What is IUPAC Nomenclature?

In Chemistry, IUPAC stands for International Union of Pure and Applied Chemistry which suggested a systematic naming approach for the organic and inorganic compounds, as in the beginning stage of nomenclature one single chemical compound was named in many ways by which lead to confusion. The need for this approach aroused as the number of chemical compounds newly discovered were increasing (approximately 32 million compounds) and the basic concept of nomenclature i.e. the trivial nomenclature and the derived system of nomenclature failed to overcome the challenge. It is an important task to name a chemical compound systematically and unambiguously which reduces lots of confusion about the newly reported compounds.

The main aim of IUPAC is to code few rules which can be used worldwide to identify and name a pure organic substance as we all depend on alphabets to represent words. However few exceptions are also observed in case of very long and tedious names, for which the trivial names are followed.   

What is Trivial System of Nomenclature or Common Names System?

Trivial nomenclature is a type of non-systematic mode which does not have any rules to be followed. This type of naming generally followed in organic chemistry and the chemical compounds is very much simplified.  

Eg: Phenol, methane, ethane, acetic acid etc.  

Further, the trivial names are short and simple to remember compared to the IUPAC name of the same compounds. Thus, this trivial names are still in existence and use till date.  

E.g.: A carboxylic acid compound present in tamarind is named tartaric acid based on the trivial nomenclature but the IUPAC name is 2,3-dihydroxy-1,4-butanedioic acid. 

In common names or trivial system naming format, the names of the compound are based on the source or property or discovery or structure of the naming compounds. Based on the source of the substance obtained, few names are given below.  

• CH₃OH – named wood/methyl spirit as it is obtained on distillation of wood.  
• NH₂CONH₂ – named urea as its source is urine.  
• CH₄ – named Marsh gas or fire damp as obtained from marsh places 
• HCOOH – formic acid which was isolated from red ants called formica in Latin 
• CH₃COOH – called vinegar or acetic acid as obtained from vinegar i.e. acetum.  
• COOH – COOH – oxalic acid as extracted from plant named oxalis.  
• CH₃CH(OH)COOH – lactic acid as it is present in milk as lactose.  
• CH₃CH₂CH₂COOH – Butyric acid as produced from butter.  
• CH₃(CH₂)₄COOH – caproic acid as present in goats.  
• C₂H₅OH – grain alcohol as obtained in barley.  

Based on the discovery of a compound, RMgX is named as “Grignard reagent” after the name of the scientist Victor Grignard who discovered it. Similarly, R₂Zn is named as “Frankland reagent” based on the scientist “Edward Frankland” who discovered it.  

Based on the basic structure or the parent compound, the numbers of carbon atoms are taken for consideration. Thus, if one carbon (1C) atom is present in the parent structures, it is termed as “meth”, two-carbon atoms as “eth”, three-carbon atoms as “prop”, 4C atoms as “but”, five-carbon atoms as “pent”, 6C atoms as “hex”, 7C atoms as “hept”, 8C atoms as “oct”, 9C atoms as “non” and 10C atoms as “dec”.  

What is the Reason Behind the Failure of the Trivial Nomenclature?  

The trivial nomenclature system could not give names to all the existing compounds i.e. the starting members of a group have trivial names but as the number of atoms increases in the parent compound, the compounds are not given trivial names. The first two compounds of carboxylic acid group have names as formic acid and acetic acid but further, the compounds do not have trivial names. Further, one compound is found to be given many trivial names like phenol is also called as hydroxybenzene or carbolic acid etc. Inorganic compounds could not be given any proper names with the trivial system.  

Naming Organic Compounds Based on IUPAC Nomenclature   

To name an organic compound based on IUPAC system it is mandatory for a compound to contain the principal element “carbon”. Other elements such as hydrogen, oxygen, nitrogen, halides, sulphur etc., are the elements associated with carbon to form various functional groups or substituents. The success of IUPAC nomenclature system was extended further towards inorganic, polymeric and biological compounds. The set of rules to be followed step wise is given below:  

1. The longest chain of carbon atoms either in a straight line or in bent form should be selected and this carbon chain acts as the parent compound. Based on the parent carbon chain, the “root” name of the compound is given. This is followed in naming simple alkane, haloalkanes etc based on IUPAC system.
2. The numbering of the parent chain should give least number to the carbon atom which contains substituent groups. 
3. When same substituent groups are present two or more times, then prefixes such as di, tri tetra etc., are used.    

Primary prefix explains if the parent structures are cyclic or acyclic in nature. In general primary prefix is seen only in cyclic compounds and secondary prefix indicates the substituent such as alkyl groups, hydroxyl group, or halogen atoms present in the compound.

The tabular column gives the secondary prefix for the functional groups such as halogen atoms (F, Cl, Br and I), hydroxyl group (OH), carbonyl group (CHO), keto group (C-O) etc., or the substituent like alkyl groups (methyl group, ethyl group phenyl group etc. Naming a molecule using the functional groups as primary or secondary prefix is termed "substitutive nomenclature".

When multiple substituent groups are present in a molecule, the substituent groups are arranged in the alphabetical order based on the IUPAC nomenclature. When functional groups such as carboxylic acid (COOH), aldehyde group or carbonyl group (CHO) are present, this carbon atom is taken first while numbering the parent structure. Based on the main functional group present the suffix of the name is given.   

When double bond or triple bond is not present in the parent hydrocarbon, they belong to alkanes and the primary suffix is "ane". When double bond or triple bond are present in the given parent hydrocarbon, they belong to alkenes and alkynes, respectively and the primary suffix are "ene" and "yne", respectively. While numbering alkenes and alkynes, the numbering should cover the double bond and triple bond i.e. the carbon atoms containing double bond and triple bond are number as first and second carbon atoms.

Primary suffix is the word return adjacent to the root word and secondary suffix which mostly represent the main functional group is the word return next to the primary suffix. Thus, the IUPAC system of nomenclature of organic molecules has three main parts, root word, prefix and suffix. Each has its own identity to represent a molecule.

While dealing with isotopes, Cahn-Ingold-Prelog rules are followed which states that, among the different substituent groups present on a carbon atom, the group having highest atomic weight is given first priority. For instance, ethyl group is given priority than methyl group. In haloalkanes, bromine is given priority than chlorine.

IUPAC Nomenclature of Inorganic Compounds 

The rules to be followed are as given below:  

• In inorganic compounds cations i.e., positively charged ions are named first followed by anion i.e. negatively charge ions.   
• Anions are named with a suffix “–ide”. H^- ion is named as hydride, F^- ion is named as fluoride.    
• For the cations, the name of the compound or element is given as such.  

         NaCl – Sodium chloride 

         MgCl₂ – Magnesium chloride 

         BaI₂ – Barium iodide  

• If cations have more than one positive charge it is given in parenthases as roman numbers or the lesser positive charge is given suffix “-ous” and higher positive charge is given suffix “-ic”.   

       FeCl₂ – Ferrous (II) chloride or cupric chloride 

         FeCl₃ – Ferric (III) chloride  

• Anions containing oxygen named as oxyanions are given suffix “ite” and “ate” for few and more number of oxygen atoms.  

                 SO₃^2- - sulphite 

                    SO₄^2- - sulphate   

• While naming complex coordination compounds, the name is given in a single word and the ligand is named first. If same ligand is present twice or thrice prefix such as di, tri are used and in case of multi ligands, alphabetical order is followed.   

Context and Applications  

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry.