4.1 IntroductionEdexcel IGCSE Chemistry: Revision notes

A hydrocarbon is a compound that contains hydrogen and carbon atoms only. Carbon can form four strong covalent bonds, including bonds to other carbon atoms, so it can build long chains, branched chains and rings. This is why there are millions of different organic compounds, even though they are built from just a few elements (mainly carbon, hydrogen, oxygen, and sometimes halogens or nitrogen).

Almost all of organic chemistry is a hydrocarbon 'backbone' with a small functional group attached that gives the molecule its characteristic chemical behaviour.

Chemists use several types of formula to describe the same molecule, each showing a different amount of detail. Using butane (C4H10) as an example:

Displayed and structural formulae are the most useful for showing isomerism, because two molecules can share a molecular formula but have different structures.

A homologous series is a family of compounds that:

• have the same general formula
• differ by a CH2 unit from one member to the next
• have the same functional group and so show similar chemical properties
• show a gradual change in physical properties (e.g. boiling point) as the chain gets longer

Compounds in the same homologous series are represented by the same general formula, for example alkanes are CnH2n+2.

Isomerism occurs when two or more compounds share the same molecular formula but have different structural formulae (different arrangements of atoms). These are called isomers. For example, C4H10 can be drawn as an unbranched chain (butane) or a branched chain (methylpropane) — same molecular formula, different structures and slightly different physical properties.

IUPAC nomenclature gives every organic compound a systematic name built from two parts:

1. A stem showing the number of carbon atoms in the longest chain
2. A suffix or prefix showing the functional group present

Stems for chains up to six carbons:

For example, an alkane with four carbons is 'butane'; an alcohol with two carbons is 'ethanol'. Branches are named as substituents (e.g. 'methyl') with a number showing which carbon they are attached to.

Reactions of organic compounds fall into three broad types:

• Substitution: an atom or group is replaced by another atom or group (e.g. an alkane reacting with a halogen, where a hydrogen is replaced by a halogen atom)
• Addition: two molecules join together to form one product, with no atoms lost (e.g. an alkene reacting with bromine)
• Combustion: a compound reacts with oxygen, releasing heat energy