Alkenes | Organic Chemistry Topic 7.3 - 7.4

Alkenes

• Cracking
• Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient: they do not flow easily and are difficult to ignite. Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules to meet demand. This is where cracking comes in.
• Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are heated to vaporise them. They are then either:
• passed over a hot catalyst, or
• mixed with steam and heated to a very high temperature.
• These processes break chemical bonds in the molecules, causing thermal decomposition reactions. Cracking produces smaller alkanes and alkenes (another type of hydrocarbon).
• Alkenes
• The products of cracking include alkenes (for example ethene and propene). The alkenes are a family of hydrocarbons that share the same general formula:
• CnH2n
• The general formula means that the number of hydrogen atoms in an alkene is double the number of carbon atoms. For example, ethene is C2H4 and propene is C3H6.
• Alkene molecules can be represented by displayed formulas in which each atom is shown as its symbol (C or H) and the chemical bonds between them by a straight line.

Testing Alkenes

• Alkenes are unsaturated hydrocarbons. They contain a double covalent bond, which is shown as two lines between two of the carbon atoms. The presence of this double bond allows alkenes to react in ways that alkanes cannot. They can react with oxygen in the air, so they could be used as fuels. But they are more useful than that: they can be used to make ethanol and polymers (plastics) - two crucial products in today's world.
• Testing for unsaturation
• Bromine water is a dilute solution of bromine, normally orange-brown in colour. It becomes colourless when shaken with an alkene, but its colour remains the same when it is shaken with alkanes.

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