Carboxylic Acids and Their Derivatives – Cambridge IGCSE / A‑Level (9701)
1. Overview
Carboxylic acids (R‑COOH) and their four main families of derivatives – acid halides, anhydrides, esters and amides – are central to organic chemistry. Their chemistry is dominated by:
- The electrophilic carbonyl carbon (sp²‑hybridised).
- The ability of the attached hetero‑atom (X) to act as a leaving group in nucleophilic acyl substitution.
2. General Structural Features
| Derivative | General formula | Leaving group (X) |
|---|
| Acid halide | R‑C(=O)‑Cl | Cl⁻ (excellent) |
| Anhydride | R‑C(=O)‑O‑C(=O)‑R | Carboxylate (RCOO⁻, good) |
| Ester | R‑C(=O)‑O‑R' | Alkoxide (RO⁻, moderate) |
| Amide | R‑C(=O)‑NR'₂ | Amide anion (NR'₂⁻, poor) |
3. Acidic Nature of Carboxylic Acids
Acidity stems from resonance stabilisation of the conjugate base (carboxylate ion):
RCOOH ⇌ RCOO⁻ + H⁺
- The negative charge is delocalised over two oxygens (≈50 % on each).
- Typical pKa values: 4–5 (stronger than alcohols, weaker than mineral acids).
4. IUPAC Nomenclature (Section 13.1)
4.1 Naming Algorithm (parent chain ≤ 6 C)
- Identify the longest continuous carbon chain that contains the carbonyl carbon.
- Number the chain to give the carbonyl carbon the lowest possible locant.
- Assign the highest‑priority functional group the suffix**;** all other groups become prefixes.
- Write the name: parent‑chain‑locant‑suffix + any prefixes (e.g., 2‑methylpropanoic acid).
4.2 Functional‑Group Priority (Cambridge syllabus)
| Priority (high → low) | Suffix / Prefix used in naming |
|---|
| 1 | -oic acid (carboxylic acid) |
| 2 | -oyl chloride (acid halide) |
| 3 | -anhydride (acid anhydride) |
| 4 | -oate (ester) |
| 5 | -amide (amide) |
| 6 | -ol, -al, -one, etc. (lower‑priority groups) |
4.3 Worked Naming Examples
- Acid halide:
CH₃‑CH₂‑C(=O)‑Cl → propionyl chloride (systematic) or propionic acid chloride.
- Anhydride:
(CH₃‑C(=O))₂O → acetic anhydride.
- Ester:
CH₃‑C(=O)‑O‑CH₂CH₃ → ethyl acetate (systematic: ethanoic acid, ethyl ester).
- Amide:
CH₃‑C(=O)‑NH₂ → acetamide (systematic: ethanamide).
4.4 Naming Practice Box (mixed‑functional‑group molecules, ≤ 6 C)
| Structure |
Systematic name |
Common name (if any) |
| CH₃‑CH₂‑C(=O)‑OH |
propanoic acid |
– |
| CH₃‑CH(OH)‑CH₂‑C(=O)‑OH |
3‑hydroxybutanoic acid |
β‑hydroxybutyric acid |
| CH₃‑C(=O)‑O‑CH₂CH₃ |
ethyl acetate (ethanoic acid, ethyl ester) |
ethyl acetate |
| CH₃‑C(=O)‑Cl |
propionyl chloride |
– |
| CH₃‑C(=O)‑NH‑CH₃ |
N‑methylacetamide (ethanamide, N‑methyl‑) |
– |
5. Bonding, Hybridisation & Resonance (Section 13.3)
- The carbonyl carbon is sp²‑hybridised. The C=O bond consists of one σ‑bond (sp²–sp²) and one π‑bond (p–p).
- Resonance in carboxylate ion:
R‑C(=O)O⁻ ↔ R‑C(–O)=O⁻
Delocalisation reduces the basicity of the conjugate base.
- In amides the nitrogen lone pair donates into the carbonyl π*‑orbital, giving partial C–N double‑bond character and markedly lowering electrophilicity.
6. Isomerism (Section 13.4)
- Structural (constitutional) isomerism: e.g., methyl propanoate (CH₃CH₂COOCH₃) vs. ethyl acetate (CH₃COOCH₂CH₃).
- Geometric (cis/trans) isomerism can appear in cyclic anhydrides or in α‑substituted acids where the carbonyl carbon participates in a C=C system.
- Tautomerism is not a major feature of the four derivatives, but keto–enol tautomerism is relevant to aldehydes/ketones (outside the present scope).
7. Core Reactions & Terminology (Section 13.2)
- Nucleophilic acyl substitution – the universal mechanism for acid halides, anhydrides, esters and amides.
- Electrophilic addition – key step in acid‑catalysed esterification (addition of H⁺ to the carbonyl).
- Hydrolysis / Saponification – conversion of an acyl derivative to the parent acid (or its salt).
- Reduction – LiAlH₄ (or NaBH₄ for esters) converts carbonyl derivatives to alcohols or amines.
- Dehydration – P₂O₅ or POCl₃ converts amides to nitriles.
8. General Mechanism of Nucleophilic Acyl Substitution
- Attack: A nucleophile (ν⁻) attacks the electrophilic carbonyl carbon, forming a tetrahedral intermediate (TI).
R‑C(=O)‑X + ν⁻ → R‑C(–O⁻)(‑X)(‑ν) (TI)
- Collapse: The TI collapses, re‑forming the C=O bond and expelling the leaving group (X⁻).
