IUPAC Nomenclature of Organic Chemistry

In chemical nomenclature, the IUPAC nomenclature of organic chemistry is a method of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). It is published in the nomenclature of organic chemistry (informally called the blue book). Ideally, every possible organic compound should have a name from which an unambiguous structural formula can be created. There is also an IUPAC nomenclature of inorganic chemistry.

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To avoid long and tedious names in normal communication, the official IUPAC naming recommendations are not always followed in practice, except when it is necessary to give an unambiguous and absolute definition to a compound. IUPAC names can sometimes be simpler than older names, as with ethanol, instead of ethyl alcohol. For relatively simple molecules they can be more easily understood than non-systematic names, which must be learned or looked over. However, the common or trivial name is often substantially shorter or clearer, and so preferred. These non-systematic names are often derived from a source of the compound. Also, very long names may be less clear than structural formulas.

In this article, we will cover the topic (IUPAC Nomenclature of Organic Chemistry). This topic falls under the broader category of (Some Basic Principles of Organic Chemistry), which is a crucial chapter in (Class 11 Chemistry). It is not only essential for board exams but also for competitive exams like the Joint Entrance Examination (JEE Main), National Eligibility Entrance Test (NEET), and other entrance exams such as SRMJEE, BITSAT, WBJEE, BCECE, and more.

Nomenclature of Straight-Chain Hydrocarbons

The names of such compounds are based on their chain structure, and end with suffix ‘-ane’ and carry a prefix indicating the number of carbon atoms present in the chain (except from $\mathrm{CH}_4$ to $\mathrm{C}_4 \mathrm{H}_{10}$ where the prefixes are derived from trivial names). The IUPAC names of some straight-chain saturated hydrocarbons are given in the Table below. The alkanes in this Table differ from each other by merely the number of $\mathrm{CH}_2$ groups in the chain. They are homologues of alkane series.

Nomenclature of branched-chain alkanes:
The rules for naming branched-chain alkanes are as follows:

1. First of all, the longest carbon chain in the molecule is identified. In the example given below, the longest chain has nine carbons and it is considered as the parent or root chain.
   

2. The carbon atoms of the parent chain are numbered to identify the parent alkane and to locate the positions of the carbon atoms at which branching takes place due to the substitution of alkyl group in place of hydrogen atoms. The numbering is done in such a way that the branched carbon atoms get the lowest possible numbers. Thus, the numbering in the above example should be from left to right (branching at carbon atoms 2 and 6).

3. The names of alkyl groups attached as a branch are then prefixed to the name of the parent alkane and position of the substituents is indicated by the appropriate numbers. If different alkyl groups are present, they are listed in alphabetical order. Thus, name for the compound shown above is: 6-ethyl-2-methylnonane.

4. If two or more identical substituent groups are present then the numbers are separated by commas. The names of identical substituents are not repeated, instead prefixes such as di (for 2), tri (for 3), tetra (for 4), penta (for 5), hexa (for 6) etc. are used. While writing the name of the substituents in alphabetical order, these prefixes, however, are not considered.

5. If the two substituents are found in equivalent positions, the lower number is given to the one coming first in the alphabetical listing. Thus, the following compound is 3-ethyl-6-methyloctane and not 6-ethyl-3-methyloctane.
   

Organic compounds having Functional Groups
The longest chain of carbon atoms containing the functional groups is numbered in such a way that the functional group attached to the carbon atom gets the lowest number in the chain.
When there are more functional groups then a priority order is followed as:$-\mathrm{COOH},-\mathrm{SO}_3 \mathrm{H},-\mathrm{COOR}$ (R=alkyl group), $\mathrm{COCl},-\mathrm{CONH}_2,-\mathrm{CN},-\mathrm{HC}=\mathrm{O},>\mathrm{C}=\mathrm{O},-\mathrm{OH}_1,-\mathrm{NH}_2,>\mathrm{C}=\mathrm{C}<,-\mathrm{C}=\mathrm{C}$

For example, the IUPAC name of this given compound is written as:

• The functional group present is an alcohol (OH). Hence the suffix is ‘-ol’.
• The longest chain containingOH has eight carbon atoms. Hence the corresponding saturated hydrocarbon is octane.
• The -OH is on carbon atom 3. In addition, a methyl group is attached at 6^th carbon.

Hence, the systematic name of this compound is 6-Methyloctan-3-ol.

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Solved Examples Based IUPAC Nomenclature of Organic Chemistry

Q.1 The IUPAC name of the following compound is :

(1) 4-methyl-3-ethylhex-4-ene

(2) 3-ethyl-4-methylhex-4-ene

(3) 4-ethyl-3-methylhex-2-ene

(4) 4,4-diethyl-3-methylbut-2-ene

Solution:

As we learned

Nomenclature of organic compounds -

• The longest chain is the main chain, branched-chain is the substituent.
• The locant value or position number should be minimum.

IUPAC system of nomenclature

4-ethyl-3-methylhex-2-ene

Therefore, option (3) is correct.

Q.2 The IUPAC name of $\mathrm{CH}_3 \mathrm{COCH}\left(\mathrm{CH}_3\right)_2$ is

(1) Isopropylmethyl ketone

(2) 2­-methyl-­3-­butanone

(3) 4-­methylisopropyl ketone

(4) 3­-methyl­-2­-butanone.

Solution:

As we learned

The IUPAC name willl be

Therefore, option (4) is correct.

Conclusion

The IUPAC system of nomenclature was created to create a worldwide standard for naming substances in order to improve communication. The goal of the system is to give each structure a unique and unambiguous name with a unique and unambiguous structure. The IUPAC nomenclature system is based on designating the longest chain of carbons joined by single bonds in a molecule, whether in a continuous chain or a ring. Prefixes and suffixes are used to denote all deviations, whether multiple bonds or atoms other than carbon and hydrogen, according to a certain set of priorities.