Edexcel A-Level Chemistry: Advanced Organic Chemistry — Complete Revision Guide (9CH0)
Edexcel A-Level Chemistry: Advanced Organic Chemistry — Complete Revision Guide (9CH0)
Advanced organic chemistry is where Edexcel 9CH0 stops feeling like a list of mechanisms and starts feeling like a coherent framework for building molecules. Aldehydes are oxidation products of primary alcohols, but they are also nucleophilic-addition substrates. Carboxylic acids are oxidation products of aldehydes, but they are also building blocks for esters, amides and polymers. Amines are nucleophiles that connect to halogenoalkanes from organic foundations and to acyl chlorides in this topic. Once you see how each functional group interconverts, organic chemistry becomes a network you can navigate.
This network is also what synthesis questions are testing. Edexcel will show you a starting material and a target, and ask you to draw the route. The reasoning is mechanical once you know the moves: every transformation either adds, removes, oxidises, reduces or substitutes a functional group, and the choice of reagent for each step is constrained.
This guide walks through the advanced organic content in 9CH0 topic by topic. It covers aldehydes and ketones (Tollens, Fehling, 2,4-DNP, NaBH4, HCN); carboxylic acids (acidity, esterification, LiAlH4); esters; acyl chlorides and acid anhydrides; amines (basicity, preparation); amino acids and proteins (zwitterions, peptide bonds, isoelectric pH); condensation polymers (PET, nylon); organic synthesis routes; and a full reaction mechanism summary. For each topic you will find the core ideas, common pitfalls, a worked example and a link into the LearningBro Advanced Organic Chemistry course.
What the Edexcel 9CH0 Specification Covers
Edexcel A-Level Chemistry (9CH0) is examined through Paper 1 (Inorganic and Physical, 1h45, 90 marks), Paper 2 (Organic and Physical, 1h45, 90 marks) and Paper 3 (General and Practical, 2h30, 120 marks). Advanced organic is examined predominantly on Paper 2, and synoptic synthesis routes appear in extended-response questions on Paper 3.
| Sub-topic | Spec area | Typical Paper 2 weight |
|---|---|---|
| Aldehydes and ketones | Topic 16 | 4-6 marks |
| Carboxylic acids | Topic 17 | 4-6 marks |
| Esters and esterification | Topic 17 | 3-5 marks |
| Acyl chlorides and acid anhydrides | Topic 17 | 3-5 marks |
| Amines | Topic 18 | 4-6 marks |
| Amino acids and proteins | Topic 18 | 4-6 marks |
| Condensation polymers | Topic 18 | 3-5 marks |
| Organic synthesis routes | Topic 19 | 8-12 marks |
These weights are estimates, modelled on the Edexcel 9CH0 paper format. What is reliable is that a multi-step synthesis question — typically eight to twelve marks — appears on essentially every Paper 2 and that test reactions for carbonyls and carboxylic acids appear on every Paper 3.
Aldehydes and Ketones
Aldehydes (RCHO) and ketones (RCOR') both contain the C=O carbonyl group, but the carbonyl carbon in an aldehyde has at least one H attached, while in a ketone it has two carbon substituents. The C=O bond is highly polar (O is more electronegative), so the carbonyl carbon is δ+ and attacks by nucleophiles (nucleophilic addition).
Tests for carbonyls. 2,4-DNPH (2,4-dinitrophenylhydrazine, Brady's reagent) gives an orange/yellow precipitate with both aldehydes and ketones — the precipitate's melting point identifies the specific carbonyl. Tollens' reagent (ammoniacal silver nitrate) oxidises aldehydes to carboxylic acids and gives a silver mirror; ketones give no reaction. Fehling's solution (or Benedict's) gives a brick-red precipitate of Cu2O with aldehydes; ketones give no reaction.
| Reagent | Aldehyde | Ketone |
|---|---|---|
| 2,4-DNPH | Orange precipitate | Orange precipitate |
| Tollens' | Silver mirror | No reaction |
| Fehling's | Red precipitate | No reaction |
| NaBH4 | Reduced to primary alcohol | Reduced to secondary alcohol |
| HCN | Hydroxynitrile (Markovnikov-like) | Hydroxynitrile |
Reduction with NaBH4 (or LiAlH4) reduces both aldehydes and ketones to alcohols by nucleophilic addition of hydride (H-) to the carbonyl carbon. Aldehydes give primary alcohols; ketones give secondary alcohols.
