Explain the term – 'Nomenclature'. State it's need with reference to organic compounds. State the basic rules of Nomenclature by the trivial system with suitable examples. Explain the longest chain rule and the smallest number for functional groups rule of Nomenclature by the IUPAC system with suitable examples.

Nomenclature is the system of assignment of names to organic compounds.

Need for Nomenclature — Large number of organic compounds due to:

1. Varying molecular structures of organic compounds.
2. Isomerism in organic compounds

increases need for correct, methodical and systematic naming of each compound.

Basic rules of Nomenclature by the trivial system:

The basis of naming organic compounds by the trivial system is its :

1. Source
2. Properties
3. Latin or greek origin of compounds

For example,
Methane (CH₄) was named marsh gas since it was obtained from marshy places.
The name Acetic acid is derived from it's source 'vinegar' [Latin : acetum]

Longest Chain Rule — The longest continuous chain of 'C' atoms, known as parent chain is selected. The longest chain need not be straight.
For example,

(i) The longest chain is of 5 carbon atoms, so root word is 'pent'

(ii) Longest chain is of 7 carbon atoms, so the root word is 'hept'. The reamining carbon atom (unnumbered) is considered a branch.

Smallest number for functional groups rule — In case, any functional group is also present in the chain, then the carbon atom are numbered in such a way that the functional group gets the smallest possible number.
For example,