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Carboxylicacidsare moreacidic than phenolsandalchols. Explin. Why ? |
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Answer» Solution : (1) Carboxylicacid losea proton as compared to phenol.Consider the ionizationof carboxylic acidandphenol. `underset("alcohol")(R-OH) hArr underset("alkoxide ion")(R-O^(-)) +H^(+)`  Dueto delocalizationthe negativechargeoverthe ortho andparapositions of aromaticring, phenoxideanion is morestablethanphenol. THUS phenoleasily undergoes ionization.  However, alcohol and alkoxide ion are single STRUCTURES. In an alkoxide ANION the negative charge is localized on a single oxygen atom. Hence, phenols are more acidic than alcohols. Carboxylic acid has two resonarice hybride non equivalent structures (I & II) while carboxylate anion has two resonance hybrid equivalent structures (III & IV). The carboxylate ion is more stable than carboxylic acid and equilibrium is shifted towards the direction of increased ionization.  Carboxylate ion has two equivalent resonance structures with negative charge is delocalized over two electronegative oxygen atoms. Phenoxide anion has non-equivalent resonance structures in which negative charge is delocalized over one oxygen atom and less electronegative CARBON atom. As a result carboxylate anion is more stable than phenoxide ion. Hence carboxylic acids ionize to the greater extent than phenol furnishing higher concentration of H ions. Therefore, carboxylic acids are more acidic than phenols and alcohols. |
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