Carboxylic Acids

1. Reactions of Carboxylic Acids

• Definition of carboxylic acids
  • Organic compounds containing the –COOH (carboxyl) functional group.
  • General formula: CₙH₂ₙ₊₁COOH for unbranched monocarboxylic acids.
  • Examples: methanoic acid (HCOOH), ethanoic acid (CH₃COOH), propanoic acid (C₂H₅COOH).

1(a) Reaction with Metals

• General reaction:
  Carboxylic acid + Metal → Salt of the acid + Hydrogen gas
  Equation (example with ethanoic acid and magnesium):
  CH₃COOH + Mg → (CH₃COO)₂Mg + H₂↑
• Observations: Effervescence (bubbling) due to hydrogen gas release, metal gradually dissolves.
• Test for hydrogen gas: Brings a lighted splint near the gas → “pop” sound.
• Naming the salt: Metal name + ethanoate (if using ethanoic acid) or corresponding alkanoate for other acids.
• Other examples:
  • 2CH₃COOH + 2Na → 2CH₃COONa + H₂
  • 2C₂H₅COOH + Ca → (C₂H₅COO)₂Ca + H₂

1(b) Reaction with Bases

• General reaction:
  Carboxylic acid + Base → Salt of the acid + Water
• Example with sodium hydroxide:
  CH₃COOH + NaOH → CH₃COONa + H₂O
• Type of reaction: Neutralisation reaction (acid + base → salt + water).
• No effervescence (since no gas is produced).
• Common bases: Sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂).

1(c) Reaction with Carbonates

• General reaction:
  Carboxylic acid + Metal carbonate → Salt + Water + Carbon dioxide
• Example with sodium carbonate:
  2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑
• Example with calcium carbonate:
  2CH₃COOH + CaCO₃ → (CH₃COO)₂Ca + H₂O + CO₂↑
• Observations: Effervescence due to CO₂ production, carbonate gradually disappears.
• Test for carbon dioxide: Bubble gas through limewater (Ca(OH)₂ solution) → turns milky due to CaCO₃ formation.

2. Formation of Ethanoic Acid from Ethanol

2(a) Oxidation with Acidified Potassium Manganate(VII)

• Reagents:
  • Ethanol (CH₃CH₂OH)
  • Acidified potassium manganate(VII) solution (KMnO₄ + dilute H₂SO₄)
• Conditions:
  • Heat under reflux to prevent loss of volatile products.
• Colour change:
  • Purple KMnO₄ → colourless (Mn²⁺ ions formed).
• Equation:
  CH₃CH₂OH + [O] → CH₃COOH + H₂O
  (Where [O] represents an oxidising agent).

2(b) Oxidation by Bacterial Action (Vinegar Production)

• Process:
  • Exposure of ethanol (in wine, beer, or cider) to air → bacteria Acetobacter oxidise ethanol to ethanoic acid.
• Conditions:
  • Presence of oxygen.
  • Warm environment.
  • Natural fermentation or controlled industrial setup.
• Equation:
  CH₃CH₂OH + O₂ → CH₃COOH + H₂O
• Applications:
  • Produces vinegar (aqueous ethanoic acid ~5%).
  • Used in food preservation and flavouring.

3. Reaction of Carboxylic Acid with Alcohol to Form Esters

Esterification Reaction

• General reaction:
  Carboxylic acid + Alcohol ⇌ Ester + Water
• Catalyst: Concentrated sulfuric acid (H₂SO₄).
• Conditions: Heat under reflux.
• Example: Ethanoic acid + Ethanol → Ethyl ethanoate + Water
  CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O
• Naming esters:
  • First part (alkyl) comes from alcohol (e.g., ethanol → ethyl).
  • Second part (alkanoate) comes from acid (e.g., ethanoic acid → ethanoate).
• Properties of esters:
  • Pleasant fruity smell.
  • Used in flavourings, perfumes, and solvents.
• Example list of esters:
  • Methanol + ethanoic acid → Methyl ethanoate.
  • Propanol + ethanoic acid → Propyl ethanoate.