Nomenclature of Saturated Hydrocarbons
Compounds containing only carbon and hydrogen atoms are known as 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 :
We are given the following compound
Let's do the numbering on the given compound
Clearly, the compound contains 5 carbon atoms; therefore, the word root of the given compound is pent (word roots are discussed in nomenclature introduction).
IUPAC name of the compound (word root + ane) = Pentane.
Similarly, straight chain alkanes containing 1, 2 and 3 carbon atoms are methane (meth+ane) , ethane (eth+ane) and propane (prop+ane) respectively.
The common names of straight chain alkanes are the same as their IUPAC names except that the prefix n-(normal) is used. e.g. n-Butane, n-Pentane etc.
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.
Carbon chains (or a single carbon) attached to the principal chain are called alkyl groups.
Principal chain is also known as parent chain.
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 :
Explanation : In the above example, numbering has been done in different ways. The chain containing 7 carbon atoms is the longest; therefore, it is selected as the principal chain.
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.
Here, methyl is attached to 3rd or 4th carbon depending upon whether the numbering is done from right to left or from left to right. Since '3' is the lower of the two, the numbering in this case should be done from right to left.
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 :
In the above compound, the sets of locants are (2,3,5) and (2,4,5) depending upon whether we do the numbering from left to right or from right to left. The first set (2,3,5) is preferred because the second term i.e., 3 is lower than the second term 4 in the second set (2,4,5). Notice that the first term i.e., 2 is the same and hence cannot be used to decide the direction of numbering.
First point of difference rule is also known as lowest set of locant rule.
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.
Here, the principal chain has 7 carbons; therefore, the word root of the compound is hept. The alkyl group is ethyl and is attached to the 4th carbon of the principal chain; ane at the end, suggests that the compound is an alkane.
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.
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.
Nomenclature of Saturated Cyclic Hydrocarbons
While naming cyclic hydrocarbons put prefix 'cyclo' before word root.
We will learn more about cyclic compounds later in nomenclature of alicyclic compounds.
Important points to be noted
Case 1 : Study the following example :
Find the longest chain in the following compound :
Solution 1 : The longest chain contains 5 carbon :
Solution 2 : But, there is one more way of getting the longest chain
In situations like this, where two different chains of equal lengths are possible, the chain containing greater number of side chains is selected as the principal chain. Hence, the first solution with two side chains is the correct one.
Case 2 : Di, tri, tetra etc., are not considered while deciding the alphabetical order of alkyl groups.
4-Ethyl-2,3-dimethylheptane (not 2,3-Dimethyl-4-ethylheptane)
Case 3 : Study the following example :
The IUPAC name of the following structure is 2-methylbutane.
You may be wondering, why numbering cannot be done like this :
The answer to this question is, "Yes, it is also correct". In fact, you can write the structure in any way, provided it satisfies its name. For example : all the following structures represent 2-methylbutane.
However, no other way is simpler than this one :
Hence, this way of writing structures is preferred.
More examples on saturated hydrocarbons