• Some organic compounds contain only C and H as the constituent elements. They are known as hydrocarbons.

• On the basis of the structure, hydrocarbons are divided into two main groups called aliphatic and aromatic.

• The set of hydrocarbons consisting of open carbon chains only are named as acyclic aliphatic hydrocarbons.

• The aliphatic hydrocarbons are classified as alkanes, alkenes, and alkynes.

• The cyclic organic compounds which are stabilized by forming a cyclic delocalized cloud of π electrons are called aromatic compounds.

• Benzene which is indicated by the molecular formula C6H6 is the simplest of aromatic hydrocarbon compounds.

• Compounds formed by replacing a hydrogen atom of an aliphatic hydrocarbon by a halogen atom are referred to as alkyl halides.

• Compounds formed by replacing a hydrogen atom of a benzene ring by a halogen atom are referred to as aryl halides.

• In many organic compounds, when hetero atoms such as nitrogen and oxygen combine with the carbon chain, due to the difference in electro negativity between the carbon and the combined atoms, this group of atoms will impart to the compound a characteristic reactivity. Such a group of atoms is called a functional group. The compounds are classified according to the functional group present in a molecule.

Common functional groups and the names of the corresponding homologous series are given below.

* In the IUPAC nomenclature, halogen is not considered a functional group