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Alcohols, Phenols and Ethers - Acidic nature of phenol

Grade 12ICSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Phenol (C6H5OHC_6H_5OH) is more acidic than alcohols but less acidic than carboxylic acids. Its acidic nature is demonstrated by its reaction with active metals (like NaNa) and aqueous NaOHNaOH to form phenoxide salts.

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The acidity of phenol is due to the formation of a stable phenoxide ion (C6H5O−C_6H_5O^-) after losing a proton. The negative charge on the oxygen atom is delocalized over the benzene ring through resonance, which stabilizes the ion.

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In contrast, alcohols (R−OHR-OH) form alkoxide ions (R−O−R-O^-) upon losing a proton. Alkoxide ions are not resonance-stabilized, and the +I+I effect of the alkyl group increases electron density on the oxygen, making the ion less stable.

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The presence of Electron Withdrawing Groups (EWG) like −NO2-NO_2, −CN-CN, or −Cl-Cl at ortho and para positions increases the acidity of phenol by stabilizing the phenoxide ion through −M-M and −I-I effects.

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The presence of Electron Donating Groups (EDG) like −CH3-CH_3 or −OCH3-OCH_3 decreases the acidity of phenol because they increase the electron density on the oxygen atom, destabilizing the phenoxide ion through +I+I and +M+M effects.

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Phenols do not react with sodium bicarbonate (NaHCO3NaHCO_3), unlike carboxylic acids, because they are weaker acids than carbonic acid (H2CO3H_2CO_3).

📐Formulae

C6H5OH+H2O⇌C6H5O−+H3O+C_6H_5OH + H_2O \rightleftharpoons C_6H_5O^- + H_3O^+

Ka=[C6H5O−][H3O+][C6H5OH]K_a = \frac{[C_6H_5O^-][H_3O^+]}{[C_6H_5OH]}

pKa=−log⁡10KapK_a = -\log_{10} K_a

2C6H5OH+2Na→2C6H5ONa+H2↑2C_6H_5OH + 2Na \rightarrow 2C_6H_5ONa + H_2 \uparrow

C6H5OH+NaOH→C6H5ONa+H2OC_6H_5OH + NaOH \rightarrow C_6H_5ONa + H_2O

💡Examples

Problem 1:

Arrange the following compounds in increasing order of their acidic strength: Phenol, pp-Nitrophenol, pp-Cresol, and Ethanol.

Solution:

Ethanol < pp-Cresol < Phenol < pp-Nitrophenol

Explanation:

Ethanol is the least acidic because it is an alcohol. pp-Cresol contains a methyl group (−CH3-CH_3) which is an EDG, decreasing acidity compared to phenol. pp-Nitrophenol contains a nitro group (−NO2-NO_2), which is a strong EWG that stabilizes the phenoxide ion, making it the most acidic.

Problem 2:

Why is oo-nitrophenol less acidic than pp-nitrophenol despite both having a nitro group?

Solution:

pp-Nitrophenol is slightly more acidic than oo-nitrophenol.

Explanation:

In oo-nitrophenol, intramolecular hydrogen bonding occurs between the −OH-OH and −NO2-NO_2 groups. This makes it more difficult for the proton (H+H^+) to be released compared to pp-nitrophenol, where no such intramolecular bonding exists.