Carboxylic Acids Notes

(a) Order of Relative Acidity of Carboxylic Acids, Phenols, Alcohols, and Water

Order of Acidity:

Carboxylic acids > Phenols > Water > Alcohols

Resonance stabilization is the stabilisation offered due to the delocalisation of electrons in a molecule.

Explanation:

Carboxylic acids (pKa ≈ 4-5) are the most acidic due to the resonance stabilisation of the carboxylate anion (RCOO⁻).

Phenols (pKa ≈ 10) are less acidic since their conjugate base (phenoxide ion) is resonance-stabilized but less effectively than carboxylates.

Water (pKa ≈ 15.7) is slightly more acidic than alcohol due to the better solvation of hydroxide ions.

Alcohols (pKa ≈ 16-18) are the least acidic as the alkoxide ion (RO⁻) is not resonance-stabilized.

Demonstration:

• React each compound with sodium hydroxide (NaOH); only carboxylic acids and phenols will react.

• React with sodium carbonate (Na₂CO₃); only carboxylic acids will produce CO₂ bubbles.

• Measure pH of aqueous solutions (carboxylic acids will have the lowest pH).

(b) Formation of Carboxylic Acids by Oxidation of Alcohols and Aldehydes

Primary Alcohols → Aldehydes → Carboxylic Acids

Reagent: Acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄)

Orange Cr₂O₇²⁻ reduces to green Cr³⁺

Example: CH3CH2OH + [O] → CH3CHO + [O] → CH3COOHCH₃CH₂OH + [O]

(c) Reduction of Carboxylic Acids using LiAlH₄

Reagent: Lithium aluminium hydride (LiAlH₄) in dry ether

Reaction: RCOOH + 4[H] → RCH2OH + H2ORCOOH + 4[H]

LiAlH₄ is strong enough to reduce carboxylic acids to primary alcohols.

(d) Formation of Aromatic Carboxylic Acids by Oxidation of Methyl Side-Chains

Reagent: Alkaline KMnO₄, heat, then acidification

Reaction:
C₆H₅CH₃ + 3[O] → C₆H₅COOH + H₂OC₆H₅CH₃ + 3[O]

Works for any alkyl side-chain on benzene (e.g., ethylbenzene → benzoic acid).

(e) Decarboxylation of Carboxylic Acids

Thermal decarboxylation:
RCOOH → RH + CO₂

Sodium salt decarboxylation with soda lime (NaOH + CaO, heat):
RCOONa + NaOH → RH + Na₂CO₃

(f) Conversion of Carboxylic Acids to Esters and Acid Chlorides & Hydrolysis

Esterification:

Reagents: Carboxylic acid + Alcohol + conc. H₂SO₄ (catalyst)

Example: CH₃COOH + CH₃OH → CH₃COOCH₃ + H₂O

Formation of Acid Chlorides:

Reagent: SOCl₂ (thionyl chloride)

Reaction: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl

Hydrolysis:

Acid chlorides hydrolyze rapidly in water to reform the carboxylic acid.

Esters undergo acidic or alkaline hydrolysis:

CH₃COOCH₃ + H₂O→ CH₃COOH + CH₃OH (Acidic)
CH₃COOCH₃+ OH⁻→ CH₃COO⁻ + CH₃OH (Alkaline)

(g) Conversion of Carboxylic Acids to Amides and Nitriles

To Amides:

Reagent: NH₃ (ammonia) and heat

Reaction: RCOOH + NH₃→ RCONH₂ + H₂O

To Nitriles (via Amide Dehydration):

Reagent: P₄O₁₀ (phosphorus pentoxide) or SOCl₂

Reaction: RCONH₂ → RCN + H₂O

(h) Formation of Nitriles from Halogenoalkanes and Hydroxynitriles from Aldehydes/Ketones

From Halogenoalkanes:

Reagent: KCN in ethanol (nucleophilic substitution)

Reaction: RBr + CN⁻ → RCN + Br⁻

From Aldehydes/Ketones (Cyanohydrin Formation):

Reagent: HCN + NaCN (catalyst)

Reaction: RCHO + HCN→ RCH(OH)CN

(i) Hydrolysis of Nitriles and Amides

Hydrolysis of Nitriles:

• Acidic (H⁺, heat):
  RCN + 2H₂O + H⁺ → RCOOH + NH₄⁺

• Alkaline (OH⁻, heat):
  RCN + OH⁻ + H₂O → RCOO⁻ + NH₃

Hydrolysis of Amides:

• Acidic (H⁺, heat):
  RCONH₂ + H₂O + H⁺→ RCOOH + NH₄⁺

• Alkaline (OH⁻, heat):
  RCONH₂ + OH⁻ → RCOO⁻ + NH₃

(j) Reduction of Nitriles using LiAlH₄

• Reagent: LiAlH₄ in dry ether

• Reaction: RCN + 4[H] → RCH₂NH₂

• Product: Primary Amine (RCN → RCH₂NH₂)