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Nomenclature of Organic Compounds

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Nomenclature of Organic Compounds

The system of naming organic compounds is termed as nomenclature of organic compounds. The following systems are used for naming organic compounds.

  1. Common system or trivial system
  2. IUPAC (International Union of Pure and Applied Chemistry) system

Common System

The common name of an organic compound is the general name given to the compound. Common name system does not follow rules of any formal system.

IUPAC System

There are millions of organic compounds present on earth; therefore, we cannot remember all of them by their common names. To overcome this problem, a better and systematic nomenclature system has been introduced. This system is known as IUPAC system.

Are common names any useful

Common names are particularly useful when the alternate IUPAC names are lengthy and complicated. For example,
It is easier to say 'citric acid' than its IUPAC name '2-hydroxypropane-1,2,3-tricarboxylic acid', isn't it?

General Rules for IUPAC System

Take a look at the following formula :

Prefix + Word root + Suffix

The easiest way of learning something is through examples. Hence, we are taking the example of 4-Chlorocyclohexan-1-ol to teach you what each word of the above formula means.

4-Chlorocyclohexan-1-ol

Explanation of the above formula

Word root

Word root denotes the number of carbon atoms present in the principal chain (the longest possible continuous chain of carbon atoms including the functional group and the multiple bonds) of an organic compound.

The word roots of principal chain containing up to 10 carbon atoms are listed in the table below :

Compound Word Root
C1 meth
C2 eth
C3 prop
C4 but
C5 pent
Compound Word Root
C6 hex
C7 hept
C8 oct
C9 non
C10 dec
Compound Word Root
C1 meth
C2 eth
C3 prop
C4 but
C5 pent
C6 hex
C7 hept
C8 oct
C9 non
C10 dec

4-Chlorocyclohexan-1-ol

hex is the word root in 4-chlorocyclohexan-1-ol, which means the principal chain of this compound contains 6 carbons.

C C C C C C

Suffix

There are two types of suffixes.

Primary suffix

The primary suffix indicates if the carbon chain is saturated or unsaturated.

Saturated carbon chains : In saturated carbon chains, all the carbon atoms are linked to one another by only single bonds.

Unsaturated carbon chains : Carbon chains containing carbon-carbon double or triple bonds in their molecules are called unsaturated carbon chains.

Primary suffixes of saturated and unsaturated carbon chains are written in the table below :

TYPE OF CARBON CHAINPRIMARY SUFFIXGENERAL NAME
Saturated-aneAlkane
Unsaturated with one double bond-eneAlkene
Unsaturated with one triple bond-yneAlkyne

4-Chlorocyclohexan-1-ol

Primary suffix an(e) in 4-chlorocyclohexan-1-ol suggests that the carbon chain is saturated i.e all carbon atoms are linked to one another by a single bond.

CH3-CH2-CH2-CH2-CH2-CH3

Note : We skip 'e' from the end of the primary suffix when secondary suffix (explained later) starts with a vowel. e.g. hexanol (not hexaneol), butan-3-al (not butane-3-al).

Secondary suffix

The secondary suffix indicates the nature of the functional group present in an organic compound. Secondary suffixes of some compounds are given below :

COMPOUND FUNCTIONAL GROUP SECONDARY SUFFIX
Alcohols -OH -ol
Aldehyde -CHO -al
Ketones >C=O -one
Carboxylic acids -COOH -oic acid
Acid amides -CONH2 -amide
Acid chlorides -COCl -oyl chloride
Esters -COOR -oate
Nitriles -CN nitrile
Thiol -SH thiol
Amines -NH2 amine

4-Chlorocyclohexan-1-ol

Secondary suffix ol in 4-chlorocyclohexan-1-ol indicates that the compound contains an -OH group as functional group; the number -1- suggests that the functional group is attached to the first carbon of the chain (from either side).

CH3CH2CH2CH2CH2CH2-OH

Prefix

There are two types of prefixes.

Primary prefix

The primary prefix is used to distinguish cyclic compounds from acyclic compounds. Cyclic compounds contain prefix cyclo before the word root.

4-Chlorocyclohexan-1-ol

Primary prefix cyclo indicates that 4-chlorocyclohexan-1-ol is actually a cyclic compound.

IUPAC Nomenclature Primary prefix example : cyclohexanol

Secondary Prefix

There are certain groups that are not considered as functional groups, but instead are treated as substituents. Some of such substituents are given below :

-F Fluoro
-Cl Chloro
-Br Bromo
-I Iodo
-NO2 Nitro
-OR Alkoxy
-R Alkyl
-CH2CH2CH3 n-Propyl
-CH(CH3)2 Isopropyl
-CH(CH3)3 tertiary butyle
-F Fluoro
-Cl Chloro
-Br Bromo
-I Iodo
-NO2 Nitro
-OR Alkoxy
-R Alkyl
-CH2CH2CH3 n-Propyl
-CH(CH3)2 Isopropyl
-CH(CH3)3 tertiary butyle

-R represents carbon chains.

-alk represents the word root. For example, OCH3 means methoxy OC2H5 means ethoxy etc.

4-Chlorocyclohexan-1-ol

Secondary prefix Chloro indicates that 4-chlorocyclohexan-1-ol contains Cl; the number 4- suggests that the Cl group is placed at the 4th carbon from the functional group (why? How? Explained in later chapters).

IUPAC Nomenclature Secondary suffix example : 4-Chlorocyclohexanol

Remember

Now, you have got acquainted with the terminology used for writing IUPAC names. We will explore the steps involved for writing the IUPAC names of different compounds in later chapters.

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