the acidity of a primary alcohol compare to that of a tertiary alcohol?

Primary alcohols are generally more acidic than tertiary alcohols. This is because the resulting primary alkoxide ion is less sterically hindered, allowing for better stabilization through solvation by solvent molecules.

Why is phenol significantly more acidic than ethanol?

Phenol is more acidic because its conjugate base, the phenoxide ion, is stabilized by resonance, allowing the negative charge to be delocalized over the aromatic ring. Ethanol's conjugate base, ethoxide, has no resonance stabilization.

Between water and ethanol, which is the stronger acid and why?

Water is a slightly stronger acid than ethanol. The alkyl group in ethanol is weakly electron-donating, which destabilizes the ethoxide ion relative to the hydroxide ion formed from water.

What is a common mistake when predicting the effect of multiple alkyl groups on alcohol acidity?

A common error is assuming more carbons increase acidity. In reality, more alkyl groups (as in tertiary alcohols) decrease acidity due to increased electron donation (induction) and decreased solvation of the alkoxide ion.

Why can't sodium hydroxide ($NaOH$) be used to completely convert ethanol into sodium ethoxide?

Because water ($pK_a$ 15.7) is a stronger acid than ethanol ($pK_a$ 16), the equilibrium favors the reactants. A much stronger base, like $NaH$, is needed to drive the reaction to completion.

What error occurs if one ignores the distance of a halogen substituent from the hydroxyl group?

The inductive effect is distance-dependent; a halogen on the alpha-carbon increases acidity much more than one on the beta or gamma carbon. Ignoring this leads to incorrect acidity rankings.

Define an 'alkoxide ion' in the context of alcohol acidity.

An alkoxide ion ($R-O^-$) is the conjugate base formed when an alcohol loses its hydroxyl proton. It is a strong base and a powerful nucleophile.

What does the $pK_a$ value represent for an alcohol?

The $pK_a$ is the negative logarithm of the acid dissociation constant ($K_a$). It quantifies acid strength; for alcohols, values usually fall between 15 and 18, with lower values indicating higher acidity.

What is the general formula for the reaction of an alcohol with sodium metal?

The reaction is $2R-OH + 2Na \rightarrow 2R-O^-Na^+ + H_2$. It produces a metal alkoxide and hydrogen gas through a redox process where the alcohol acts as an acid.

How do electron-withdrawing groups (EWGs) affect the $pK_a$ of an alcohol?

EWGs lower the $pK_a$ of an alcohol. By withdrawing electron density, they stabilize the negative charge on the conjugate base (alkoxide), making the parent alcohol more acidic.

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AS-Level

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Chemistry

16. Hydroxy Compounds

Acidity of Alcohols

Summary

Alcohols act as weak Brønsted-Lowry acids by donating a proton from the hydroxyl group to form an alkoxide ion. Their acidity is governed by the stability of the resulting conjugate base, which is influenced by inductive effects, steric hindrance, and solvation efficiency.

1. Definition & Core Concepts

Diagram showing the dissociation of an alcohol into an alkoxide ion and a proton.

2. Underlying Principles: Inductive Effects

Electron-Withdrawing Groups (EWGs): Atoms like Fluorine or Chlorine pull electron density away from the negatively charged oxygen through sigma bonds. This disperses the negative charge, stabilizing the alkoxide and increasing acidity.
Electron-Donating Groups (EDGs): Alkyl groups are weakly electron-donating. They push electron density toward the oxygen, intensifying the negative charge and destabilizing the alkoxide, which decreases acidity.
Distance Dependency: The inductive effect weakens significantly as the distance between the electronegative substituent and the hydroxyl group increases.

3. Steric Effects & Solvation

Solvation Stabilization: In solution, alkoxide ions are stabilized by solvation, where solvent molecules surround the ion and dissipate the charge. This is a major factor in determining relative acidity.
Steric Hindrance: Bulky alcohols (like tertiary alcohols) produce bulky alkoxide ions. These large groups prevent solvent molecules from closely approaching the oxygen atom, leading to poor solvation.
Acidity Trend: Because they are more easily solvated, primary alcohols are generally more acidic than secondary alcohols, which are more acidic than tertiary alcohols ( $1^\circ > 2^\circ > 3^\circ$ ).

4. Key Distinctions

5. Reactions with Metals and Bases

6. Exam Strategy & Tips