Chapter 11: Organic Chemistry — Part 5

Topic 11.5 · Alkenes

Bonding and Manufacture

Bonding: Alkenes contain at least one double carbon–carbon covalent bond and are unsaturated hydrocarbons.
Cracking: Alkenes are manufactured by the cracking of larger, less useful alkane molecules.
- Process: Long-chain hydrocarbons are heated and vaporised, then passed over a hot catalyst or mixed with high-temperature steam.
- Products: Produces alkenes, shorter-chain alkanes, and hydrogen.
- Reasons: Helps meet the high demand for short-chain fuels and provides alkenes as chemical feedstock.

Exam Traps

Do not confuse cracking with fractional distillation — cracking is a chemical reaction that breaks C–C bonds; distillation is physical separation.
Do not say cracking produces only alkenes — shorter alkanes and hydrogen are also formed.

In addition reactions, two reactants combine to form only one product; the double bond is broken and becomes a single bond.

With Bromine: Alkenes decolourise aqueous bromine from orange to colourless. This serves as the test to distinguish saturated alkanes from unsaturated alkenes.
With Hydrogen: In the presence of a nickel catalyst, alkenes react to form alkanes.
With Steam: In the presence of an acid catalyst (e.g., phosphoric acid), alkenes react to form alcohols.

Exam Traps

Do not say bromine decolourisation proves an alkane — only alkenes (and other unsaturated compounds) decolourise aqueous bromine without UV light.
Do not confuse nickel (H₂ addition) with phosphoric acid (steam addition) — mixing catalysts loses marks on conditions questions.
Do not call alkene + bromine a substitution reaction — bromine adds across the double bond; no atom is replaced.

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