Carboxylic Acids — Part 2: Reactions, tests and uses: visual lesson, ISC Class 12 Chemistry

Lesson

Reactions of carboxylic acids

Part 2 — acidic reactions, acid derivatives, losing CO₂, α-halogenation, ring substitution, tests and uses.

In this lesson you will

Write reactions with active metals, alkalis, carbonates and hydrogencarbonates
Write equations for forming acid chlorides, anhydrides, esters and amides
Write decarboxylation, Kolbe electrolysis and HVZ reactions; predict ring substitution of benzoic acid
Describe tests for and uses of formic, acetic and benzoic acids

Concept

Reactions as acids

Carboxylic acids react with active metals (H₂), alkalis (salt + water), carbonates and hydrogencarbonates (CO₂ effervescence — a test that distinguishes them from phenols).

\ce{2CH3COOH + 2Na -> 2CH3COONa + H2}

\ce{2CH3COOH + Na2CO3 -> 2CH3COONa + H2O + CO2}

\ce{C6H5COOH + NaHCO3 -> C6H5COONa + H2O + CO2}

Table of acidic reactions
Reagent	Products	Phenol reacts?
Na	Salt + H₂	Yes
NaOH	Salt + H₂O	Yes
Na₂CO₃	Salt + H₂O + CO₂	No CO₂
NaHCO₃	Salt + H₂O + CO₂	No

Concept

Acid derivatives

Replacing –OH gives derivatives. Acid chlorides: PCl₅, PCl₃ or SOCl₂. Anhydrides: heat with P₂O₅ (dehydration of two molecules). Esters: alcohol + conc. H₂SO₄ (reversible). Amides: NH₃ gives the ammonium salt, which loses water on heating.

\ce{CH3COOH + SOCl2 -> CH3COCl + SO2 + HCl}

\ce{3CH3COOH + PCl3 -> 3CH3COCl + H3PO3}

\ce{2CH3COOH ->[P2O5][\Delta] (CH3CO)2O + H2O}

\ce{CH3COONH4 ->[\Delta] CH3CONH2 + H2O}

Table of acid derivatives
Derivative	Reagent	Example
Acid chloride	PCl₅, PCl₃, SOCl₂	Ethanoyl chloride
Anhydride	P₂O₅, heat	Ethanoic anhydride
Ester	R′OH, H⁺	Ethyl ethanoate
Amide	NH₃, then heat	Ethanamide

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