Topic 13: Introduction to Organic Chemistry
AS Level · Organic Chemistry · CIE 9701 · 2025–2027 syllabus
Topic 13: Introduction to Organic Chemistry
AS Level · Organic Chemistry · Topic 13 of 22
Organic Reaction Types & Mechanisms
The foundation of all organic chemistry — understanding how and why reactions happen. Covers the six core reaction types (substitution, addition, elimination, condensation, hydrolysis, oxidation/reduction), bond fission mechanisms, and the key reagents for each. Tested heavily in Paper 1 (MCQ) and Paper 2 (Structured).
6Reaction types
2Fission types
P1+P2Papers assessed
HighExam frequency
What this topic covers
Bond fission
Homolytic (equal split → radicals) and heterolytic (unequal split → ions). Determines the reaction mechanism.
Reagent types
Free radicals, nucleophiles and electrophiles — what they are, how they attack, and typical examples of each.
Substitution
Free radical substitution (alkanes + Cl₂/UV) and nucleophilic substitution (halogenoalkanes + OH⁻, NH₃, CN⁻).
Addition
Electrophilic addition to alkenes (Br₂, HBr, H₂/Ni, H₂O/H₃PO₄) and nucleophilic addition to carbonyls (HCN/KCN).
Elimination & Condensation
Elimination (OH⁻/alc, heat) forms double bonds. Condensation (esterification) joins two molecules with loss of H₂O.
Oxidation & Reduction
Oxidation: add O or remove H. Reduction: add H or remove O. Key oxidising agents: K₂Cr₂O₇/H⁺, KMnO₄/H⁺; reducing agent: NaBH₄, LiAlH₄.
Why this topic matters: Every subsequent organic topic (Topics 14–21) builds directly on these reaction types. If you can classify a reaction and name its reagents and conditions, you can answer most organic questions. Learn the six types and their conditions cold.
Definitions you must be able to reproduce exactly
Key Terms
Free radical
A species with an unpaired electron. Highly reactive.
e.g. Cl• · CH₃•
Nucleophile
A lone pair donor — attacks electron-deficient (δ+) centres.
e.g. OH⁻, NH₃, CN⁻, H₂O
Electrophile
A lone pair acceptor — attacks electron-rich centres.
e.g. HBr, Br₂, H⁺, NO₂⁺
Homolytic fission
Bonding electrons are equally shared when a covalent bond breaks, producing two free radicals (one electron each).
Cl–Cl →UV 2 Cl•
Heterolytic fission
Bonding electrons are unequally shared when a covalent bond breaks, producing a cation and an anion.
HBr → H⁺ + Br⁻
Functional group
An atom or group of atoms responsible for the characteristic reactions of a molecule.
e.g. –OH (alcohol), C=O (carbonyl), –COOH (carboxylic acid)
The six reaction types — one-line definitions
Substitution An atom or group in an organic molecule is replaced by another atom or group.
Addition One molecule is added to another by breaking a double bond — the product contains all atoms of both reactants.
Elimination A small molecule is removed from an organic compound, resulting in formation of a double bond.
Condensation Two molecules join together with the elimination of a small molecule (usually H₂O).
Hydrolysis A compound is split into two molecules by reaction with water (or aqueous acid/alkali).
Oxidation Addition of oxygen or removal of hydrogen from an organic compound.
Reduction Addition of hydrogen or removal of oxygen from an organic compound.
All reactions from your notes · reagents shown in red
Reaction Types
Substitution chlorination · bromination · nucleophilic substitution
An atom or group of atoms in an organic molecule is replaced by another atom or group.
Free radical substitution — Alkane + halogen
Chlorination of methane (UV light required)
CH₄ + Cl₂ UV light→ CH₃Cl + HCl
Mechanism: Initiation (Cl–Cl →UV 2 Cl•) → Propagation → Termination. Only homolytic fission produces radicals.
Nucleophilic substitution — Halogenoalkane
With OH⁻ (aqueous) → alcohol
CH₃CH₂Br + OH⁻(aq) reflux→ CH₃CH₂OH + Br⁻
With NH₃ (alcoholic) → amine
CH₃CH₂Br + NH₃(alc) heat under pressure→ CH₃CH₂NH₂ + HBr
With CN⁻ (alcoholic) → nitrile (chain lengthened by 1C)
CH₃CH₂Br + CN⁻(alc) reflux→ CH₃CH₂CN + Br⁻
Nucleophilic substitution — Alcohol
Alcohol + HCl → halogenoalkane
CH₃CH₂OH + HCl heat→ CH₃CH₂Cl + H₂O
Addition electrophilic · nucleophilic · polymerisation
One molecule is added to another by breaking a double bond. The product contains all atoms of both reactants.
