Alkenes

1. Bonding in alkenes and definition as unsaturated hydrocarbons

Structure of alkenes:
- Alkenes are hydrocarbons, meaning they contain only hydrogen (H) and carbon (C) atoms.
- They are part of a homologous series with the general formula CₙH₂ₙ.
- Each alkene molecule contains at least one carbon–carbon double covalent bond (C=C).
Nature of bonding:
- The C=C bond consists of:
  - One sigma (σ) bond – formed by head-on overlap of atomic orbitals.
  - One pi (π) bond – formed by sideways overlap of p orbitals.
- The presence of the π bond makes alkenes more reactive than alkanes.
Saturated vs. unsaturated hydrocarbons:
- Saturated: all carbon–carbon bonds are single bonds (e.g., alkanes).
- Unsaturated: at least one C=C double bond (e.g., alkenes).
- The unsaturation allows addition reactions to occur.

Cracking: the process of breaking large hydrocarbon molecules into smaller, more useful molecules.
Why cracking is necessary:
- Large alkanes from petroleum have low economic value and are not in high demand.
- Smaller alkanes and alkenes are more useful as fuels and feedstock for making plastics.
- Cracking increases the supply of petrol and also produces alkenes for chemical industries.
Conditions for cracking:
- High temperature (typically 600–900 °C).
- Catalyst: commonly aluminium oxide (Al₂O₃) or silica (SiO₂) for catalytic cracking.
Products of cracking:
- Smaller alkanes (used as fuels).
- Alkenes (used to make polymers, e.g., polyethene).
- Hydrogen gas (used in manufacturing ammonia, hydrogenation of oils).
Example reaction:
- C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene)

Economic reasons:
- High demand for petrol (C₅–C₉ range alkanes) and alkenes for plastics.
- Lower demand for heavy fuel oils (C₁₅+ range).
Practical reasons:
- Produces alkenes needed for manufacturing a wide range of products such as alcohols, detergents, and synthetic materials.
- Produces hydrogen gas for industrial uses.

Test: Reaction with aqueous bromine (bromine water, orange-brown in colour).
Procedure:
- Add bromine water to the hydrocarbon.
- Shake the mixture.
Observation:
- If unsaturated (alkene): the orange-brown colour decolourises rapidly at room temperature → indicates addition reaction with the C=C bond.
- If saturated (alkane): no change in colour under the same conditions.
Equation example:
- C₂H₄ + Br₂ → C₂H₄Br₂ (ethene + bromine → 1,2-dibromoethane)

Addition reaction: a reaction in which two reactants combine to form a single product.
In alkenes, the π bond in the C=C is broken, and atoms are added to each carbon of the double bond.
No other products are formed apart from the single organic product.

Reaction:
- With bromine (Br₂): rapid reaction at room temperature, forming a dibromoalkane.
- With aqueous bromine: also forms a dibromo product but in the presence of water, halohydrins can form in some cases.
Equation:
- CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br (ethene + bromine → 1,2-dibromoethane)
Observation:
- Orange-brown bromine decolourises → test for C=C bond.

Reaction: Hydration of alkenes to produce alcohols.
Conditions:
- Catalyst: concentrated phosphoric acid (H₃PO₄).
- High temperature (~300 °C) and high pressure (~60 atm).
Equation:
- CH₂=CH₂ + H₂O → CH₃–CH₂OH (ethene + steam → ethanol)
Industrial use:
- Production of ethanol without fermentation.