Combustion of Organic Compounds
Complete Combustion
Excess O₂ → CO₂ + H₂O
C₈H₁₈ + 12½O₂ → 8CO₂ + 9H₂O
Highly exothermic. Basis for fuels
Incomplete Combustion
Limited O₂ → CO or C + H₂O
C₄H₁₀ + 4O₂ → 4CO + 5H₂O
CO is toxic (binds to haemoglobin); soot causes global dimming
Addition Reactions of Alkenes
The C=C double bond is a region of high electron density (π electrons) that is susceptible to attack by electrophiles.
| Reaction | Reagent | Conditions | Product |
|---|---|---|---|
| Hydrogenation | H₂ | Ni catalyst, 150°C | Alkane |
| Halogenation | Br₂ (or Cl₂) | Room temperature | Dihalogenoalkane |
| Hydration | H₂O (steam) | H₃PO₄ catalyst, 300°C | Alcohol |
| Hydrohalogenation | HBr (or HCl) | Room temperature | Halogenoalkane |
Bromine Water Test for Unsaturation
Add bromine water (orange/brown). Alkenes decolourise it (addition reaction across C=C). Alkanes do NOT react. No colour change.
🔬 HL. Markovnikov's Rule
When adding HX to an asymmetric alkene, the H atom adds to the C of the double bond with more H atoms already attached.
Why? The reaction proceeds via the most stable carbocation intermediate (3° > 2° > 1°). This determines the major product.
🔬 HL. Electrophilic Addition Mechanism
- Step 1: The π electrons from the C=C attack the δ⁺ end of the polarised electrophile → heterolytic fission of the electrophile bond → carbocation intermediate formed
- Step 2: The remaining anion (nucleophile) attacks the positively charged carbon → final addition product formed
Show curly arrows from C=C to electrophile and from anion to C⁺ in your mechanism.
Substitution Reactions
Substitution (SL)
One atom/group replaces another. Typical for alkanes and halogenoalkanes.
CH₄ + Cl₂ → CH₃Cl + HCl (UV light)
🔬 HL. Free-Radical Substitution Mechanism
Conditions: UV light or high heat (alkanes are generally unreactive due to strong, non-polar C−C and C−H bonds)
Initiation: UV light causes homolytic fission of the halogen molecule:
Cl₂ → 2Cl•
Propagation (two steps. Chain reaction):
Cl• + CH₄ → •CH₃ + HCl
•CH₃ + Cl₂ → CH₃Cl + Cl•
Termination: Two radicals combine:
Cl• + Cl• → Cl₂ or •CH₃ + •CH₃ → C₂H₆
⚠️ Produces a mixture of products (poly-substitution possible: CH₂Cl₂, CHCl₃, CCl₄)
🔬 HL. Nucleophilic Substitution (SN1 vs SN2)
A nucleophile (electron-pair donor) attacks an electrophilic carbon, displacing the leaving group.
| SN2 | SN1 | |
|---|---|---|
| Substrate | Primary halogenoalkanes | Tertiary halogenoalkanes |
| Steps | One step (concerted) | Two steps (carbocation intermediate) |
| Stereochemistry | Inversion of configuration | Racemic mixture |
Secondary halogenoalkanes can undergo both SN1 and SN2.
Oxidation of Alcohols
Oxidation with Acidified K₂Cr₂O₇
Colour change: orange → green when oxidation occurs.
- Primary alcohol → aldehyde (distil) → carboxylic acid (reflux)
- Secondary alcohol → ketone
- Tertiary alcohol → No oxidation (resistant. No colour change)
Condensation & Esterification
Esterification
Alcohol + carboxylic acid → ester + H₂O
Requires acid catalyst (e.g. Conc. H₂SO₄) and heat (reflux). This is a condensation reaction.
Hydrolysis of esters:
- Acid hydrolysis: ester + H₂O (with acid catalyst) → alcohol + carboxylic acid
- Base hydrolysis (saponification): ester + NaOH → alcohol + sodium carboxylate
🔬 HL. Elimination Reactions
Halogenoalkanes can undergo elimination (competing with substitution) to form alkenes.
- Favoured by heat and strong bases (e.g. Ethanolic NaOH)
- Tertiary substrates favour elimination over substitution
- E1 mechanism: two-step via carbocation; E2 mechanism: concerted one-step
⚠️ Common Exam Mistakes
- Forgetting to state conditions for each reaction (catalyst, temperature, UV light)
- Confusing addition (one product, unsaturated) with substitution (swap, saturated)
- Drawing curly arrows from the nucleophile to the electrophilic centre, not the other way around
- Stating that tertiary alcohols "can't be oxidised" without noting they resist oxidation
Study this topic on the go. Get flashcards and quizzes in ChemEasy, or plan your revision with ChemPlan IB. See our apps →