3.6. CARBOXYLIC ACIDS AND DERIVATIVES

How is benzoic acid produced from an alkylbenzene such as methylbenzene?

Oxidation of Alkylbenzenes

• Reagents: Hot alkaline KMnO₄ followed by dilute acid (HCl).

• Conditions: Reflux.

• Observation: The purple Mn⁷⁺ ions are reduced to Mn⁴⁺, forming a brown MnO₂ precipitate.

• Final Step: Acidification with dilute HCl protonates the organic product → Benzoic acid (C₆H₅COOH) is formed.

Equation: C₆H₅CH₃ + [O] → C₆H₅COOH

How do carboxylic acids react with PCl₃, PCl₅, or SOCl₂ to form acyl chlorides?

Formation of Acyl Chlorides (-COCl)

• Carboxylic acids react with:

  1. Solid PCl₅ → Acyl chloride + POCl₃ + HCl.

  2. Liquid PCl₃ + Heat → Acyl chloride + H₃PO₃.

  3. Liquid SOCl₂ → Acyl chloride + SO₂ + HCl.

Example – Formation of Ethanoyl Chloride:

CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl

How is methanoic acid (HCOOH) further oxidised, and what observations can be made?

Oxidation of Methanoic Acid (HCOOH):

• Methanoic acid acts as a reducing agent and undergoes oxidation to CO₂.

• Oxidising Agents Used:

  1. Fehling’s solution: Cu²⁺ reduced to Cu₂O (red precipitate).

  2. Tollens’ reagent: Ag⁺ reduced to metallic silver (mirror effect).

  3. Acidified KMnO₄ or K₂Cr₂O₇: Mn⁷⁺ → Mn²⁺ (Purple → Colourless) / Cr⁶⁺ → Cr³⁺ (Orange → Green).

Equation: HCOOH + [O] → CO₂ + H₂O

How is ethanedioic acid (HOOCCOOH) oxidised, and what is the role of KMnO₄?

Oxidation of Ethanedioic Acid:

• Reagent: Warm acidified KMnO₄.

• Products: Carbon dioxide (CO₂) & water.

• Observations: Mn⁷⁺ ions are reduced to Mn²⁺, causing the purple KMnO₄ solution to turn colourless.

Equation: HOOCCOOH + [O] → 2CO₂ + H₂O

How do the acidities of carboxylic acids, phenols, and alcohols compare, and what factors influence their relative strengths?

Factors Affecting Acidity:

1. Strength of O-H Bond:

   • In carboxylic acids, the carbonyl (-C=O) group withdraws electron density, weakening the O-H bond.

   • Alcohols lack this electron-withdrawing effect, making them weaker acids.

2. Stability of Conjugate Base:

   • Carboxylate ions (-COO⁻) are highly stable due to charge delocalisation, making carboxylic acids stronger acids.

   • Phenoxide ions (C₆H₅O⁻) are stabilised but not as effectively as carboxylates → Weaker acidity than carboxylic acids.

   • Alkoxide ions (C₂H₅O⁻) are destabilised due to electron donation from the alkyl group → Ethanol is the weakest acid.

How do electron-withdrawing chlorine atoms affect the acidity of carboxylic acids?

Why More Chlorine = Stronger Acidity?

1. Chlorine atoms withdraw electron density from the O-H bond, making it weaker and facilitating proton loss.

2. Delocalisation of charge in the carboxylate ion is further extended by Cl atoms → Greater stability → Lower tendency to recombine with H⁺.

Example – Trichloroethanoic Acid (Strongest Acid):

• Three electronegative Cl atoms withdraw electron density.

• Highly stabilised carboxylate ion (-COO⁻) reduces attraction to H⁺ → Very strong acid.

What are esters, and why are acyl chloride reactions preferred for their synthesis?

Structure & Uses of Esters:

• Esters have the -COOR functional group.

• Used in perfumes, cosmetics, and solvents due to their characteristic smells.

• Named based on their reactants:

  • Alcohol contributes the first part of the name.

  • Acyl chloride contributes the second part (e.g., Ethyl ethanoate = ethanol + ethanoyl chloride).

Why Use Acyl Chlorides Instead of Carboxylic Acids?

