Carboxylic Acids

Definition: Organic compounds containing -COOH as a functional group are called carboxylic acids (carb from carbonyl and oxyl from hydroxyl).

General Formula: For monocarboxylic acids: R-COOH or CₙH₂ₙO₂ (CₙH₂ₙ₊₁COOH)

Structural Formula	Common Name	IUPAC Name
H-COOH	Formic acid	Methanoic acid
CH₃-COOH	Acetic acid	Ethanoic acid
CH₃-CH₂-COOH	Propionic acid	Propanoic acid
CH₃-CH₂-CH₂-COOH	Butyric acid	Butanoic acid
CH₃-CH(COOH)-CH₃	Iso-butyric acid	2-Methylpropanoic acid

Depending upon the group attached to the carboxylic group, acids are classified as:

Based on number of carboxyl groups:

• Monocarboxylic acids: One COOH group (e.g., Formic acid, Acetic acid)
• Dicarboxylic acids: Two COOH groups (e.g., Oxalic acid, Malonic acid)
• Polycarboxylic acids: More than two COOH groups (e.g., Citric acid)

• Odor: C₁ to C₄ have pungent smell; C₄ to C₆ have unpleasant smell
• Solubility: C₁ to C₄ are very soluble in water due to hydrogen bonding. Solubility decreases with increase in molecular mass
• Boiling Points: Relatively high due to intermolecular hydrogen bonding
  HCOOH (373K) < CH₃COOH (391K) < C₂H₅COOH (424K)
• Melting Points: Increase irregularly with molecular mass. Even-numbered carbon acids have higher melting points than odd-numbered neighbors
• Dimerization: Exist as cyclic dimers in non-polar solvents like benzene
• Glacial Acetic Acid: Pure acetic acid freezes at 17°C to an ice-like solid

1. Carbonation of Grignard Reagent:

2. Hydrolysis of Nitriles:

3. Oxidation of Primary Alcohols:

4. Oxidation of Aldehydes:

5. Oxidation of Alkyl Benzene:

Three types of reactions:

1. Salt Formation (involving H atom of COOH)
   • With active metals: 2R-COOH + 2Na → 2R-COONa + H₂
   • With bases: R-COOH + NaOH → R-COONa + H₂O
   • With carbonates: 2R-COOH + Na₂CO₃ → 2R-COONa + CO₂ + H₂O
2. OH Group Replacement
   • Esterification: R-COOH + R’-OH ⇌ R-COO-R’ + H₂O
   • Acid Chloride Formation: R-COOH + PCl₅ → R-COCl + POCl₃ + HCl
   • Amide Formation: R-COOH + NH₃ → R-COONH₄ → R-CONH₂ + H₂O
3. Reactions Involving COOH Group as Whole
   • Reduction to Alcohols: R-COOH + 4[H] → R-CH₂OH + H₂O (using LiAlH₄)
   • Reduction to Alkanes: R-COOH + 6HI → R-CH₃ + 2H₂O + 3I₂
   • Decarboxylation: R-COONa + NaOH → R-H + Na₂CO₃ (with soda lime)

Order of Reactivity: Acid halide > Acid anhydride > Ester > Amide

Derivative	Formula	Preparation	Key Reaction
Acyl Halide	R-CO-X	R-COOH + PCl₅/SOCl₂	Hydrolysis to acid, Alcoholysis to ester, Ammonolysis to amide
Acid Anhydride	(R-CO)₂O	2R-COOH + P₂O₅ or R-COCl + R-COONa	Hydrolysis to acids, Alcoholysis to ester + acid
Ester	R-COO-R’	R-COOH + R’-OH (esterification)	Hydrolysis (acidic/basic) to acid + alcohol
Amide	R-CONH₂	R-COOH + NH₃ or R-COCl + NH₃	Hydrolysis to acid + NH₃, Dehydration to nitrile

Carboxylic acids are weak acids (pKa ≈ 5) compared to mineral acids but stronger than phenols and alcohols.

Carboxylic Acid	Formula	pKa
Formic acid	H-COOH	3.75
Acetic acid	CH₃-COOH	4.76
Propanoic acid	CH₃-CH₂-COOH	4.88
Fluoroacetic acid	F-CH₂-COOH	2.66
Chloroacetic acid	Cl-CH₂-COOH	2.86
Trichloroacetic acid	Cl₃C-COOH	0.70

Factors affecting acidic strength:

• Electron Withdrawing Groups (EWG): Increase acidity by stabilizing carboxylate anion
  Order: F > Cl > Br > I > NO₂ > CN
• Electron Donating Groups (EDG): Decrease acidity by destabilizing carboxylate anion
  Order: CH₃ > C₂H₅ > etc.
• Inductive Effect: More pronounced with proximity to COOH group

Practical Applications of Carboxylic Acids:

• Food Industry: Acetic acid (vinegar), citric acid (preservative), tartaric acid (baking powder)
• Pharmaceuticals: Aspirin (acetylsalicylic acid), ibuprofen, paracetamol synthesis
• Textile Industry: Acetic acid in dyeing, formic acid in leather tanning
• Polymers: Terephthalic acid for PET bottles, adipic acid for nylon
• Soaps & Detergents: Fatty acids as raw materials
• Agriculture: 2,4-D (herbicide), gibberellic acid (plant growth hormone)
• Perfumes & Flavors: Esters derived from carboxylic acids
• Solvents: Acetic acid as industrial solvent