Naming of Organic Compounds | Common vs IUPAC Nomenclature Notes

The naming of organic compounds:

Nomenclature:

The systematic process of naming organic compounds is called nomenclature.

At the beginning of organic chemistry, each new compound was given an individual name.

These names were based on :

1.its source

2.property

3.on the name of the discoverer

4.based on history

5.method of preparation

For Example:

Formic acid, from Formica(Latin) = Red ants

Lactic acid, from Lactum(Latin) = Fermented milk

Butyric acid, from Butyrum(Latin) =Butter

There are two systems of naming organic compounds.

(i) Common system or Trivial Names:

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

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

Common System of Naming first twelve Alkanes are:

(1) CH₄– Methane (4) C₄H₁₀-Butane (7) C₇H₁₆-Heptane (10) C₁₀H₂₂-Decane

(2) C₂H₆-Ethane (5) C₅H₁₂-Pentane (8) C₈H₁₈-Octane (11) C₁₁H₂₄-Undecane

(3) C₃H₈-Propane (6) C₆H₁₄-Hexane (9) C₉H₂₀-Nonane (12) C₁₂H₂₆-Dudecane

Names of alkanes having five carbons and more have Greek prefixes indicating a number of carbons in the alkanes.

The common system failed to name every compound as it is not systematic.

(ii) IUPAC System:

With the increase in the number of organic compounds, it became impossible to give the name to such a large number of organic compounds.

Moreover, trivial names do not give any information about the structure of the organic compound. The need was, Therefore felt to name organic compounds systematically.

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

Geneva System was revised by the 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 them in 1967. This system is now called IUPAC System.

Rules of IUPAC System:

IUPAC System involves the following steps:

Step-I Longest Chain selection:

· Select the longest possible continuous carbon chain containing the carbon of the functional group and if the functional group has no carbon in it then the carbon to which the functional group is attached should be included in the selected chain.

· If more than one longest chain of an equal number of carbons is 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 the 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 the same for both ends then numbering is carried out from that end which gives the smallest locant to the first substituent.

· If the locant of the first substituent is the same for both ends then numbering is carried out from that end which gives the smaller sum of locants of all the substituents.

· If the sum of locants of all substituents is also the 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 the locant of the more electronegative group is also the same for both ends then the last preference is given to the larger alkyl group.

Step-III Naming:

· Names of substituents are written before the parent name and names of substituents are preceded by their locants. In the end, the 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 the same substituent is occurring more than one time then prefixes di, tri, tetra, penta etc are used with their 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 a 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.