Addition Reactions (Alkenes only)
Addition reactions occur across the C=C double bond. The double bond breaks and atoms add to both carbons. Alkenes are unsaturated — they can accommodate additional atoms by converting C=C to C–C.
| Reagent | Reaction type | Conditions | Products |
|---|
| H2 | Hydrogenation | Ni/Pt catalyst, heat | Alkane (saturated) |
| HX (HBr, HCl) | Hydrohalogenation | Room temp, no catalyst | Halogenoalkane |
| H2O | Hydration | H3PO4/H2SO4 catalyst, heat | Alcohol |
| X2 (Br2, Cl2) | Halogenation | Room temp (test: bromine water decolourises) | Dihalogenoalkane |
Markovnikov’s Rule: When HX adds to an unsymmetrical alkene, the H atom adds to the carbon with more hydrogen atoms already attached (the less substituted carbon), and X adds to the carbon with fewer H atoms (the more substituted carbon). “The rich get richer.”
CH2=CH2 + Br2 → CH2BrCH2Br (1,2-dibromoethane)
CH2=CHCH3 + HBr → CH3CHBrCH3 (Markovnikov — Br on C2)
Elimination Reactions
Elimination is the reverse of addition. Atoms are removed to form (or extend) a double bond, producing an unsaturated compound. Two main types:
- Dehydration of alcohols: Alcohol + concentrated H2SO4, heat → Alkene + H2O. An OH and an adjacent H are eliminated. (If the double bond position is ambiguous, Zaitsev’s rule gives the more substituted alkene.)
- Dehydrohalogenation: Halogenoalkane + KOH dissolved in ethanol, heat → Alkene + KX + H2O. An H and a halogen on adjacent carbons are eliminated.
CH3CH2OH ⟶[conc H2SO4, 180°C] CH2=CH2 + H2O (dehydration)
CH3CHBrCH3 + KOH(ethanol) ⟶[heat] CH3CH=CH2 + KBr + H2O (dehydrohalogenation)
Substitution Reactions
In substitution, one atom or group is replaced by another without changing the carbon skeleton’s connectivity.
- Free-radical halogenation of alkanes: CH4 + Cl2 ⟶[UV light] CH3Cl + HCl. UV light initiates the reaction by homolytically splitting Cl2 into Cl• radicals. Alkanes are unreactive under most conditions — UV light provides the activation energy.
- Nucleophilic substitution of halogenoalkanes: R–X + NaOH(aq) ⟶[heat] R–OH + NaX. The OH− nucleophile attacks and replaces the halogen.
CH3Br + NaOH(aq) → CH3OH + NaBr (nucleophilic substitution)
Esterification
An ester is formed when a carboxylic acid reacts with an alcohol in the presence of an acid catalyst. The reaction is reversible:
Carboxylic acid + Alcohol ⇌ Ester + Water (H2SO4 catalyst, reflux)
CH3COOH + CH3CH2OH ⇌ CH3COOCH2CH3 + H2O
(ethanoic acid + ethanol ⇌ ethyl ethanoate + water)
Esters have fruity smells and are used in flavourings, perfumes, and solvents. Naming: the alcohol part names the alkyl group (ethyl), and the acid part becomes -oate (ethanoate).
Hydrolysis of Esters
Hydrolysis is the reverse of esterification — water breaks the ester bond. Two types:
- Acid hydrolysis: Ester + H2O + dilute H2SO4 → Carboxylic acid + Alcohol (slow, reversible).
- Base hydrolysis (Saponification): Ester + NaOH(aq) → Sodium carboxylate salt + Alcohol. This is the basis of soap making — fats (triglycerides) are saponified to give fatty acid salts (soaps) and glycerol.
CH3COOCH2CH3 + NaOH(aq) → CH3COO−Na+ + CH3CH2OH (saponification)
Cracking
Large alkane molecules from crude oil are broken into smaller, more useful alkenes and alkanes by supplying energy:
- Thermal cracking: Very high temperature (500–900°C), high pressure, no catalyst. Gives a mixture of products including alkenes.
- Catalytic cracking: Moderate temperature (~500°C), zeolite catalyst, lower pressure. More controlled; used in refineries to produce petrol-range hydrocarbons and feedstocks.
C10H22 → C5H12 + C5H10 (one possible cracking product mix)
Combustion
- Complete combustion (excess O2): Hydrocarbon + O2 → CO2 + H2O. Clean burning; releases maximum energy.
- Incomplete combustion (limited O2): Produces CO (toxic) and/or C (soot) in addition to H2O. CO is dangerous because it binds to haemoglobin; soot causes air pollution.
C3H8 + 5O2 → 3CO2 + 4H2O (complete)
2C3H8 + 7O2 → 6CO + 8H2O (incomplete)
Polymerisation
- Addition polymerisation: Many alkene monomers join together — the double bond opens and the monomers link into a long chain. No by-product is produced. Example: n CH2=CH2 → (–CH2–CH2–)n (polyethene/polyethylene).
- Condensation polymerisation: Monomers with two functional groups react repeatedly, releasing a small molecule (water or HCl) at each step. Examples: nylon (polyamide from diamine + dicarboxylic acid), polyester (from diol + dicarboxylic acid), like PET.
Summary Table of Organic Reactions
| Reaction type | Reactant | Key conditions | Product |
|---|
| Addition (hydrogenation) | Alkene + H2 | Ni catalyst, heat | Alkane |
| Addition (hydration) | Alkene + H2O | H3PO4, heat | Alcohol |
| Addition (halogenation) | Alkene + Br2 | Room temp | Dihalogenoalkane |
| Elimination (dehydration) | Alcohol | Conc H2SO4, heat | Alkene + H2O |
| Substitution (free radical) | Alkane + X2 | UV light | Halogenoalkane + HX |
| Substitution (nucleophilic) | Halogenoalkane + NaOH(aq) | Heat | Alcohol + NaX |
| Esterification | Carboxylic acid + Alcohol | H2SO4, reflux | Ester + H2O |
| Hydrolysis (base) | Ester + NaOH(aq) | Heat | Salt + Alcohol |
| Combustion (complete) | Hydrocarbon + excess O2 | Ignition | CO2 + H2O |
| Cracking | Long-chain alkane | High T or catalyst | Short alkane + alkene |
⭐ IEB Extension — Reaction Mechanisms & Regioselectivity
SN1 vs SN2 (Nucleophilic Substitution):
- SN2 (bimolecular): One-step; nucleophile attacks as leaving group departs simultaneously. Favoured by primary (1°) halogenoalkanes and strong nucleophiles. Causes inversion of configuration (Walden inversion).
- SN1 (unimolecular): Two-step; first forms a carbocation intermediate, then nucleophile attacks. Favoured by tertiary (3°) halogenoalkanes in polar protic solvents. Gives racemisation at chiral centres.
E1 vs E2 (Elimination):
- E2 (bimolecular): Concerted; base removes H while halogen departs simultaneously. Requires anti-periplanar geometry. Favoured by strong bases (KOH in ethanol), primary and secondary substrates.
- E1 (unimolecular): Two-step via carbocation; favoured by tertiary substrates, weak bases, polar protic solvents.
Regioselectivity: When multiple products are possible (e.g., different alkene positions in elimination), the major product follows Zaitsev’s rule — the more substituted (more stable) alkene is the major product.