R‑C(–O⁻)(‑X)(‑ν) → R‑C(=O)‑ν + X⁻
Overall: RCOX + ν⁻ → RCONu + X⁻
9. Individual Derivatives – Key Reactions & Examples
9.1 Acid Halides (RCOCl)
- Hydrolysis (fast, exothermic)
RCOCl + H₂O → RCOOH + HCl
- Esterification (with alcohol)
RCOCl + R'OH → RCOOR' + HCl
(Pyridine or Et₃N used to trap HCl.)
- Amide formation (with amine)
RCOCl + R'NH₂ → RCONHR' + HCl
- Reduction (LiAlH₄) → primary alcohol
RCOCl + 4 [H] → RCH₂OH + HCl
- Typical reagents for preparation: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl.
9.2 Anhydrides ((RCO)₂O)
9.3 Esters (RCOOR′)
- Acid‑catalysed hydrolysis (reverse Fischer esterification)
RCOOR' + H₂O ⟶H⁺ RCOOH + R'OH
- Base‑catalysed hydrolysis (saponification) – gives carboxylate salt.
RCOOR' + OH⁻ → RCOO⁻ + R'OH
- Reduction (LiAlH₄) – affords two alcohols.
RCOOR' + 4 [H] → RCH₂OH + R'OH
- Fischer esterification (acidic esterification)
RCOOH + R'OH ⟶H⁺, Δ RCOOR' + H₂O
(Use excess alcohol or a Dean‑Stark trap to drive equilibrium.)
9.4 Amides (RCONR′₂)
10. Comparative Reactivity (Nucleophilic Acyl Substitution)
| Derivative | Leaving‑group ability (X⁻) | Relative reactivity |
|---|
| Acid halide | Cl⁻ (excellent) | Very high |
| Anhydride | Carboxylate (good) | High |
| Ester | Alkoxide (moderate) | Moderate |
| Amide | Amide anion (poor) | Low |
Trend explained by resonance donation from X: the more electron‑donating the leaving group, the less electrophilic the carbonyl carbon becomes.
11. Laboratory Preparations (Section 13.5)
- Acid halide:
RCOOH + SOCl₂ → RCOCl + SO₂ + HCl
(Gaseous by‑products drive the reaction forward.)
- Anhydride:
2 RCOOH + (COCl)₂ → (RCO)₂O + 2 HCl
(Oxalyl chloride is common; the reaction is catalysed by pyridine.)
- Ester (Fischer esterification):
RCOOH + R'OH ⟶H⁺, Δ RCOOR' + H₂O
(Dean‑Stark trap or excess alcohol removes water.)
- Amide (direct amidation):
RCOCl + R'NH₂ → RCONHR' + HCl
(Et₃N or pyridine scavenges HCl.)
12. Links to Other Syllabus Sections
12.1 Hydrocarbons (Section 14.1)
- Acid halides are the most reactive halogen derivatives; they are prepared from carboxylic acids using thionyl chloride, analogous to the preparation of alkyl chlorides from alkanes.
- Complete combustion of alkanes gives CO₂ and H₂O, which can be oxidised (e.g., KMnO₄) to carboxylic acids – the starting point for the derivative series.
12.2 Alkenes (Section 14.2)
- Electrophilic addition of HCl to an alkene yields a chloro‑alkane; subsequent oxidation (KMnO₄, hot) gives a carboxylic acid, which can be converted to any derivative.
- The acid‑catalysed esterification mechanism mirrors the electrophilic activation of carbonyls in alkene addition reactions.
12.3 Halogen Compounds (Section 15)
- Acid halides undergo nucleophilic acyl substitution, a distinct pathway from the SN1/SN2 mechanisms of alkyl halides.
- The leaving‑group trend (Cl⁻ > RCOO⁻ > RO⁻ > NR₂⁻) parallels the reactivity order of alkyl halides (I > Br > Cl > F).
12.4 Hydroxy Compounds (Section 16)
- Alcohols act as nucleophiles in esterification and acid‑halide reactions.
- Phenols are less nucleophilic because of aromatic resonance, reacting more slowly with acid halides – an illustration of functional‑group selectivity.
- Ethers are generally inert under nucleophilic acyl substitution conditions, highlighting the chemoselectivity of the reactions.
13. Typical Examination Questions (Cambridge style)
- Predict the major product when benzoic acid is treated with thionyl chloride.
Answer: benzoyl chloride (C₆H₅COCl).
- Write a step‑by‑step mechanism for the saponification of ethyl acetate with NaOH, including the tetrahedral intermediate.
Key points:* attack of OH⁻, formation of TI, collapse to acetate ion + ethanol, neutralisation.
- Explain why acetamide is far less reactive towards nucleophiles than acetyl chloride.
Answer:* resonance donation from the amide nitrogen reduces the electrophilicity of the carbonyl carbon; the leaving group (amide anion) is a very poor base.
- Given the reaction:
CH₃COCl + 2 C₂H₅OH → ? Identify the product(s) and state the role of each reagent.
Answer:* ethyl acetate (ester) is formed; excess ethanol acts as both nucleophile and solvent; HCl produced is scavenged by pyridine.
- Compare the reactivity of the following towards NaOH at 25 °C: (i) ethyl acetate, (ii) acetic anhydride, (iii) acetamide.
Answer:* acetic anhydride > ethyl acetate > acetamide (based on leaving‑group ability).
14. Quick Revision Checklist
- Identify the carbonyl‑containing functional group and assign the correct suffix.
- Remember the priority order for naming (acid > acid halide > anhydride > ester > amide).
- For each derivative, know:
- Key leaving group
- Typical preparation method
- One characteristic reaction (hydrolysis, esterification, amide formation, reduction, etc.)
- Reactivity trend: acid halide > anhydride > ester > amide.
- Resonance stabilisation of the carboxylate ion explains acidity; resonance donation from X explains decreasing electrophilicity.
- Be able to draw the tetrahedral intermediate for any nucleophilic acyl substitution.