Electrophilic addition — Alkene
Bromination
H₂C=CH₂ + Br₂ → BrCH₂CH₂Br (1,2-dibromoethane)
H₂C=CH₂ + HBr → CH₃CH₂Br
Hydrogenation
H₂C=CH₂ + H₂ Ni catalyst→ CH₃CH₃
Hydration (produces ethanol)
H₂C=CH₂ + H₂O(g) conc. H₃PO₄, 300°C→ CH₃CH₂OH
Nucleophilic addition — Carbonyl compound
Ketone/aldehyde + HCN → hydroxynitrile
CH₃COCH₃ + HCN KCN catalyst→ (CH₃)₂C(OH)CN
The CN⁻ acts as the nucleophile, attacking the δ+ carbon of the carbonyl group. HCN alone is too weak an acid — KCN provides CN⁻.
Addition polymerisation
Many monomers join via repeated addition across double bonds
n CH₂=CHCH₃ → –(CH₂–CH(CH₃))ₙ– (polypropene)
Elimination dehydrohalogenation · dehydration
A small molecule is eliminated from an organic compound, resulting in the formation of a double bond.
Halogenoalkane → alkene (dehydrohalogenation)
CH₃CHBrCH₃ OH⁻(alc), reflux→ CH₃CH=CH₂ + HBr
When more than one alkene product is possible, a mixture is formed — the more substituted alkene (Zaitsev's product) is major.
Alcohol → alkene (dehydration)
CH₃CH(OH)CH₃ conc. H₂SO₄, heat→ CH₃CH=CH₂ + H₂O
Condensation esterification
Two molecules join together with the elimination of a small molecule (H₂O in esterification).
Alcohol + carboxylic acid → ester + water
CH₃CH₂OH + HCOOH conc. H₂SO₄, heat⇌ HCOOCH₂CH₃ + H₂O
The reaction is reversible (⇌). Yield is improved by removing water or using excess of one reactant.
Hydrolysis ester · nitrile · halogenoalkane
A compound is split into two molecules by reaction with water (often with acid or alkali catalyst).
Ester hydrolysis
Acid hydrolysis (reversible)
HCOOCH₂CH₃ + H₂O H₂SO₄(aq), heat⇌ CH₃CH₂OH + HCOOH
Base hydrolysis / saponification (irreversible — gives carboxylate salt)
HCOOCH₂CH₃ + OH⁻ NaOH(aq), heat→ CH₃CH₂OH + HCOO⁻
Nitrile hydrolysis
Acid hydrolysis → carboxylic acid + NH₄⁺
CH₃CN + 2H₂O H⁺, heat→ CH₃COOH + NH₄⁺
Base hydrolysis → carboxylate salt + NH₃
CH₃CN + H₂O + OH⁻ OH⁻, heat→ CH₃COO⁻ + NH₃
Halogenoalkane hydrolysis
CH₃CH₂Br + OH⁻(aq) reflux→ CH₃CH₂OH + Br⁻
Note: this is the same reaction as nucleophilic substitution with OH⁻ — it is also classified as hydrolysis when water/OH⁻ is the reagent breaking the C–X bond.
Oxidation dehydrogenation · [O] reagents
Addition of oxygen or removal of hydrogen from an organic compound. Written as [O] over the arrow.
Oxidation of alkene → aldehyde/ketone + CO₂
Terminal alkene (propene) → ethanal + methanal
H₃C–CH=CH₂ [O] KMnO₄/H⁺→ CH₃CHO + HCHO
Only KMnO₄/H⁺ can oxidise alkenes. K₂Cr₂O₇/H⁺ cannot.
Oxidation of alcohols
Primary alcohol → aldehyde (distil to prevent further oxidation)
CH₃CH₂OH K₂Cr₂O₇/H⁺, distil→ CH₃CHO + H₂O
Primary alcohol → carboxylic acid (reflux allows full oxidation)
CH₃CH₂OH K₂Cr₂O₇/H⁺, reflux→ CH₃COOH + H₂O
Secondary alcohol → ketone
CH₃CH(OH)CH₃ K₂Cr₂O₇/H⁺, reflux→ CH₃COCH₃ (propanone)
Aldehyde → carboxylic acid
CH₃CHO [O] reflux→ CH₃COOH
K₂Cr₂O₇/H⁺, KMnO₄/H⁺, Tollens' reagent and Fehling's solution can all oxidise aldehydes. Ketones cannot be oxidised further under normal conditions.
Reduction hydrogenation · [H] reagents
Addition of hydrogen or removal of oxygen from an organic compound.
Reduction of alkene → alkane
H₂C=CH₂ + H₂ Ni catalyst, heat→ CH₃CH₃
Only H₂/Ni can reduce an alkene. NaBH₄ and LiAlH₄ cannot.
Reduction of carbonyl compounds
Aldehyde → primary alcohol
CH₃CHO NaBH₄→ CH₃CH₂OH
Ketone → secondary alcohol
CH₃COCH₃ NaBH₄→ CH₃CH(OH)CH₃
LiAlH₄, H₂/Ni and NaBH₄ can all reduce ketones and aldehydes.
