What is meant by aliphetic hydrocarbons?

Aliphatic hydrocarbons

Aliphatic hydrocarbons do not contain delocalised electron systems (benzene rings, primarily). They can be sub-divided into straight chains, branched chains.

Straight chain hydrocarbons are actually a zig-zag shape, as the bond angle formed between carbon-carbon-carbon atoms is actually 109.5º. However, it is more convenient to represent the structure as though it were straight with right angles. The term ‘straight chain’ is used to differentiate the structures from branched chain molecules.

This is shown below using the straight chain molecule octane, C8H18.

Octane representations
3-dimensional representation
2-dimensional representation

Although the 2-dimensional (flat) structure is not strictly speaking correct, it contains all of the information that is needed.

Branched chains have carbon containing chains branching off the main chain, just like branches of the main trunk of a tree. Once again, in reality the carbon atoms are all arranged in a zig-zag fashion.

2-methyloctane showing the branched structure
3-dimensional representation
2-dimensional representation