Organic Chemistry – Alkenes (Cambridge IGCSE 0620)
1. Definition & General Formula
- Alkenes are unsaturated hydrocarbons that contain at least one carbon‑carbon double bond (C=C).
- For acyclic alkenes the molecular formula is CnH2n.
- The double bond consists of:
- one σ‑bond (head‑to‑head overlap, same as in alkanes)
- one π‑bond (side‑by‑side overlap of p‑orbitals). The π‑bond is a region of high electron density and is the reactive site.
- Visual aid: a simple diagram showing the σ‑bond as a single line and the π‑bond as a shaded “cloud” above and below the bond helps to interpret structures (AO2).
2. Manufacture of Alkenes – Cracking
Industrial alkenes are obtained by breaking larger alkanes into smaller fragments (cracking). Two main types are used:
| Type of Cracking |
Typical Conditions |
Key Features |
| Thermal cracking |
~900 °C, no catalyst required |
High temperature alone supplies enough energy to break C–C σ‑bonds; product mixture is broad. |
| Catalytic cracking |
400–600 °C, solid acid catalyst (Al₂O₃, SiO₂, zeolites) |
Catalyst lowers the activation energy, gives higher selectivity for useful alkenes; the process used in modern refineries. |
Typical example (thermal cracking):
$$\mathrm{C_8H_{18}\;(octane)\;\xrightarrow{900^\circ\!C,\;Al_2O_3}\; C_4H_8\;(butene) + C_4H_{10}\;(butane)}$$
- High temperature provides the energy to break strong C–C σ‑bonds.
- The catalyst (when used) speeds the reaction and steers it toward desired products.
- Cracking is favoured because it increases the number of molecules → higher entropy.
- After cracking the mixture must be separated (usually by fractional distillation) – a link to the “separation & purification” topic (Section 12.4).
3. Test for Unsaturation – Aqueous Bromine Test
- Colour of bromine water: orange‑brown (due to dissolved Br₂).
- When an alkene is present the double bond adds Br₂, consuming the bromine and de‑colourising the solution.
- Reaction (example with ethene):
$$\mathrm{CH_2=CH_2 + Br_2 \;\longrightarrow\; CH_2Br–CH_2Br}$$
- Observation: orange‑brown → colourless = positive test for a C=C bond (AO2).
- Safety tip: Bromine is corrosive and toxic; handle in a fume‑hood, wear gloves and eye protection.
4. Addition Reactions of Alkenes
In an addition reaction the π‑bond is broken and two new σ‑bonds are formed. For the IGCSE syllabus the essential point is that **only one product is formed** when a simple addition reagent adds across a double bond under the standard conditions.
4.1 Why Only One Product?
- The double bond is planar; the two carbon atoms are equivalent in the transition state.
- The reagent adds **simultaneously** to both carbons, so there is no opportunity for the formation of regio‑isomers or stereoisomers under the normal (non‑catalysed) conditions.
- For symmetrical reagents (H₂, Br₂, H₂O) and simple alkenes, no rearrangement occurs, giving a single, well‑defined product.
- When the reagent is unsymmetrical (e.g., HBr, HCl) the addition follows **Markovnikov’s rule** – the more electronegative group (X) attaches to the more substituted carbon, still yielding a single major product.
4.2 Typical Addition Reagents
| Reagent |
Conditions |
Product (example with ethene) |
Type of Addition |
Key Notes |
| H₂ / Pt (or Pd, Ni) |
300–400 °C, solid metal catalyst |
Ethane, $\mathrm{C_2H_6}$ |
Hydrogenation (syn‑addition) |
Both H atoms add to opposite faces → single saturated alkane. |
| Br₂ (or Cl₂) |
Room temperature, no catalyst |
1,2‑Dibromoethane, $\mathrm{C_2H_4Br_2}$ |
Halogenation (anti‑addition) |
Halogen atoms add to opposite sides of the double bond. |
| H₂O (acid‑catalysed hydration) |
H₂SO₄, 60–80 °C |
Ethanol, $\mathrm{C_2H_5OH}$ |
Hydration (Markovnikov addition) |
OH attaches to the more substituted carbon. |
| HX (e.g., HBr, HCl) |
H₂SO₄ catalyst, 30–60 °C |
2‑Bromo‑propane (from propene) |
Hydrohalogenation (Markovnikov) |
H adds to the less substituted carbon, X to the more substituted carbon. |
4.3 Detailed Examples
Hydrogenation of Ethene
$$\mathrm{CH_2=CH_2 + H_2 \xrightarrow{Pt} CH_3-CH_3}$$
- Both hydrogen atoms add across the C=C bond at the same time (syn‑addition).
- Only one product – ethane – is obtained.
Halogenation of Propene
$$\mathrm{CH_3-CH=CH_2 + Br_2 \longrightarrow CH_3-CH(Br)-CH_2Br}$$
- Anti‑addition: the two bromine atoms add to opposite faces of the double bond.
- No positional isomer is formed under standard conditions; the product is 1,2‑dibromo‑propane.
Acid‑Catalysed Hydration of Propene (Markovnikov)
$$\mathrm{CH_3-CH=CH_2 + H_2O \xrightarrow{H_2SO_4} CH_3-CH(OH)-CH_3}$$
- The –OH group attaches to the more substituted carbon (the middle carbon), giving isopropanol.
- Only one constitutional isomer is produced because of Markovnikov’s rule.
Hydrohalogenation of Propene (HBr)
$$\mathrm{CH_3-CH=CH_2 + HBr \xrightarrow{H_2SO_4} CH_3-CH(Br)-CH_3}$$
- H adds to the terminal carbon, Br to the more substituted carbon – the Markovnikov product.
- Again, a single major product is obtained.
5. Summary
- Alkenes contain a planar C=C double bond (σ + π) and have the formula CnH2n (acyclic).
- Industrial production is by cracking of larger alkanes:
- Thermal cracking (high temperature only) and catalytic cracking (solid acid catalyst).
- Products are later separated by distillation.
- The aqueous bromine test detects unsaturation: orange‑brown bromine water becomes colourless when an alkene is present. Handle Br₂ with care.
- In addition reactions the π‑bond is broken and two new σ‑bonds form. Because the addition occurs simultaneously at the two carbon atoms, **only one product** is obtained for the simple reagents covered in the syllabus.
- Key addition reactions required for IGCSE 0620:
- Hydrogenation (H₂/Pt) → saturated alkane.
- Halogenation (Br₂ or Cl₂) → vicinal dihalide (anti‑addition).
- Acid‑catalysed hydration (H₂O/H₂SO₄) → alcohol (Markovnikov).
- Acid‑catalysed hydrohalogenation (HX/H₂SO₄) → alkyl halide (Markovnikov, also single product).