Nomenclature of Organic Compounds - Definition, Different Classes, FAQs

Nomenclature definition: It is used for assigning proper name to a particular organic compound on the basis of certain standard rules so that the study of these compounds may become systematic and easy to follow. In case of aliphatic compounds, two systems of naming system are generally used,

(A) Trivial system (B) IUPAC system

(A) Trivial system.

In previous days, the organic compounds were named after the source from which they were obtained, as in example, urea got its name because the compound was obtained from the urine of the mammals. Similarly, methyl alcohol was known as wood spirit since it could be obtained as one of the products during the destructive distillation of wood and formic acid derived its name from the Greek word formicus (red ants) since the acid could be obtained from red ants’ species. These names are without any systematic basis or rules and are known as common names or trivial matters. Such a type of a system of nomenclature is known as a trivial system.

(B) What is the full form of IUPAC?

IUPAC system of nomenclature of carbon compounds

Nomenclature meaning in Tamil is Peyargal

In an attempt to rationalise the system of naming an International Congress of Chemists was held in Geneva in 1892. They adopted certain uniform rules for naming or nomenclature of the organic compounds/ naming organic compound: The system of nomenclature was named as the Geneva system and since then the system of naming has been modified by the International Union of Pure and Applied Chemistry which is full form of IUPAC in chemistry.

The new system is called IUPAC system of naming the compound. Thus this system of nomenclature of organic compound was first introduced in 1947 and was changed from time to time. The most comprehensive rules for nomenclature of organic compounds and names were first published in 1979 and later revised, modified and updated in 1993.

The rules discussed are based on guide books published by IUPAC in 1979 (Nomenclature of Organic Chemistry by J Rigandy and S.P. Klesney). With the help of this above naming system, an organic compound having any number of carbon atoms can be easily named.

Many of the books discuss nomenclature based on the IUPAC recommendations of 1979, we include here the recommendations of 1993 also to provide LATEST UPDATED RULES.

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IUPAC System of Naming Organic Compounds:

In the IUPAC system, the naming of an organic compound consists

(i) Word root. The word root is the number of carbon atoms present in the chain and for chains containing up to four carbon atoms, special word roots (meth-C₁, eth-C₂. prop-C₃) have been used.

(ii) Suffix: In the nomenclature of organic compounds and naming the compound the word root is linked to the suffix which may be primary or secondary or both.

1. Primary suffix. In the naming, IUPAC nomenclature of organic chemistry it indicates the nature of linkages in the carbon atoms. For example, if the carbon atoms are linked by single covalent bonds (C-C), the primary suffix -ane is used.
2. In nomenclature of organic compounds double bond (C=C) between the carbon atoms, the suffix is -ene while for the triple bonded carbon atoms (C=C suffix -yne is used. Thus, the suffixes are:

ane -for single bonded compounds, C-C

ene- for double bonded compounds, C=C

yne - for triple bonded compounds, C=C

If the parent chain contains two, three or more double or triple bonds in the organic compound, by IUPAC name then the numerical prefixes such as di(for two),tri(for three),tetra(for four).etc are added to the primary suffix.

(b) Secondary suffix. The nomenclature of organic compounds by IUPAC name it indicates the presence of functional secondary suffixes used to represent functional groups are:

It may be reported that while adding the secondary suffix to the primary suffix in the nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry, the terminal 'e' of the primary suffix (i.e., ane, ene or yne) is omitted if the complete secondary suffix (the suffix plus multiplying affix, if any, such as di, tri, tetra) begins with a vowel (a, e, i, o, u) and however, the terminal'-e' is retained if the secondary suffix begins with a consonant.

(iii) Prefixes. The nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry, there are many groups which are not regarded as functional groups in the IUPAC name of the compound and these are regarded as substituents or side chains. These are represented as prefixes and are placed before the word root while naming a compound. These may be

(a) Alkyl groups. In IUPAC naming rules, these groups in organic compounds contain one hydrogen atom less than the alkane which are named by substituting the suffix ane of the name of the corresponding alkane by yl.

(b) The nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry functional groups not regarded as principal functional groups and if a compound contains more than one functional group, then one of the functional groups is regarded as principal functional group and is treated as a secondary suffix. The other functional groups are regarded as substituents and are indicated by prefixes according to the IUPAC naming system.

These are the iupac nomenclature rules to be followed.

