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Acidic Nature of Alcohols | chemca

Acidic Nature of Alcohols | chemca
Organic Chemistry

Acidic Order of Alcohols

Understanding Inductive Effects, Solvation, and Stability of Alkoxide Ions.

By chemca Team • Updated Jan 2026

Alcohols are Brønsted acids because they can donate a proton ($H^+$) from the hydroxyl group. The acidity of an alcohol is primarily determined by the stability of the resulting Alkoxide ion ($RO^-$).

$$ R-O-H \rightleftharpoons R-O^- \text{ (Alkoxide)} + H^+ $$

More stable the Alkoxide ion $\rightarrow$ Equilibrium shifts right $\rightarrow$ Higher Acidity.

1. Inductive Effect (+I Effect)

Electron Releasing Groups

Alkyl groups ($R-$) are electron-releasing in nature (+I effect). They push electrons towards the oxygen atom.

  • This increases the electron density on the Oxygen atom in the alkoxide ion.
  • This destabilizes the negative charge on the Oxygen.
  • Consequently, the acidity decreases.
Acidity Order:
$$ \underset{\text{Methanol}}{CH_3OH} > \underset{1^\circ}{CH_3CH_2OH} > \underset{2^\circ}{(CH_3)_2CHOH} > \underset{3^\circ}{(CH_3)_3COH} $$
Summary: As the number of alkyl groups increases (from primary to tertiary), the +I effect increases, making the alkoxide less stable and the alcohol less acidic.

2. Solvation Effect (Steric Hindrance)

Stabilization by Water

In aqueous solution, alkoxide ions are stabilized by solvation (hydrogen bonding with water molecules).

Small Ions (e.g., Methoxide): Less sterically hindered. Extensive hydrogen bonding. Highly stabilized. More Acidic parent alcohol.

Bulky Ions (e.g., tert-Butoxide): Alkyl groups cause steric hindrance, preventing water molecules from approaching the Oxygen. Poorly solvated. Less stabilized. Less Acidic parent alcohol.

Stability of $RO^-$ (in water): $1^\circ > 2^\circ > 3^\circ$

3. Effect of Electron Withdrawing Groups (EWG)

-I Effect Increases Acidity

Atoms like Halogens ($F, Cl$) or groups like $NO_2, CN$ withdraw electrons from the O-H bond.

  • This disperses the negative charge on the alkoxide oxygen.
  • This stabilizes the ion.
  • Acidity increases.
Example:
$$ CF_3CH_2OH > CCl_3CH_2OH > CH_3CH_2OH $$

4. Important Comparisons

A. Water vs Alcohols

Water is generally a stronger acid than alcohols (except Methanol). The hydroxide ion ($OH^-$) is more stable than most alkoxide ions due to less +I effect (H vs R) and better solvation.

$$ CH_3OH > H_2O > CH_3CH_2OH > (CH_3)_2CHOH $$

Note: Methanol is slightly stronger than water due to higher polarizability, but for general purposes, Water > Ethanol.

B. Alcohols vs Phenols

Phenols are much more acidic ($pK_a \approx 10$) than alcohols ($pK_a \approx 16-18$).

  • Reason: In phenoxide ion, the negative charge is delocalized over the benzene ring via Resonance. Alkoxide ions have no resonance stabilization.

Knowledge Check

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