Aldehydes, Ketones & Carboxylic Acids
SRMJEEE – Chemistry (Organic)
Aldehydes, ketones, and carboxylic acids are the most important functional groups in organic chemistry. They are the “core” of many reactions and industrial processes.
1. Simple Explanation
| Functional Group | General Formula | Key Feature | Typical Nomenclature |
|---|---|---|---|
| Aldehyde | R–CHO | Carbonyl carbon attached to at least one H | –al (ex. ethanal → acetaldehyde) |
| Ketone | R–CO–R′ | Carbonyl carbon attached to two R groups | –one (ex. propanone → acetone) |
| Carboxylic Acid | R–COOH | Carbonyl + hydroxyl on same carbon | –oic acid (ex. ethanoyl → acetic acid) |
- Carbonyl (C=O) is the defining feature.
- Aldehyde: one side is H → more reactive (can be oxidized).
- Ketone: both sides are alkyl → less reactive, harder to oxidize.
- Carboxylic Acid: extra –OH → strongly acidic, prone to acid‑base reactions.
2. Key Points
| Category | Highlights |
|---|---|
| Preparation | • Aldehydes: oxidation of primary alcohols, reduction of acid chlorides, addition of Grignard reagents to CO. • Ketones: oxidation of secondary alcohols, addition of Grignard reagents to CO. • Carboxylic acids: oxidation of aldehydes, reduction of acid chlorides, hydrolysis of acid halides. |
| Physical Properties | • Lower boiling points than alcohols of similar MW (C–H vs C–O). • Aldehydes are usually more volatile, liquids at RT. • Carboxylic acids: high boiling points, often solid, strong IR C=O stretch (~1700 cm⁻¹). |
| Acidity | • Carboxylate anion is resonance‑stabilised → pKₐ ~4–5. • Aldehydes & ketones are neutral. |
| Reactivity | • Nucleophilic addition (to C=O) dominates for aldehydes & ketones. • Oxidation: Aldehydes → acids; ketones are resistant. • Reduction: All can be reduced to alcohols (NaBH₄, LiAlH₄). |
| Common Reagents | • NaBH₄ (mild, stops at alcohol). • LiAlH₄ (strong, reduces CO₂ to H₂O). • PCC, Jones, O₂/Cl₂ for oxidation. • Acid & base for protonation/neutralisation. |
| IUPAC Naming | • Aldehyde = –al suffix. • Ketone = –one suffix. • Carboxylic acid = –oic acid suffix. • Numbers designate the position of the functional group (lowest number). |
3. Worked Example
Problem
Reduce 2‑butanone (CH₃–CO–CH₂–CH₃) to the corresponding alcohol using NaBH₄. Draw the product and provide the mechanism.
Solution
-
Identify the substrate
- 2‑Butanone is a ketone: CH₃–CO–CH₂–CH₃.
-
Choose the reducing agent
- NaBH₄ → strong hydride donor but stops at alcohol (no over‑reduction).
-
Mechanism
- Step 1: Hydride (H⁻) from NaBH₄ attacks the electrophilic carbonyl carbon → tetrahedral alkoxide intermediate.
- Step 2: Protonation of the alkoxide by the solvent (e.g., ethanol or water) → alcohol.
CH3–CO–CH2–CH3 + NaBH4 → CH3–CH(OH)–CH2–CH3 (tetrahedral intermediate) → 2‑butanol -
Product
- 2‑Butanol (CH₃–CH(OH)–CH₂–CH₃).
- Stereochemistry: formation of a new chiral centre → racemic mixture (R and S).
-
Key points
- NaBH₄ reduces only to alcohol; no further reduction to alkane.
- Reaction is usually performed in anhydrous ethanol or THF at 0–25 °C.
4. Quick Revision Summary
| Topic | Quick Take‑away |
|---|---|
| Aldehyde | R–CHO; easily oxidised to acid; undergoes nucleophilic addition. |
| Ketone | R–CO–R′; harder to oxidise; also undergoes addition. |
| Carboxylic Acid | R–COOH; acidic, high boiling point, reacts in esterification, amidation. |
| Reduction | NaBH₄ → alcohol; LiAlH₄ → alcohol + CO₂ to H₂O. |
| Oxidation | Aldehyde → acid (PCC, Jones, O₂). Ketone → no oxidation (unless strong). |
| Key Reagents | NaBH₄, LiAlH₄, PCC, Jones reagent, acid (for esterification), base (for deprotonation). |
| Naming | –al (aldehyde), –one (ketone), –oic acid (acid). |
| Physical | Carboxylic acids > 2× boiling point of alcohols of same MW. |
| Typical Reactions | 1. N |