Different Classes of Organic Compounds

These are the organic compounds containing carbon and hydrogen atoms only. They may be further classified into two classes: saturated and unsaturated hydrocarbons. (a) Saturated Hydrocarbons-Alkanes

1. Hydrocarbons: The nomenclature of organic compounds which contain only carbon-carbon single bonds where these were earlier named as paraffins (Latin: meaning little affinity) due to their least chemical reactivity and according to IUPAC these are named as alkanes (ane is suffix with word root). Various members of the series are given below as named by IUPAC system.

General formula=C, H₂_n+2 Suffix: ane

IUPAC name list/ IUPAC name chart

Classification of carbon atoms in alkanes. The carbon atoms in an alkane molecule may be classified into four types as primary (1°), secondary (2), tertiary (3°) and quaternary (4°).

The nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry, the nomenclature can be done by

(1) A carbon atom connected to one other (or no other) carbon is called primary carbon

(written as 1° carbon).

(2) A carbon atom connected to two other carbon atoms is called secondary carbon

(written as 2º carbon).

(3) A carbon atom connected to three other carbon atoms is called tertiary carbon

(written as 3º carbon).

(4) A carbon atom connected to four other carbon atoms is called quaternary carbon

(written as 4º carbon).

Hydrogen atoms attached to 1°, 2° and 3° carbon atoms are referred to as primary, secondary and tertiary H atoms respectively. The following example clearly differentiates the various carbon atoms and it is useful in naming the compounds.

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General rules for naming organic compounds

We have discussed the simple rules for naming organic compounds for different families of compounds. The nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry. But many organic compounds have typical branching or substituent and due to branching at different positions in a molecule, it is not possible to name them directly.

Thus, the nomenclature includes simple branched chain compounds that can be named according to common names. In the common system, all the isomeric alkanes (having the same molecular formula) have the same parent’s name and the names of various isomers are distinguished by prefixes where the prefix indicates the type of branching in the molecule. Example,

(i) In nomenclature, prefix n-(normal) is used for those alkanes in which all the carbon atoms form a continuous chain where there is no branching.

CH₃CH₂CH₂CH₃

n-Butane

(ii)In nomenclature prefix iso- is employed for those alkanes during which one methyl group is connected to the next-to-end carbon atom (second last) of the continuous chain.

(iii) )In nomenclature prefix neo- is employed for those alkanes which have two methyl groups connected to the second last carbon atom of the continuous chain.

In naming highly branched alkanes, the system becomes complicated. However, they can be easily named for IUPAC Nomenclature of Branched Chain Saturated Hydrocarbons (Alkanes) according to IUPAC system as discussed below:

Rules for IUPAC Nomenclature of Branched Chain Saturated Hydrocarbons (Alkanes)

The following rules are used for naming/ nomenclature of the branched chain alkanes :

Longest chain rule-The nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry, the nomenclature can be done by

1. choose the longest continuous chain of carbon atoms within the molecule. the chosen chain, containing the utmost variety of carbon atoms, is considered the parent or root chain and it provides the name of the parent organic compound. Carbon atoms that don't seem to be enclosed within the parent chain are known as substituents or branched chains.

Position of the substituent-The nomenclature of compounds and naming compounds by IUPAC nomenclature in nomenclature chemistry, the nomenclature can be done by

2. Number the carbon atoms of the parent chain as 1, 2, 3... etc. start from the end which gives lower number to the carbon atoms having the substituents in the compound.
3. Lowest set of locants- When two or more substituents are present, then the end of the parent chain which gives the lowest set of the locants is preferred for numbering.
   This rule is called lowest set of locants which means that when two or more different sets of locants are present then that set of locants when compared term by term with other sets. This rule is used irrespective of the nature of the substitute.

4. Presence of more than one same substituent-In IUPAC nomenclature of the same substituent or side chain occurs more than once, the prefixes di (for 2), tri (for 3), tetra (for 4) etc.
5. Naming different substituent -In IUPAC nomenclature of two or more different substituents or side chains are present in the m, they are summed in the alphabetical order along with their appropriate positions.
6. Naming different substituents at equivalent positions- In IUPAC nomenclature of two different substituents are in equivalent positions from the two ends of the chain (two sets of locants are equally possible), then the numbering of the chain done in such a way that the group which comes first in the alphabetical order (written first in the name) ats lower number.

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