What this chapter is about
This chapter introduces three important classes of organic compounds that contain the carbonyl group (C=O): aldehydes, ketones and carboxylic acids. In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom; in ketones, it is bonded to two carbon-containing groups; in carboxylic acids, the carbonyl group is attached to a hydroxyl group (–OH), forming the carboxyl group (–COOH). These functional groups determine the chemical behaviour and reactivity of each class.
You study this chapter now because these compounds are central to organic chemistry and biochemistry. Aldehydes and ketones undergo nucleophilic addition reactions at the carbonyl carbon, while carboxylic acids show acidic character and undergo substitution at the carboxyl group. Understanding their preparation, physical properties, chemical reactions and interconversions builds the foundation for studying biomolecules such as carbohydrates, amino acids and fats.
After completing this chapter, you should be able to write IUPAC names and structures, explain the reactivity of the carbonyl and carboxyl groups, predict products of common reactions, and distinguish between aldehydes, ketones and carboxylic acids using simple chemical tests.
Key ideas
- The carbonyl group (C=O) is polar; the carbon carries a partial positive charge (δ+) and the oxygen a partial negative charge (δ−), making the carbon susceptible to attack by nucleophiles.
- Aldehydes have the general structure R–CHO (where R may be H or an alkyl/aryl group); ketones have the structure R–CO–R′ (both R groups are carbon-containing).
- Nucleophilic addition is the characteristic reaction of aldehydes and ketones: nucleophiles such as HCN, NaHSO₃, alcohols, ammonia derivatives and Grignard reagents add across the C=O bond.
- Aldehydes are easily oxidised to carboxylic acids; ketones resist oxidation under mild conditions. This difference is the basis of Tollens' test (silver mirror) and Fehling's test (red precipitate of Cu₂O).
- Carboxylic acids are weak acids; the carboxyl hydrogen ionises because the resulting carboxylate ion (RCOO⁻) is resonance-stabilised.
- Electron-withdrawing groups (such as –Cl, –NO₂) increase acid strength; electron-donating groups (such as –CH₃) decrease it.
- Important reactions of carboxylic acids include esterification (with alcohols), reduction (to primary alcohols), decarboxylation (loss of CO₂) and formation of acid derivatives (acid chlorides, amides, anhydrides).
Formulas and facts to remember
1. IUPAC naming: Replace the terminal –e of the parent alkane with –al for aldehydes (ethanal), –one for ketones (propan-2-one), –oic acid for carboxylic acids (ethanoic acid).
2. Boiling points: Aldehydes and ketones have higher boiling points than comparable alkanes (due to dipole–dipole attraction) but lower than alcohols of similar mass (no hydrogen bonding between molecules). Carboxylic acids have the highest boiling points because they form strong hydrogen-bonded dimers.
3. Aldol condensation: Two molecules of an aldehyde (or ketone) with an α-hydrogen combine in dilute alkali to give a β-hydroxy aldehyde (aldol), which can lose water to form an α,β-unsaturated carbonyl compound.
4. Cannizzaro reaction: Aldehydes without an α-hydrogen (e.g., HCHO, C₆H₅CHO) undergo self-oxidation-reduction in concentrated alkali, giving an alcohol and a carboxylate salt.
5. Tollens' test: Aldehydes reduce ammoniacal silver nitrate to metallic silver (mirror); ketones do not react.
6. Fehling's test: Aldehydes reduce the blue Cu²⁺ complex to a red precipitate of Cu₂O; aromatic aldehydes and ketones give a negative test.
7. Esterification (Fischer): RCOOH + R′OH ⇌ RCOOR′ + H₂O (acid catalyst, reversible).
8. Hell–Volhard–Zelinsky reaction: Carboxylic acids with an α-hydrogen react with Cl₂ or Br₂ in the presence of red phosphorus to give α-halo acids.
Worked examples
### Example 1: Writing the IUPAC name
Problem: Give the IUPAC name of CH₃–CH₂–CH₂–CHO.
Solution: 1. Identify the functional group: –CHO indicates an aldehyde. 2. Find the longest carbon chain containing –CHO: 4 carbons. 3. Number from the aldehyde carbon (it is always carbon 1). 4. Parent alkane is butane; replace –e with –al. 5. Name: Butanal.
### Example 2: Predicting the product of nucleophilic addition
Problem: What product forms when propanone reacts with HCN in the presence of a base?
Solution: 1. Propanone structure: CH₃–CO–CH₃. 2. CN⁻ (from HCN and base) attacks the electrophilic carbonyl carbon. 3. The π-bond breaks; oxygen accepts the electron pair and becomes negatively charged. 4. Protonation by HCN gives the cyanohydrin. 5. Product: 2-hydroxy-2-methylpropanenitrile, (CH₃)₂C(OH)CN.
### Example 3: Comparing acid strengths
Problem: Arrange these acids in order of increasing acid strength: CH₃COOH, ClCH₂COOH, HCOOH.
Solution: 1. Consider the stability of the conjugate base (carboxylate ion). 2. Chlorine is electron-withdrawing (−I effect); it stabilises the negative charge by pulling electron density away. 3. ClCH₂COO⁻ is more stable than CH₃COO⁻; hence ClCH₂COOH is a stronger acid than CH₃COOH. 4. In HCOOH, the H atom exerts no electron-donating effect; its acidity lies between the other two. 5. Order of increasing acid strength: CH₃COOH < HCOOH < ClCH₂COOH.
Common mistakes
- Writing the carbonyl carbon as nucleophilic → remember it is electrophilic (δ+) because oxygen is more electronegative.
- Attempting Tollens' test on ketones and expecting a positive result → only aldehydes reduce Tollens' reagent; ketones do not.
- Forgetting that aldol condensation needs at least one α-hydrogen → compounds like benzaldehyde (C₆H₅CHO) cannot undergo aldol; they undergo Cannizzaro instead.
- Confusing esterification with neutralisation → esterification is a slow, reversible reaction between an acid and an alcohol, not a fast acid–base reaction.
- Ignoring the inductive effect when comparing acid strengths → electron-withdrawing substituents increase acidity; electron-donating ones decrease it.
Quick revision
- Aldehydes (–CHO) are oxidised easily; ketones (–CO–) resist mild oxidation.
- Nucleophilic addition at C=O: the nucleophile attacks carbon, the electrophile (or proton) goes to oxygen.
- Carboxylic acids are acidic because the carboxylate ion is stabilised by resonance.
- Tollens' test (silver mirror) and Fehling's test (red Cu₂O) distinguish aldehydes from ketones.
- Aldol condensation requires an α-hydrogen; Cannizzaro reaction occurs when there is no α-hydrogen.
- Esterification: acid + alcohol ⇌ ester + water (acid catalyst, reversible).