Carboxylic acid → primary alcohol
CH₃COOH LiAlH₄→ CH₃CH₂OH
Only LiAlH₄ can reduce a carboxylic acid. NaBH₄ is not strong enough.
Quick-reference table — reagent, conditions, and what it does
Reagent Summary
Oxidising agents
| Reagent | Can oxidise | Cannot oxidise | Key condition |
|---|---|---|---|
| K₂Cr₂O₇/H⁺ | Primary alcohols → aldehyde or acid; secondary alcohols → ketone; aldehydes → acid | Alkenes; ketones | Distil (aldehyde); reflux (acid) |
| KMnO₄/H⁺ | Alkenes → carbonyl + CO₂; primary alcohols; aldehydes | Ketones | Acidic conditions |
| Tollens' reagent | Aldehydes → acid (silver mirror) | Ketones | Warm gently |
| Fehling's solution | Aldehydes → acid (brick-red ppt) | Ketones; aromatic aldehydes | Warm gently |
Reducing agents
| Reagent | Can reduce | Cannot reduce | Notes |
|---|---|---|---|
| H₂/Ni | Alkenes → alkanes; aldehydes/ketones → alcohols | Carboxylic acids | Needs Ni catalyst and heat/pressure |
| NaBH₄ | Aldehydes → primary alcohols; ketones → secondary alcohols | Alkenes; carboxylic acids | Mild reducing agent; aqueous/alcoholic solvent |
| LiAlH₄ | Aldehydes; ketones; carboxylic acids → primary alcohols; esters; nitriles | Alkenes | Strong; use dry ether only — reacts violently with water |
Nucleophilic substitution reagents
| Nucleophile | Conditions | Product from halogenoalkane |
|---|---|---|
| OH⁻(aq) | Reflux | Alcohol |
| NH₃(alc) | Heat under pressure (sealed tube) | Amine (+HBr) |
| CN⁻(alc) | Reflux | Nitrile — chain extended by 1 carbon |
Memory tip for reducing agents: "NaBH₄ for carbonyls, LiAlH₄ for acids and everything." LiAlH₄ is the stronger reagent and the only one that can reduce C=O in a carboxylic acid. Use in dry ether — it reacts explosively with water.
Common mistakes · mark scheme language · Paper 1 + 2 strategy
Exam Technique
DO — Mark scheme habits
- Classify the reaction type first before writing conditions — it saves time and prevents errors
- Always include state symbols for reagents when given (aq), (alc), (g)
- For oxidation, specify the oxidising agent: [O] alone scores 0 in Paper 2 — write K₂Cr₂O₇/H⁺ or KMnO₄/H⁺
- For elimination vs substitution: OH⁻(aq) = substitution; OH⁻(alc) = elimination — the solvent determines the product
- State "distil" vs "reflux" for alcohol oxidation — these are separate mark points
DON'T — Common errors
- Don't use NaBH₄ to reduce carboxylic acids — only LiAlH₄ works
- Don't use K₂Cr₂O₇ to oxidise alkenes — only KMnO₄/H⁺ works
- Don't forget UV light is required for free radical substitution — without it the reaction won't start
- Don't confuse esterification (condensation, reversible ⇌) with hydrolysis — they are reverse reactions
- Don't draw base hydrolysis of ester as reversible — it is irreversible because it produces the carboxylate anion (RCOO⁻)
The OH⁻ solvent rule — exam favourite
OH⁻ (aqueous) + halogenoalkane
→ Substitution → alcohol
Water stabilises the transition state, favours SN2 pathway.
Water stabilises the transition state, favours SN2 pathway.
OH⁻ (alcoholic) + halogenoalkane
→ Elimination → alkene
Alcoholic solvent allows OH⁻ to act as a base rather than nucleophile.
Alcoholic solvent allows OH⁻ to act as a base rather than nucleophile.
Syllabus checklist — key outcomes
- Define free radical, nucleophile and electrophile with examples
- Distinguish homolytic and heterolytic fission; state products of each
- Define the six reaction types: substitution, addition, elimination, condensation, hydrolysis, oxidation, reduction
- Write equations for free radical substitution of alkanes (initiation, propagation, termination steps)
- Write equations for nucleophilic substitution of halogenoalkanes with OH⁻, NH₃ and CN⁻
- Write equations for electrophilic addition to alkenes (Br₂, HBr, H₂/Ni, H₂O/H₃PO₄)
- Write equation for nucleophilic addition of HCN to carbonyl compounds
- Distinguish conditions for elimination (OH⁻/alc) vs substitution (OH⁻/aq)
- Write equations for oxidation of primary and secondary alcohols, stating correct reagent and conditions
- State which reducing agent is required for each substrate (alkene → H₂/Ni; carbonyl → NaBH₄; acid → LiAlH₄)
- Write equations for acid and base hydrolysis of esters and nitriles