Nomenclature of organic compounds:

   There are two systems of naming organic compounds.

• Common system or Trivial Names:

In this system names are derived from source or their property or their structure etc.

e.g Urea from urine (source), Citric acid from citrus plant (source).Glucose from the greek word meaning sweet (property, Valyric acid from Greek word valyr meaning powerful (property).

The name pentane is from Greek word penta meaning five. The common system failed to name every compound as it is not systematic.

• IUPAC System:

To develop a systematic system of naming compounds International Chemical congress met in Geneva in 1892 and formulated a system of naming called Geneva System.

Geneva System was revised by International Union of Chemistry at Liege in 1930 and is known as IUC System.

IUC later change its name to IUPAC i.e. International Union of Pure and Applied Chemistry and modified the rules in 1957 and revised in 1967. This system now called IUPAC System.

Rules of IUPAC System:

IUPAC System involves following steps:

                Step-I    Chain selection:

• Select the longest possible continuous carbon chain containing the carbon of functional group and if functional group has no carbon in it then the carbon to which functional group is attached should be included in the selected chain.
• If more than one longest chains of equal number of carbons are there then the one which has more branches or substituents or side chains, is selected.

                Step-II   Numbering:

• Carbon atoms of the selected chain (parent chain) are numbered from one end to other end such that the locant or sum of locants of the functional group(s) should be smaller.
• If the locant or sum of locants of the functional group(s) is same for both ends then numbering is carried out from that end which gives smallest locant to the first substituent.
• If the locant of first substituent is same for both ends then numbering is carried out from that end which gives smaller sum of locants of all the substituents.
• If sum of locants of all substituents is also same for both ends then numbering is carried out from that end which gives smaller locant to more electronegative group(s) like halogen, NH₂ etc.
• If locant of more electronegative group is also same for both ends then last preference is given to larger alkyl group.

                Step-III Naming:

• Names of substituents are written before parent name and names of substituents are preceded by their locants. At end parent name is written according to carbons in the selected chain and the parent name is preceded by locant(s) of functional group(s).
• If same substituent is occurring more than one time then prefixes di, tri, tetra, penta etc are used with its name and locant of each is written each time.
• If substituents of more than one kind are there then their names are written in alphabetical order.

Two numbers are separated by comma between them; a number and a name are separated by a hyphen between them while two names are separated by a space between them. 

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