Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond ($C=C$). While alkanes are generally unreactive, the double bond in alkenes is a region of high electron density that readily undergoes addition reactions. This chemical reactivity is the foundation of the plastics industry and the production of synthetic alcohols. The IGCSE 0654 Extended syllabus requires a detailed explanation of alkene manufacture via cracking, a descriptive analysis of three primary addition reactions, and a distinction between the two mechanisms of polymerization.

Alkenes are not found in significant quantities in crude oil. Instead, they are produced from long-chain alkanes through a process called catalytic cracking.

• Process: Large alkane molecules from the heavier petroleum fractions are heated and passed over a catalyst (e.g., silica or alumina).
• Result: The large molecules "crack" into smaller, more useful molecules:
• Smaller Alkanes: Used as high-demand fuels (like gasoline).
• Alkenes: Used as feedstocks for the chemical industry.
• Hydrogen: Often produced as a byproduct.
• Conditions: High temperature and the presence of a catalyst are essential to provide the activation energy needed to break the strong C-C single bonds.

In an addition reaction, the $C=C$ double bond "opens up" to become a single bond, allowing two new atoms to be added to the carbon atoms.

(a) Hydrogenation (Adding $H_2$)

• Reaction: Alkene + Hydrogen $\rightarrow$ Alkane
• Conditions: Nickel catalyst, temperature of approximately 60°C.
• Use: Converting liquid vegetable oils into solid margarine.

(b) Halogenation (Adding $Br_2$)

• Reaction: Alkene + Bromine $\rightarrow$ Dibromoalkane
• Test: This is the standard test for unsaturation. The orange-brown aqueous bromine is decolorized instantly as the bromine adds across the double bond.

(c) Hydration (Adding Steam, $H_2O$)

• Reaction: Alkene + Steam $\rightarrow$ Alcohol
• Conditions: High temperature (300°C), high pressure (60 atm), and a phosphoric acid catalyst.
• Use: The primary industrial method for the production of ethanol ($C_2H_5OH$) from ethene.

A polymer is a giant molecule (macromolecule) built from many small repeating units called monomers.

Addition Polymerization

Occurs when many alkene monomers join together by opening their double bonds.

• Product: Only the polymer is formed.
• Example: Poly(ethene) is formed from ethene monomers.

Condensation Polymerization

Occurs when monomers with two functional groups join together.

• Product: The polymer is formed plus a small molecule (usually water or $HCl$) is released (lost) for every link made.
• Nylon (A Polyamide): Formed when a dicarboxylic acid reacts with a diamine. The link between the monomers is an amide linkage ($-CO-NH-$). Nylon is used extensively in synthetic fibers and high-strength engineering plastics.

The double bond of alkenes is the "chemical switch" that allows for the creation of an immense variety of synthetic materials. By manipulating this bond through cracking and addition reactions, we can transform simple petroleum fractions into the fuels, alcohols, and polymers—including addition plastics and condensation nylons—that define the material reality of the 21st century.