HCN addition is a nucleophilic addition that gives a hydroxynitrile. The mechanism: CN- (formed from KCN/H+ or NaCN/H2SO4) attacks the δ+ carbonyl carbon; the C=O bond breaks heterolytically, giving an alkoxide; protonation gives the hydroxynitrile. This reaction adds a carbon to the chain, so it is a useful synthesis step.
Worked example. Predict the product of propanal + HCN. Nucleophilic addition gives 2-hydroxybutanenitrile, CH3CH2CH(OH)CN. The carbinol carbon is now chiral, so the product is a racemate (1:1 mixture of enantiomers).
A common pitfall is forgetting that HCN addition to an asymmetric carbonyl produces a racemate because the planar carbonyl can be attacked from either face. Another is over-reducing with LiAlH4 — both reagents reduce carbonyls to alcohols, not back to alkanes. See the carbonyls lesson.
Carboxylic Acids
Carboxylic acids (RCOOH) are weak acids because the carboxylate anion (RCOO-) formed on deprotonation is stabilised by resonance delocalisation of the negative charge across both oxygens. Stronger acids (lower pKa) result from electron-withdrawing substituents that further stabilise the carboxylate. Trichloroethanoic acid (CCl3COOH, pKa 0.7) is a much stronger acid than ethanoic acid (CH3COOH, pKa 4.76).
Carboxylic acids react with the standard base reagents to form salts: with sodium hydroxide gives sodium salt + water; with sodium carbonate gives sodium salt + water + CO2 (effervescence is the test); with sodium hydrogen carbonate gives sodium salt + water + CO2; with metals gives salt + H2.
The NaHCO3 test — adding solid sodium hydrogencarbonate to a suspected carboxylic acid produces fizzing as CO2 is released — distinguishes carboxylic acids from phenols and most other acidic species.
Reduction with LiAlH4 reduces carboxylic acids to primary alcohols (NaBH4 is too mild). The reaction goes via the aldehyde, but cannot be stopped at the aldehyde stage in practice.
A common pitfall is to forget that carboxylic acids are weak — only about 1 percent dissociate in solution at ordinary concentrations. Another is to expect NaBH4 to reduce a carboxylic acid (it does not). See the carboxylic acids lesson.
Esters and Esterification
Esters (RCOOR') are formed by condensation of a carboxylic acid and an alcohol, with concentrated sulfuric acid as catalyst. The reaction is a reversible equilibrium with a typical Kc of about 4-5, so excess of one reagent is needed for good yield.
RCOOH + R'OH ⇌ RCOOR' + H2O
The mechanism: protonation of the C=O oxygen activates the carbonyl for nucleophilic attack; the alcohol oxygen attacks the δ+ carbonyl carbon; the OH from the acid leaves as water; deprotonation gives the ester.
Esters have characteristic fruity smells (pear-drop, banana, pineapple) and are extensively used in flavourings and perfumes. They can be hydrolysed back to the acid and alcohol by acid (reverse of esterification, equilibrium) or base (irreversible, gives carboxylate salt + alcohol). Base hydrolysis is also called saponification — the basis of soap manufacture from triglyceride esters.
Worked example. Write the equation for the formation of ethyl ethanoate from ethanol and ethanoic acid. CH3COOH + CH3CH2OH ⇌ CH3COOCH2CH3 + H2O, with H2SO4 as catalyst.
A common pitfall is to omit the water from the equation, or to write the ester as RCOOH-OR' (the leaving water comes from the acid OH and the alcohol H). See the esters lesson.
Acyl Chlorides and Acid Anhydrides
Acyl chlorides (RCOCl) are derivatives of carboxylic acids in which OH is replaced by Cl. They are far more reactive than carboxylic acids because Cl is a much better leaving group and because the C=O is more electrophilic. Acyl chlorides react vigorously with water (hydrolysis to acid + HCl), with alcohols (esterification, irreversibly, fast), with ammonia (amide formation), and with amines (substituted amide formation).
Acid anhydrides (RCOOCOR) react similarly to acyl chlorides but more controllably. The classic example is ethanoic anhydride, used to make aspirin (esterification of salicylic acid).
| Reagent | Product with RCOCl |
|---|---|
| H2O | RCOOH + HCl (vigorous, steamy fumes) |
| R'OH | RCOOR' + HCl (irreversible esterification) |
| NH3 | RCONH2 + HCl (amide) |
| R'NH2 | RCONHR' + HCl (substituted amide) |
A common pitfall is to use carboxylic acid + amine to make an amide directly — at room temperature this just gives the salt; the amide forms only on heating or via the acyl chloride. See the acyl chlorides lesson.
