### Carboxylic Acids: Structure, Properties, and Reactions
#### Definition and Structure
- **Carboxylic Acids**: Organic compounds containing a carboxyl group (−COOH).
- **General Formula**: R−COOH, where R can be a hydrogen atom, an alkyl group, or
an aryl group.
- **Functional Group**: Carboxyl group consists of a carbonyl group (C=O) and a
hydroxyl group (−OH) attached to the same carbon atom.
#### Nomenclature
- **IUPAC Names**: Named by replacing the "e" in the name of the parent alkane with
"oic acid".
- Example: Methane becomes methanoic acid (common name: formic acid).
- **Common Names**: Many carboxylic acids are known by their common names which
often derive from Latin or Greek words related to the source of the acid.
- Example: Ethanoic acid is commonly known as acetic acid.
#### Physical Properties
1. **Polarity**: Carboxylic acids are highly polar due to the presence of both a
carbonyl and a hydroxyl group.
2. **Hydrogen Bonding**: Capable of forming strong hydrogen bonds with water and
with each other, leading to high boiling and melting points compared to other
organic compounds of similar molecular weight.
3. **Solubility**:
- **Lower Members (1-4 carbons)**: Highly soluble in water due to hydrogen
bonding.
- **Higher Members**: Solubility decreases with increasing carbon chain length.
#### Chemical Properties
1. **Acidity**:
- Carboxylic acids are weak acids and dissociate partially in water to form a
carboxylate anion (RCOO⁻) and a hydrogen ion (H⁺).
- **Acidity Strength**: The presence of electron-withdrawing groups increases
acidity, while electron-donating groups decrease it.
2. **Reactivity**:
- **Reduction**: Carboxylic acids can be reduced to primary alcohols using
reducing agents like lithium aluminum hydride (LiAlH₄).
- **Esterification**: React with alcohols in the presence of an acid catalyst to
form esters (Fischer esterification).
- **Formation of Acid Chlorides**: React with thionyl chloride (SOCl₂) to form
acyl chlorides.
- **Decarboxylation**: Carboxylic acids can lose a carbon dioxide molecule under
certain conditions.
#### Reactions of Carboxylic Acids
1. **Formation of Esters**:
- **Reaction**: Carboxylic acid + Alcohol → Ester + Water
- **Catalyst**: Typically an acid like sulfuric acid (H₂SO₄).
- **Example**: Ethanoic acid + Methanol → Methyl ethanoate + Water.
2. **Reduction to Primary Alcohols**:
- **Reducing Agent**: Lithium aluminum hydride (LiAlH₄).
- **Reaction**: R−COOH → R−CH₂OH.
- **Example**: Ethanoic acid → Ethanol.
3. **Formation of Acid Chlorides**:
- **Reaction**: Carboxylic acid + Thionyl chloride (SOCl₂) → Acyl chloride + SO₂
+ HCl.
- **Example**: Ethanoic acid + SOCl₂ → Acetyl chloride + SO₂ + HCl.
4. **Decarboxylation**:
- **Reaction**: Carboxylic acid → Hydrocarbon + CO₂.
- **Conditions**: Often requires heating, sometimes in the presence of a
catalyst.
5. **Formation of Amides**:
- **Reaction**: Carboxylic acid + Amine → Amide + Water.
- **Example**: Ethanoic acid + Ammonia → Acetamide + Water.
#### Uses and Applications
1. **Industrial Applications**:
- **Acetic Acid**: Used in the production of vinegar, as a solvent, and as a
precursor to various chemicals.
- **Formic Acid**: Used in leather processing, as a preservative, and in rubber
production.
- **Benzoic Acid**: Used as a food preservative (sodium benzoate) and in the
manufacture of various pharmaceuticals.
2. **Biological Significance**:
- **Fatty Acids**: Long-chain carboxylic acids are essential components of
lipids.
- **Amino Acids**: Building blocks of proteins contain carboxyl groups.
3. **Pharmaceuticals**: Many drugs are carboxylic acids or their derivatives,
playing crucial roles in medicinal chemistry.
#### Synthesis of Carboxylic Acids
1. **Oxidation of Primary Alcohols and Aldehydes**:
- **Reaction**: Primary alcohol → Aldehyde → Carboxylic acid.
- **Oxidizing Agents**: Potassium permanganate (KMnO₄), Chromium trioxide
(CrO₃).
2. **Hydrolysis of Nitriles**:
- **Reaction**: Nitrile (R−C≡N) + Water → Carboxylic acid.
- **Catalyst**: Acidic or basic conditions.
- **Example**: CH₃CN + H₂O → CH₃COOH.
3. **Carbonation of Grignard Reagents**:
- **Reaction**: Grignard reagent (R−MgX) + CO₂ → Carboxylic acid.
- **Example**: CH₃MgBr + CO₂ → CH₃COOH.
### Summary
Carboxylic acids are versatile organic compounds with significant industrial,
biological, and pharmaceutical importance. They are characterized by their carboxyl
group (−COOH), high boiling points due to hydrogen bonding, and moderate acidity.
Key reactions include esterification, reduction, and the formation of acid
chlorides. Their synthesis can be achieved through various methods, including the
oxidation of primary alcohols and the carbonation of Grignard reagents.
