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