Nomenclature of Saturated Hydrocarbons
Hydrocarbons
Compounds containing only carbon and hydrogen atoms are known as hydrocarbons.
Saturated Hydrocarbons
Hydrocarbons in which all the carbon atoms are linked to one another by only single bonds are called saturated hydrocarbons.
Saturated hydrocarbons may be either cyclic (closed chain) or acyclic (open chained). Saturated acyclic hydrocarbons are called alkanes.
Primary suffix: ane.
Types of Alkanes
Alkanes are of the following types :
Straight chain alkanes
In straight chain alkanes, no carbon atom is linked to more than two carbon atoms.
Butane : CH3-CH2-CH2-CH3
Pentane : CH3-CH2-CH2-CH2-CH3
Nomenclature of saturated straight chain alkanes
The IUPAC names of straight chain alkanes are obtained by adding the suffix ane to word root.
Study the following example carefully :
Similarly, straight chain alkanes containing 1, 2 and 3 carbon atoms are methane (meth+ane) , ethane (eth+ane) and propane (prop+ane) respectively.
Branched chain alkanes
In branched chain alkanes, at least one carbon atom is linked to more than 2 carbon atoms.
Principal chain in alkanes
The longest continuous chain is the principal chain.
Alkyl group
Carbon chains (or a single carbon) attached to the principal chain are called alkyl groups.
Rules for IUPAC nomenclature of Branched Chain Alkanes
The following rules are used for naming branched chain alkanes.
Longest chain rule
The longest continuous chain present in an alkane is the principal chain.
Take a close look at the following example :
Answer the following question and see if you have understood the concept or not.
Lowest locant rule
Do the numbering in such a way that the alkyl group (branched chain) gets the lowest possible number.
First point of difference rule
When two or more alkyl groups are present, that direction is preferred, which has a lower number at the first point of difference. Take a look at the following example for better understanding :
Name of the branched chain alkane
Prefix the name of the alkyl group to the name of the parent alkane and indicate its position by writing before it the number of the carbon atom carrying the alkyl group.
Alphabetical order of the side chains.
When two or more alkyl groups are present on the parent chain then their names are to be written in alphabetical order. Remember, numbering is done from the side, which is nearer to the alkyl group.
4-Ethyl-2-methylhexane
More than one occurrence
When the number of occurrence of the same substituent is more than one on the parent chain at different positions, the position number of each occurrence is separated by commas and suitable numerical prefixes like di for two, tri for three, tetra for four are used.
2,2,3-Trimethylbutane
Nomenclature of Saturated Cyclic Hydrocarbons
While naming cyclic hydrocarbons put prefix 'cyclo' before word root.
Cycloalkanes
Cyclopropane
Cyclobutane
Cyclopentane
We will learn more about cyclic compounds later in nomenclature of alicyclic compounds.
Important points to be noted
Case 1 : Study the following example :
Case 2 : Di, tri, tetra etc., are not considered while deciding the alphabetical order of alkyl groups.
Case 3 : Study the following example :
More examples on saturated hydrocarbons