Carboxylic acids and derivatives: properties, reactions
Carboxylic Acids & Their Derivatives
1️⃣ What is a Carboxylic Acid?
A carboxylic acid has the functional group –COOH. Think of it as a “sticky” group that loves to grab onto other molecules. The classic example is acetic acid (CH₃COOH), the main component of vinegar 🧪. Its structure looks like this: $$\ce{CH3-COOH}$$
2️⃣ Key Properties
- Acidity: Carboxylic acids donate a proton (H⁺) from the –OH part of the group, making them weak acids (pKa ≈ 4–5).
- Solubility: Small acids (like acetic acid) are soluble in water due to hydrogen bonding.
- Odor: Many acids have a sharp, pungent smell (think of the “vinegar” scent).
- Reactivity: The carbonyl carbon (C=O) is electrophilic, ready to accept nucleophiles.
3️⃣ Derivatives of Carboxylic Acids
Derivatives are formed by replacing the –OH of the carboxyl group with other atoms or groups. They share many properties but differ in reactivity.
| Derivative | General Formula | Common Example |
|---|---|---|
| Acid Chloride | $$\ce{R-COCl}$$ | Acetyl chloride (CH₃COCl) |
| Ester | $$\ce{R-COOR'}$$ | Ethyl acetate (CH₃COOCH₂CH₃) |
| Amide | $$\ce{R-CONR'}$$ | Acetamide (CH₃CONH₂) |
| Anhydride | $$\ce{R-CO-O-CO-R'}$$ | Acetic anhydride (CH₃CO)₂O |
4️⃣ Reactions of Carboxylic Acids
4.1 Acid–Base Neutralisation
Acids react with bases to form a salt and water. Example with sodium hydroxide: $$\ce{CH3COOH + NaOH -> CH3COONa + H2O}$$ The product, sodium acetate, is a salt that can be used in cooking or as a buffer.
4.2 Nucleophilic Acyl Substitution
This is the main reaction pathway for all derivatives. The carbonyl carbon is attacked by a nucleophile, displacing the leaving group.
- Acid Chloride + Alcohol → Ester + HCl
$$\ce{CH3COCl + CH3CH2OH -> CH3COOCH2CH3 + HCl}$$
- Acid Chloride + Amine → Amide + HCl
$$\ce{CH3COCl + NH2CH3 -> CH3CONHCH3 + HCl}$$
- Ester + Alcohol (Fischer Esterification) → Higher Ester + Water
$$\ce{CH3COOH + CH3CH2OH -> CH3COOCH2CH3 + H2O}$$ (acid catalyst required)
- Anhydride + Water → Two Acids (Hydrolysis)
$$\ce{(CH3CO)2O + H2O -> 2 CH3COOH}$$
4.3 Redox Reactions
Carboxylic acids can be reduced to alcohols or aldehydes. For example, reduction of acetic acid with lithium aluminium hydride (LiAlH₄) gives ethanol: $$\ce{CH3COOH + LiAlH4 -> CH3CH2OH + LiAlO2 + H2}$$
5️⃣ Quick Reference Table
| Derivative | Typical Reaction | Key Point |
|---|---|---|
| Acid Chloride | React with alcohol → ester | Very reactive, releases HCl |
| Ester | Hydrolysis → acid + alcohol | Smell like fruit (often sweet) |
| Amide | Hydrolysis → acid + amine | Less reactive, stable at room temp |
| Anhydride | Hydrolysis → two acids | Strongly reacts with water |
💡 Take‑Away Points
- Carboxylic acids are like “sticky” molecules that love to form bonds.
- Derivatives change the leaving group, altering reactivity.
- All reactions involve the electrophilic carbonyl carbon.
- Acid–base neutralisation is the simplest reaction; nucleophilic substitution is the most versatile.
- Remember the “acid‑base + nucleophile = substitution” rule of thumb!
Revision
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