Amines
Amines are derivatives of ammonia in which one or more H atoms are replaced by alkyl or aryl groups. Primary (RNH2), secondary (R2NH), tertiary (R3N), and quaternary ammonium (R4N+, always positively charged).
Basicity. Amines are weak bases because the lone pair on N can accept a proton: RNH2 + H+ → RNH3+. Aliphatic amines are more basic than ammonia (alkyl groups are electron-donating, making the lone pair more available); aromatic amines like phenylamine are less basic than ammonia (the lone pair is partially delocalised into the ring, less available for protonation).
Preparation. Amines can be made by:
- Heating a halogenoalkane with excess ammonia in ethanol (nucleophilic substitution; messy because successive substitutions give R2NH, R3N, R4N+)
- Reduction of a nitrile (R-CN + LiAlH4 → R-CH2NH2; adds a carbon to the chain)
- Reduction of a nitroarene (Ar-NO2 + Sn / conc HCl → Ar-NH2; the standard route to phenylamine)
A common pitfall is to expect the simple alkylation route (RX + NH3) to give a clean primary amine — it never does, because the product RNH2 is itself a nucleophile and reacts further. See the amines lesson.
Amino Acids and Proteins
Amino acids are 2-aminocarboxylic acids of the form H2N-CHR-COOH. The α-carbon (the C bonded to both NH2 and COOH) is chiral except in glycine (R = H). All naturally occurring amino acids are L-enantiomers.
In solution at intermediate pH, the COOH donates its proton to the NH2, giving a zwitterion with both a negative carboxylate and a positive ammonium group. The zwitterion has zero net charge.
The pH at which the average net charge is zero is the isoelectric point (pI). At pH below pI, the molecule is protonated (positive); at pH above pI, it is deprotonated (negative). At pI, the molecule has zero net charge and minimum solubility — useful for amino-acid separation by electrophoresis.
Peptide bonds form by condensation between the COOH of one amino acid and the NH2 of another, releasing water and creating a -CONH- amide linkage. A peptide is a chain of amino acids; a protein is a long peptide chain folded into a specific three-dimensional structure. Hydrolysis of a peptide bond reverses the condensation and gives back the free amino acids.
Worked example. Identify the products of acid hydrolysis of the dipeptide glycyl-alanine (Gly-Ala). Acid hydrolysis breaks the amide linkage to give the two free amino acids: glycine (H2NCH2COOH) and alanine (H2NCH(CH3)COOH).
A common pitfall is to draw the zwitterion with both groups uncharged. Another is to confuse the protonation state at different pH values. See the amino acids lesson.
Condensation Polymers
Condensation polymers are formed by condensation between monomers, releasing a small molecule (usually water or HCl) at each linkage. Two main types appear on the specification.
Polyesters (e.g. PET, polyethylene terephthalate) are formed from a diol and a dicarboxylic acid (or diacyl chloride). PET is made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid. The repeat unit contains an ester linkage -COO-.
Polyamides (e.g. nylon-6,6, Kevlar) are formed from a diamine and a dicarboxylic acid (or diacyl chloride). Nylon-6,6 is made from hexane-1,6-diamine and hexanedioic acid. The repeat unit contains an amide linkage -CONH-.
Both polyester and polyamide can be hydrolysed back to the original monomers, in contrast to addition polymers (polythene, polypropylene) which are essentially non-biodegradable. This is the basis for the recycling of PET into rPET.
Worked example. Draw the repeat unit for the polyester formed from ethane-1,2-diol and ethanedioic acid. Repeat unit: -O-CH2-CH2-O-CO-CO- (with continuation lines into the polymer backbone).
A common pitfall is to draw the polymer with the H2O still attached to the repeat unit. Another is to confuse condensation with addition polymerisation. See the polymers lesson.
Organic Synthesis Routes
Synthesis questions ask you to convert one molecule into another using the reactions covered in foundations and advanced. The standard approach is retrosynthesis: work backward from the target by identifying disconnections — which functional groups are present, and what reagent could have produced each.
