Carboxylic Acids & Derivatives: Structure, Reactions & Uses

TAKEAWAYS:

• Carboxylic acids have the –COOH group and are acidic due to hydrogen ion donation.
• Derivatives (acid chlorides, esters, amides) are formed by replacing –OH or H in –COOH.
• Esterification (acid + alcohol → ester + water) is reversible and used in perfumes.
• Hydrolysis breaks derivatives back into acids/alcohols (acidic/basic conditions matter).
• Reduction converts acids/derivatives to alcohols or aldehydes using LiAlH₄.
• Applications include soaps, dyes, and pharmaceuticals—critical for NEB practicals.

1. Carboxylic Acids: Structure & Properties

Carboxylic acids are organic compounds with the functional group –COOH (carboxyl group). They are polar (due to C=O and O–H bonds) and acidic (donate H⁺ ions).

Key Examples & Naming

Name	Formula	Structure	Source
Formic acid	HCOOH		Ant stings, bee venom
Acetic acid	CH₃COOH		Vinegar (5–8% solution)
Benzoic acid	C₆H₅COOH		Preservative in foods

Naming Rules:

• Replace -e in alkane name with -oic acid. Example: CH₃CH₂COOH → Propanoic acid (from propane).

Physical Properties

• Boiling Point: High (due to hydrogen bonding between –COOH groups).
• Solubility: Soluble in water (polar) but less soluble in nonpolar solvents.
• Smell: Pungent (e.g., acetic acid smells like vinegar).

Hydrogen bonds between acetic acid molecules raise boiling points. (Image: Jü, CC0, via Wikimedia Commons)

2. Preparation of Carboxylic Acids

Method 1: Oxidation of Primary Alcohols/Aldehydes

• Reagents: Acidified K₂Cr₂O₇ (orange → green) or KMnO₄ (purple → colorless).
• Example:

  flowchart TD
    A["Primary Alcohol (R–CH₂OH)"] -->|[K₂Cr₂O₇/H⁺] B["Aldehyde (R–CHO)"]
    B -->|[K₂Cr₂O₇/H⁺] C["Carboxylic Acid (R–COOH)"]

Method 2: Hydrolysis of Nitriles

• Reagent: Dilute HCl or H₂O (heat).
• Example:

  flowchart TD
    A["Nitrile (R–CN)"] -->|[H₂O/H⁺, heat] B["Carboxylic Acid (R–COOH)"]

Method 3: Grignard Reagent + CO₂

• Reagent: CO₂ followed by H₃O⁺.
• Example:

  flowchart TD
    A["Grignard (R–MgX)"] -->|[CO₂] B["Carboxylate Salt (R–COO⁻Mg⁺X⁻)"]
    B -->|[H₃O⁺] C["Carboxylic Acid (R–COOH)"]

3. Chemical Reactions of Carboxylic Acids

A. Reaction with Metals (Acidic Nature)

• Example: Acetic acid + Magnesium → Salt + Hydrogen gas.

  flowchart TD
    A["CH₃COOH"] + B["Mg"] --> C["(CH₃COO)₂Mg"] + D["H₂"]

B. Esterification (Reaction with Alcohols)

• Reagent: Alcohol + Acid catalyst (H₂SO₄).
• Example: Ethanoic acid + Ethanol → Ethyl ethanoate (smells like fruit).

flowchart TD
  A["CH₃COOH"] + B["C₂H₅OH"] -->|[H⁺, heat] C["CH₃COOC₂H₅"] + D["H₂O"]

C. Reduction to Primary Alcohols

• Reagent: LiAlH₄ (strong reducing agent).
• Example:

  flowchart TD
    A["R–COOH"] -->|[LiAlH₄] B["R–CH₂OH"]

D. Reaction with PCl₅/PCl₃ (to Acid Chloride)

• Example:

  flowchart TD
    A["R–COOH"] + B["PCl₅"] --> C["R–COCl"] + D["POCl₃ + HCl"]

4. Derivatives of Carboxylic Acids

Derivatives are formed by replacing –OH or H in –COOH. They are less acidic but more reactive than acids.

