Unit 14: Carboxylic Acid and its Derivatives

Introduction to Carboxylic Acids

The functional group that defines carboxylic acids is the carboxyl group, represented as -COOH. It consists of a carbonyl group (C=O) attached to a hydroxyl group (-OH). This group imparts acidic character due to the ability to donate the hydroxyl proton, forming a resonance‑stabilized carboxylate anion (RCOO⁻). Carboxylic acids are ubiquitous in nature and industry, occurring as fatty acids, amino acids, and many pharmaceutical intermediates.

Isomerism in Carboxylic Acids

Chain Isomerism

Chain isomerism arises when the carbon skeleton differs while the carboxyl group remains at the same position. Example: CH₃CH₂CH₂COOH (butanoic acid) and (CH₃)₂CHCOOH (2‑methylpropanoic acid) are chain isomers.

Position Isomerism

Position isomerism occurs when the carboxyl group is attached to different carbon atoms of the same chain. Example: CH₃CH₂CH₂COOH (butanoic acid) vs. CH₃CH(CH₃)COOH (2‑methylpropanoic acid) – actually the latter is chain; a true position example: CH₃CH₂CH₂COOH (butanoic acid) and CH₃CH₂CH(OH)CHO is not; better: HOOC‑CH₂‑CH₂‑CH₃ (butanoic) vs. HOOC‑CH(CH₃)‑CH₃ (2‑methylpropanoic) – still chain. For position we need same chain length: HOOC‑CH₂‑CH₂‑CH₃ (butanoic) and HOOC‑CH(CH₃)‑CH₃ is chain. Actually position isomerism is less common because the carboxyl group is terminal; however, in dicarboxylic acids we can have positions: HOOC‑CH₂‑CH₂‑COOH (succinic) vs. HOOC‑CH₂‑CH(CH₃)‑COOH (methylsuccinic).

Functional Isomerism

Functional isomerism occurs when compounds have the same molecular formula but different functional groups. Example: C₃H₆O₂ can be propanoic acid (CH₃CH₂COOH) or methyl acetate (CH₃COOCH₃).

Preparation of Carboxylic Acids

1. Oxidation of Aldehydes:
   RCHO + [O] → RCOOH (e.g., ethanol → acetaldehyde → acetic acid using KMnO₄ or CrO₃).
2. Hydrolysis of Nitriles:
   R‑CN + 2 H₂O → RCOOH + NH₃ (acidic or basic conditions). Example: benzonitrile → benzoic acid.
3. Partial Decarboxylation of Dicarboxylic Acids:
   Heating a dicarboxylic acid can eliminate one carboxyl group as CO₂:
   HOOC‑(CH₂)ₙ‑COOH → HOOC‑(CH₂)ₙ₋₁‑COOH + CO₂ (e.g., glutaric acid → succinic acid + CO₂).
4. Reaction of Sodium Alkoxide with Carbon Dioxide:
   RONa + CO₂ → RCOONa followed by acidification gives the acid. Example: sodium ethoxide + CO₂ → sodium propionate → propionic acid.
5. Hydrolysis of Trihaloalkanes:
   R‑CX₃ + 3 H₂O → RCOOH + 3 HX (X = Cl, Br). Example: chloroform (CHCl₃) hydrolyzes slowly to formic acid.
6. Oxidation of Alkylbenzenes to Benzoic Acid:
   C₆H₅‑CH₂R + [O] → C₆H₅‑COOH (strong oxidizing agents like KMnO₄). Example: toluene → benzoic acid.

Physical Properties

• Boiling Point: Carboxylic acids exhibit unusually high boiling points due to extensive intermolecular hydrogen bonding, leading to dimer formation in the liquid phase (two molecules linked by two H‑bonds).
• Solubility: Lower members (up to C₄) are miscible with water because of hydrogen bonding; solubility decreases with increasing hydrocarbon chain length.
• Fatty Acid Series: Saturated fatty acids follow the general formula CH₃(CH₂)ₙCOOH (n = 0–16+). Example: palmitic acid (CH₃(CH₂)₁₄COOH, n=14) and stearic acid (CH₃(CH₂)₁₆COOH, n=16).

Chemical Properties

Reaction with Bases (Neutralization)

Carboxylic acids react with alkalis to give carboxylate salts and water:
RCOOH + NaOH → RCOONa + H₂O

Reaction with Metal Oxides

Two acid molecules react with a metal oxide to give a metal carboxylate and water:
2 RCOOH + MO → (RCOO)₂M + H₂O (M = Ca, Zn, etc.).

Reaction with Metal Carbonates

Effervescence due to CO₂ evolution:
2 RCOOH + MCO₃ → (RCOO)₂M + CO₂ + H₂O

Reaction with Metal Bicarbonates

Similar to carbonates, with vigorous CO₂ release:
2 RCOOH + MHCO₃ → (RCOO)₂M + CO₂ + H₂O

Formation of Acid Chlorides (using PCl₃)

Thionyl chloride or phosphorus trichloride converts the acid to acyl chloride:
RCOOH + PCl₃ → RCOCl + H₃PO₃

Reduction with LiAlH₄

Lithium aluminium hydride reduces carboxylic acids to primary alcohols:
RCOOH + 4[H] → RCH₂OH + H₂O

Dehydration to Acid Anhydrides

Heating with a dehydrating agent (P₂O₅) yields the symmetrical anhydride:
2 RCOOH ⟶[P₂O₅/Δ] (RCO)₂O + H₂O