A small library of workhorse moves covers most synthesis questions:
| Functional group transformation | Reagent / conditions |
|---|---|
| Alkene → alkane | H2 / Ni catalyst |
| Alkene → halogenoalkane | HBr or Br2 |
| Alkene → diol | Cold dilute KMnO4 |
| Halogenoalkane → alcohol | NaOH(aq), reflux |
| Halogenoalkane → nitrile | KCN/ethanol, reflux (adds 1 carbon) |
| Halogenoalkane → amine | NH3(excess) in ethanol, sealed tube |
| Alcohol (1°) → aldehyde | Acidified K2Cr2O7, distil |
| Alcohol (1°) → carboxylic acid | Acidified K2Cr2O7, reflux |
| Alcohol (2°) → ketone | Acidified K2Cr2O7, reflux |
| Aldehyde / ketone → alcohol | NaBH4 in methanol |
| Aldehyde / ketone → hydroxynitrile | KCN/H+ (adds 1 carbon, racemate) |
| Carboxylic acid → ester | R'OH + conc H2SO4 |
| Carboxylic acid → acyl chloride | SOCl2 or PCl5 |
| Acyl chloride → amide | NH3 or RNH2 |
| Nitrile → amine | LiAlH4 or H2/Ni |
| Nitrile → carboxylic acid | Aqueous HCl, reflux |
| Nitroarene → arylamine | Sn / conc HCl, then NaOH |
Worked example. Devise a route from propan-1-ol to 2-aminobutanoic acid. (1) Oxidise propan-1-ol to propanal with acidified dichromate (distil out aldehyde). (2) Add HCN to give 2-hydroxybutanenitrile. (3) Hydrolyse with HCl to give 2-hydroxybutanoic acid. (4) Convert to 2-bromobutanoic acid with PBr3. (5) Heat with excess NH3 to give 2-aminobutanoic acid. Five steps; HCN added a carbon.
A common pitfall is to skip steps that change the chain length (HCN addition adds a carbon; many syntheses fail because students don't notice the chain difference). Another is to forget intermediate oxidation/reduction steps. See the synthesis lesson.
Reaction Mechanism Summary
Across organic foundations and advanced you should be fluent in five major mechanism types: free radical substitution, electrophilic addition, nucleophilic substitution (SN1, SN2), elimination (E1, E2), and nucleophilic addition (carbonyl chemistry). Together they cover essentially every organic reaction on the specification.
Nucleophilic addition (carbonyls): nucleophile attacks δ+ carbonyl carbon, C=O breaks heterolytically to give alkoxide, alkoxide is protonated. Examples: HCN + carbonyl → hydroxynitrile; NaBH4 + carbonyl → alcohol.
Nucleophilic addition-elimination (carboxylic acid derivatives): nucleophile attacks δ+ carbonyl carbon, intermediate forms, leaving group departs. Examples: acyl chloride + amine → amide + HCl; ester + OH- → carboxylate + alcohol.
A common pitfall is to use straight curly arrows for these mechanisms — every step is two-electron movement and arrows must go from electron source to electron sink. See the mechanism summary lesson.
Common Mark-Loss Patterns
- Forgetting that carbonyl nucleophilic addition gives a racemate when the carbonyl is asymmetric.
- Drawing the ester product without water released.
- Trying to make an amide directly from carboxylic acid + amine (gives salt only).
- Drawing zwitterions without both charges.
- Mis-classifying primary, secondary and tertiary amines.
- Confusing condensation and addition polymerisation.
- Skipping carbon-changing steps (HCN, nitrile reduction) in synthesis.
- Forgetting that LiAlH4 reduces carboxylic acids but NaBH4 does not.
- Drawing the polymer repeat unit with the H2O leaving group still attached.
- Using the same arrow style for radical and ionic mechanisms.
How to Revise This Topic
- Build a synthesis flashcard deck. Each card has a starting material on one side and the target on the other; you must draw the route. Drill until automatic.
- Memorise the carbonyl test colours and observations cold. 2,4-DNPH (orange), Tollens' (silver mirror), Fehling's (red).
- Practise mechanism drawing daily for two weeks, alternating between nucleophilic addition, addition-elimination, SN1, SN2, E1, E2.
- Drill amino acid pH behaviour by sketching protonation states above and below pI.
- Use the LearningBro practice quizzes to test under timed conditions.
Linking to Other Topics
Advanced organic builds directly on organic foundations. The mechanisms here extend the SN1/SN2/electrophilic addition framework to carbonyls and acid derivatives. Analytical chemistry is the other side of this topic — IR identifies the C=O stretch in carbonyls and acids, and NMR distinguishes aldehyde, ketone and ester products. The pH behaviour of amino acids is a special case of acids and buffers. Even bonding theory (bonding) appears here in explaining the polarity of C=O and the resonance stabilisation of carboxylates.
Final Word
Advanced organic is the topic where everything in foundations starts paying off. Master the small library of standard reactions, drill mechanisms until they are automatic, and practise multi-step synthesis routes by retrosynthesis. The full LearningBro Advanced Organic Chemistry course walks through every reaction with worked examples and AI tutor feedback. Get this section fluent and Paper 2's organic synthesis question becomes a routine puzzle rather than a maze.