1. Acyl chlorides are more reactive → Faster ester formation.

2. Reaction goes to completion → No equilibrium mixture, maximum ester yield.

How does ethanol react with ethanoyl chloride to form ethyl ethanoate?

Reaction Details:

• Reagents: Ethanol (C₂H₅OH) + Ethanoyl chloride (CH₃COCl).

• Products: Ethyl ethanoate (CH₃COOCH₂CH₃) + HCl (white fumes).

• Mechanism:

  1. Nucleophilic attack by ethanol’s -OH group on the electron-deficient carbonyl carbon.

  2. Chlorine (-Cl) atom is eliminated, forming HCl gas.

  3. Ester bond (-COO-) is formed, producing ethyl ethanoate.

Equation: CH₃COCl + C₂H₅OH → CH₃COOCH₂CH₃ + HCl

How does phenol react with benzoyl chloride to form phenyl benzoate?

Reaction Details:

• Reagents: Phenol (C₆H₅OH) + Benzoyl chloride (C₆H₅COCl).

• Products: Phenyl benzoate (C₆H₅COOC₆H₅) + HCl.

• Mechanism:

  1. Phenol’s -OH group attacks the carbonyl carbon in benzoyl chloride.

  2. Chlorine (-Cl) is eliminated, releasing HCl gas.

  3. Phenyl benzoate (ester) is formed.

Equation: C₆H₅COCl + C₆H₅OH → C₆H₅COOC₆H₅ + HCl

How are acyl chlorides synthesized from carboxylic acids, and what reagents are required?

Acyl Chlorides & Their Reactivity:

• Acyl chlorides contain the -COCl functional group.

• They are more reactive than carboxylic acids, making them valuable intermediates in organic synthesis.

Example – Formation of Propanoyl Chloride:

CH₃CH₂COOH + SOCl₂ → CH₃CH₂COCl + SO₂ + HCl

How do acyl chlorides react with water, and what is the mechanism?

Reaction Overview:

• Reagent: H₂O (room temperature).

• Products: Carboxylic acid + HCl gas.

• Observations: White fumes of HCl are released.

Mechanism:

1. Water’s lone pair attacks the carbonyl carbon.

2. Addition of H₂O molecule.

3. Elimination of HCl, forming the carboxylic acid.

Equation: CH₃CH₂COCl + H₂O → CH₃CH₂COOH + HCl

How do acyl chlorides react with alcohols and phenols to form esters?

Reaction with Alcohols:

• Reagent: Alcohol (room temperature).

• Products: Ester + HCl.

• Example: Ethanol + Ethanoyl chloride → Ethyl ethanoate + HCl.

• Equation: CH₃COCl + C₂H₅OH → CH₃COOCH₂CH₃ + HCl

Reaction with Phenols:

• Reagent: Phenol + Base (NaOH, needed for phenoxide formation).

• Products: Ester + NaCl.

• Example: Phenol + Benzoyl chloride → Phenyl benzoate + NaCl.

• Equation: C₆H₅COCl + C₆H₅O⁻ → C₆H₅COOC₆H₅ + NaCl

How do acyl chlorides react with ammonia and amines to form amides?

Reaction with Ammonia (NH₃):

• Reagents: NH₃ (room temperature).

• Products: Non-substituted amide + HCl gas.

• Example: Ethanoyl chloride + NH₃ → Ethanamide + HCl.

• Equation: CH₃COCl + NH₃ → CH₃CONH₂ + HCl

Reaction with Primary & Secondary Amines:

• Reagent: Amine (room temperature).

• Products: Substituted amide + Ammonium salt.

• Example: Ethanoyl chloride + Methylamine → N-Methylethanamide + Methylammonium chloride.

• Equation: CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + CH₃NH₃⁺Cl⁻

What is the general addition-elimination mechanism for acyl chloride reactions?

Step 1 – Addition of a Nucleophile:

• Water, alcohol, phenol, ammonia, or amine attacks carbonyl carbon using a lone pair.

• Electron transfer weakens C=O bond.

Step 2 – Elimination of a Small Molecule:

• Leaving group (-Cl) is expelled, forming HCl gas or NaCl (with phenols).

• Final Product: Carboxylic acid, ester, or amide.

Example – Hydrolysis of Acyl Chlorides:

1. Nucleophilic attack by water.

2. Formation of intermediate.

3. Elimination of HCl, forming carboxylic acid.