Organic chemistry is the study of carbon-based compounds. Carbon's unique ability to form four strong covalent bonds allows it to build an almost infinite variety of stable structures, ranging from simple fuels like methane to the complex macromolecules that constitute living organisms. The IGCSE 0654 Core syllabus provides an introduction to the fundamental "families" of organic compounds, the systematic naming of these molecules, and the industrial refining of petroleum.

The most basic organic compounds are hydrocarbons, which are defined as compounds that contain hydrogen and carbon only. These are categorized into homologous series (families) with similar structures.

Alkanes (Saturated)

Alkanes are the simplest hydrocarbons. They contain only single carbon-carbon bonds. Because every carbon atom is bonded to the maximum possible number of hydrogen atoms, they are described as saturated.

• Properties: Generally unreactive, except for combustion (burning) to release energy.
• Examples: Methane ($CH_4$) and Ethane ($C_2H_6$).

Alkenes (Unsaturated)

Alkenes contain at least one double carbon-carbon bond ($C=C$). Because they could potentially bond with more hydrogen atoms if the double bond were broken, they are described as unsaturated.

• Chemical Test: Alkenes react with aqueous bromine (bromine water). The orange-brown solution is decolorized (turns colorless) in the presence of an alkene.
• Example: Ethene ($C_2H_4$).

Organic molecules are named using a systematic prefix system that indicates the number of carbon atoms in the chain:

1. Meth-: 1 Carbon (e.g., Methane)
2. Eth-: 2 Carbons (e.g., Ethane, Ethene, Ethanol)
3. Prop-: 3 Carbons (e.g., Propane)
4. But-: 4 Carbons (e.g., Butane)

The suffix indicates the family: -ane for alkanes, -ene for alkenes, and -ol for alcohols.

Petroleum (crude oil) is a complex mixture of thousands of different hydrocarbons. To be useful, this mixture must be separated into groups of molecules with similar boiling points, known as fractions.

Fractional Distillation

This separation occurs in a tall fractionating column, which is hot at the bottom and cooler at the top.

• Process: Crude oil is heated and vaporized. As the vapors rise, they cool and condense at different heights based on their boiling points.
• Trends: As you move up the column:
• The carbon chain length decreases.
• The boiling point decreases.
• The viscosity (thickness) decreases (liquids become more runny).

Industrial Fractions and Uses

Organic chemistry is the engine of modern industry and the foundation of life. By understanding the structural differences between saturated alkanes and unsaturated alkenes, and by utilizing the physical properties of petroleum fractions, we can synthesize the fuels, plastics, and chemicals that sustain our global civilization.