Alkenes

Alkenes are hydrocarbons that contain at least one carbon–carbon double covalent bond (C=C). They are called unsaturated because more atoms can be added to the double bond. The simplest alkene is ethene (C2H4). For alkenes with one C=C and no rings, the general formula is CnH2n.

Key ideas

• Bonding: a double C=C bond. It is shorter and stronger than a single bond and makes alkenes more reactive.
• Naming: names end in “-ene” (ethene, propene). The number shows the C=C position: but-1-ene (CH2=CH–CH2–CH3), but-2-ene (CH3–CH=CH–CH3).
• Compared with alkanes: alkanes have only single bonds (saturated) and are less reactive.

Making alkenes: cracking

Large alkane molecules are broken into smaller, more useful molecules by cracking at high temperature with a catalyst (e.g., silica or alumina). Cracking produces alkenes and hydrogen. Reasons: to meet demand for fuels that burn well and to make alkenes used for plastics and chemicals.

Addition reactions (one product formed)

• With bromine (test for C=C): orange bromine water turns colourless quickly with an alkene. Example: CH2=CH2 + Br2 → CH2Br–CH2Br (1,2-dibromoethane).
• With hydrogen (Ni catalyst): an alkene becomes an alkane. Example: CH2=CH2 + H2 → CH3–CH3.
• With steam (acid catalyst): forms an alcohol. Example: CH2=CH2 + H2O → CH3–CH2OH (ethanol).

Addition polymerisation

Many small alkene molecules (monomers) join to make a long-chain polymer. Ethene forms poly(ethene), used in plastic bags.

Common misconceptions

• Bromine water does not need UV light to react with alkenes (it does for alkanes).
• Addition reactions make one product, not many.
• Cracking is not combustion; it is a chemical breakdown using heat and a catalyst.