Nomenclature of Organic Compounds
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.
- Common system or trivial system
- IUPAC (International Union of Pure and Applied Chemistry) 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.
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.
Explanation of the above formula
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 :
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
There are two types of suffixes.
The primary suffix indicates if the carbon chain is saturated or unsaturated.
Saturated carbon chains : In saturated carbon chains, all 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 CHAIN||PRIMARY SUFFIX||GENERAL NAME|
|Unsaturated with one double bond||-ene||Alkene|
|Unsaturated with one triple bond||-yne||Alkyne|
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.
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).
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|
|Carboxylic acids||-COOH||-oic acid|
|Acid chlorides||-COCl||-oyl chloride|
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).
There are two types of prefixes.
The primary prefix is used to distinguish cyclic compounds from acyclic compounds. Cyclic compounds contain prefix cyclo before the word root.
Primary prefix cyclo indicates that 4-chlorocyclohexan-1-ol is actually a cyclic compound.
There are certain groups that are not considered as functional groups, but instead are treated as substituents. Some of such substituents are given below :
-R represents carbon chains.
-alk represents the word root. For example, OCH3 means methoxy OC2H5 means ethoxy etc.
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).
- Hyphen (−) is used to separate numbers from letters.
For example : 4−Chlorocyclohexane, 1−bromo−3−methylpentane etc.
- Comma (,) is used to separate a number from another number.
For example : 2,3-Dimethylpentane
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.