Hell‑Volhard‑Zelinsky (HVZ) Reaction – α‑Bromination

In the presence of a catalytic amount of PBr₃, the α‑hydrogen is replaced by bromine:
RCH₂COOH + Br₂ + PBr₃ → RCH(Br)COOH + HBr

Electrophilic Substitution of Benzoic Acid

The carboxyl group is meta‑directing. Typical reactions:

• Nitration: C₆H₅COOH + HNO₃/H₂SO₄ → m‑NO₂C₆H₅COOH + H₂O
• Bromination: C₆H₅COOH + Br₂/FeBr₃ → m‑BrC₆H₅COOH + HBr
• Sulphonation: C₆H₅COOH + H₂SO₄ → m‑HO₃SC₆H₅COOH + H₂O

Effect of Substituents on Acidic Strength

Electron‑withdrawing groups (–NO₂, –Cl, –CF₃) increase acidity by stabilizing the carboxylate anion, lowering pKa. Electron‑donating groups (–CH₃, –OCH₃) decrease acidity, raising pKa. Example: pKa values: acetic acid 4.76; chloroacetic acid 2.86; methoxyacetic acid 3.58.

Abnormal Behaviour of Methanoic Acid (Formic Acid)

Formic acid contains both a carboxyl and an aldehyde‑like hydrogen, allowing it to act as a reducing agent:

HCOOH + 2[Ag(NH₃)₂]⁺ → CO₂ + 2Ag + 4NH₃ + H⁺ (Tollens’ test)

It also reduces Fehling’s solution and can be oxidized to carbon dioxide.

Derivatives of Carboxylic Acids

Acid Halides (Acyl Chlorides)

Most reactive derivatives; prepared by treating the acid with PCl₅, PCl₃, or SOCl₂:

• RCOOH + PCl₅ → RCOCl + POCl₃ + HCl
• RCOOH + SOCl₂ → RCOCl + SO₂ + HCl

Amides

Formed by nucleophilic substitution of acyl chlorides with ammonia (or amines):

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl
RCOCl + 2 NH₃ → RCONH₂ + NH₄Cl

Esters

Fischer esterification (acid‑catalyzed equilibrium):

RCOOH + R'OH ⇌ RCOOR' + H₂O (catalyst: H₂SO₄). Example: acetic acid + ethanol → ethyl acetate + water.

Acid Anhydrides

Prepared by dehydration of two acid molecules with P₂O₅:

2 RCOOH ⟶[P₂O₅] (RCO)₂O + 2 HPO₃ (symmetrical) or by reacting an acyl chloride with a carboxylate salt for mixed anhydrides.

Comparative Properties and Reactions of Derivatives

Derivative	Relative Reactivity	Typical Reaction with Water (Hydrolysis)	Ammonolysis	Alcoholysis	Reduction (LiAlH₄)
Acid Chloride (RCOCl)	Most reactive	RCOCl + H₂O → RCOOH + HCl	RCOCl + 2 NH₃ → RCONH₂ + NH₄Cl	RCOCl + R'OH → RCOOR' + HCl	RCOCl + 2[H] → RCHO → RCH₂OH
Acid Anhydride ((RCO)₂O)	High	(RCO)₂O + H₂O → 2 RCOOH	(RCO)₂O + 2 NH₃ → RCONH₂ + RCOONH₄	(RCO)₂O + R'OH → RCOOR' + RCOOH	(RCO)₂O + 4[H] → 2 RCH₂OH
Ester (RCOOR')	Moderate	RCOOR' + H₂O ⇌ RCOOH + R'OH	RCOOR' + NH₃ → RCONH₂ + R'OH (slow)	RCOOR' + R''OH ⇌ RCOOR'' + R'OH (transesterification)	RCOOR' + 4[H] → RCH₂OH + R'OH
Amide (RCONH₂)	Least reactive	RCONH₂ + H₂O → RCOOH + NH₃ (requires strong acid/base, heat)	—	—	RCONH₂ + 4[H] → RCH₂OH + NH₃

Claisen Condensation

Esters with α‑hydrogens undergo base‑catalyzed condensation to give β‑keto esters:

2 CH₃COOC₂H₅ ⟶[NaOEt] CH₃COCH₂COOC₂H₅ + C₂H₅OH (ethyl acetoacetate).

Hofmann Bromamide Reaction

Primary amides are converted to amines with loss of one carbon:

RCONH₂ + Br₂ + 4 NaOH → RNH₂ + Na₂CO₃ + 2 NaBr + 2 H₂O

Amphoteric Nature of Amides

Amides can act as very weak acids (donating the N‑H proton) and very weak bases (accepting a proton on the carbonyl oxygen). Example: acetamide pKa ≈ 15 (acidic) and pKb ≈ 14 (basic).

Summary

This chapter has provided an exhaustive treatment of carboxylic acids and their derivatives, covering:

• Structure and isomerism
• Laboratory and industrial preparation methods
• Physical characteristics arising from hydrogen bonding
• A rich array of chemical reactions, including acid‑base behavior, conversion to acyl halides, reductions, dehydrations, halogenations, and electrophilic aromatic substitution
• The chemistry of key derivatives—acid chlorides, amides, esters, anhydrides—and their interconversions
• Comparative reactivity patterns and important named reactions such as Hell‑Volhard‑Zelinsky, Claisen condensation, and Hofmann bromamide

Mastery of these concepts equips students to tackle advanced topics in organic synthesis, biochemistry, and polymer science.