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Aldehydes, Ketones and Carboxylic Acids - Acidity of Carboxylic Acids

Grade 12CBSEChemistry

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

🔑Concepts

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Carboxylic acids are significantly more acidic than alcohols and phenols because the carboxylate ion (RCOO−RCOO^-) is stabilized by two equivalent resonance structures where the negative charge is delocalized over two electronegative oxygen atoms.

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The strength of the acid is expressed by the acid dissociation constant (KaK_a) or its negative logarithm (pKapK_a). A higher KaK_a or a lower pKapK_a value indicates a stronger acid.

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Electron Withdrawing Groups (EWG) like −NO2-NO_2, −Cl-Cl, −F-F, and −CN-CN increase the acidity of carboxylic acids by stabilizing the carboxylate anion through the inductive effect (−I-I effect) and/or resonance effect (−M-M effect).

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Electron Donating Groups (EDG) like alkyl groups (−CH3-CH_3) or −OCH3-OCH_3 decrease the acidity by destabilizing the carboxylate anion via the +I+I or +M+M effect.

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The effect of a substituent decreases as its distance from the carboxyl group increases. For example, 22-chlorobutanoic acid is stronger than 33-chlorobutanoic acid.

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Aromatic carboxylic acids: Substituents on the benzene ring affect acidity. An EWG at the ortho or para position increases acidity, whereas an EDG decreases it. Due to the 'Ortho Effect', almost all ortho-substituted benzoic acids are stronger than benzoic acid regardless of the nature of the substituent.

📐Formulae

RCOOH+H2O⇌RCOO−+H3O+RCOOH + H_2O \rightleftharpoons RCOO^- + H_3O^+

Ka=[RCOO−][H3O+][RCOOH]K_a = \frac{[RCOO^-][H_3O^+]}{[RCOOH]}

pKa=−log⁡10KapK_a = -\log_{10} K_a

Acidity∝Stability of RCOO−∝Electron Withdrawing PowerElectron Donating Power\text{Acidity} \propto \text{Stability of } RCOO^- \propto \frac{\text{Electron Withdrawing Power}}{\text{Electron Donating Power}}

💡Examples

Problem 1:

Arrange the following in increasing order of their acidic strength: ClCH2COOHClCH_2COOH, Cl2CHCOOHCl_2CHCOOH, Cl3CCOOHCl_3CCOOH, CH3COOHCH_3COOH.

Solution:

CH3COOH<ClCH2COOH<Cl2CHCOOH<Cl3CCOOHCH_3COOH < ClCH_2COOH < Cl_2CHCOOH < Cl_3CCOOH

Explanation:

The chlorine atom is an electron-withdrawing group (−I-I effect). As the number of chlorine atoms increases, the electron-withdrawing effect increases, which further stabilizes the carboxylate ion and increases the acidity. Thus, trichloroacetic acid is the strongest.

Problem 2:

Between CH3CH2CH(Cl)COOHCH_3CH_2CH(Cl)COOH and CH3CH(Cl)CH2COOHCH_3CH(Cl)CH_2COOH, which one is a stronger acid and why?

Solution:

CH3CH2CH(Cl)COOHCH_3CH_2CH(Cl)COOH is a stronger acid.

Explanation:

The inductive effect (−I-I effect) of the chlorine atom is distance-dependent. In 22-chlorobutanoic acid (CH3CH2CH(Cl)COOHCH_3CH_2CH(Cl)COOH), the ClCl atom is closer to the −COOH-COOH group compared to 33-chlorobutanoic acid, resulting in greater stabilization of the carboxylate ion.

Problem 3:

Compare the acidity of Benzoic acid and 44-nitrobenzoic acid.

Solution:

4-nitrobenzoic acid>Benzoic acid4\text{-nitrobenzoic acid} > \text{Benzoic acid}

Explanation:

The nitro group (−NO2-NO_2) is a strong electron-withdrawing group (both −I-I and −R-R effects). It withdraws electron density from the carboxylate group, stabilizing the anion and increasing the acidity compared to unsubstituted benzoic acid.

Acidity of Carboxylic Acids - Revision Notes & Key Formulas | CBSE Class 12 Chemistry