Pyramid showing acid chloride > anhydride > ester > amide in reactivity. (Image: Minihaa, CC0, via Wikimedia Commons)

5. Key Reactions of Derivatives

A. Hydrolysis (Breaking Down)

• Acid Chloride + Water → Acid + HCl

  flowchart TD
    A["R–COCl"] + B["H₂O"] --> C["R–COOH"] + D["HCl"]

• Ester + Water → Acid + Alcohol (requires H⁺ or OH⁻ catalyst).

  flowchart TD
    A["R–COOR'"] + B["H₂O"] -->|[H⁺/OH⁻] C["R–COOH"] + D["R'OH"]

B. Reduction

• LiAlH₄ reduces all derivatives to alcohols.

  flowchart TD
    A["R–COCl"] -->|[LiAlH₄] B["R–CH₂OH"]
    C["R–COOR'"] -->|[LiAlH₄] D["R–CH₂OH + R'OH"]

C. Amide Formation (Nucleophilic Substitution)

• Acid Chloride + Ammonia → Amide + HCl

  flowchart TD
    A["R–COCl"] + B["NH₃"] --> C["R–CONH₂"] + D["HCl"]

6. Applications in Daily Life

7. NEB Exam Tips

1. Memorize Naming:

   • Always replace -e with -oic acid for acids.
   • For derivatives, use -oyl chloride (acid chloride), -oate (ester), -amide (amide).

2. Reaction Conditions Matter:

   • Esterification: Acid catalyst (H₂SO₄), heat.
   • Hydrolysis: Acidic (H⁺) or basic (OH⁻) conditions.
   • Reduction: LiAlH₄ (strong) vs. NaBH₄ (milder).

3. Draw Mechanisms for Key Reactions:

   • Esterification (protonation of –COOH, nucleophilic attack by alcohol).
   • Hydrolysis (nucleophilic attack by H₂O).

4. Practical Questions:

   • Preparation: How to convert ethanol to ethanoic acid? (Oxidation with K₂Cr₂O₇).
   • Identification: How to distinguish between acetic acid and ethanol? (Use NaHCO₃ test—acetic acid gives CO₂ bubbles).

5. Common Mistakes to Avoid:

   • Forgetting heat in esterification.
   • Confusing acid chloride (R–COCl) with acid anhydride ((R–CO)₂O).
   • Not balancing redox reactions (e.g., oxidation of alcohols).

Solved NEB-Style Questions

Q1. Write the IUPAC name of the following compound:

CH₃–CH₂–CH(Cl)–COOH Answer:

1. Longest chain: 4 carbons (butanoic acid base).
2. Substituent: Chloro (Cl) at 2nd carbon. Final Name: 3-Chlorobutanoic acid

Q2. How will you prepare propanoic acid from ethanol?

Answer:

1. Oxidize ethanol to ethanal (using K₂Cr₂O₇/H⁺).
2. Further oxidize ethanal to propanoic acid (same reagent).

   flowchart TD
     A["C₂H₅OH"] -->|[K₂Cr₂O₇/H⁺] B["CH₃CHO"]
     B -->|[K₂Cr₂O₇/H⁺] C["CH₃COOH"]

Q3. Write the reaction of acetic acid with sodium bicarbonate.

Answer: Acetic acid (weak acid) reacts with NaHCO₃ to give CO₂ gas (effervescence).

flowchart TD
  A["CH₃COOH"] + B["NaHCO₃"] --> C["CH₃COONa"] + D["H₂O"] + E["CO₂"]

Q4. Compare the reactivity of acid chloride, ester, and amide.

Answer:

Summary Table for Quick Revision

Final Note:

• Carboxylic acids are the parent compounds; their derivatives are more reactive intermediates.
• Esterification is reversible—use excess alcohol to push equilibrium forward.
• LiAlH₄ is a super-reducer—it converts acids/derivatives to alcohols.

Good luck for your NEB exams! Practice drawing mechanisms and naming compounds daily.