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Nomenclature of Saturated Hydrocarbons

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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 :

We are given the following compound

Straight chain alkane iupac nomenclature example

Let's do the numbering on the given compound

Straight chain alkane iupac nomenclature example

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.

Branched chain alkanes example : Isobutane
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.

 alkyl group is a branched carbon chain in a compound

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 :

Longest chain is the principal chain in iupac nomenclature of saturated hydrocarbons

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.

Identify the principal chain.

Question : Identify the principal chain

Lowest locant rule

Do the numbering in such a way that the alkyl group (branched chain) gets the lowest possible number.

IUPAC nomenclature : lowest locant rule

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 :

IUPAC nomenclature : First point of difference rule

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.

true/false

The following compound is numbered correctly :

IUPAC nomenclature :True or false

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.

Naming of the saturated hydrocarbon : 4-Ethylheptane

4-Ethylheptane

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.

Substituents are named alphabetically in iupac nomenclature : 4-ethyl-2-methylhexane

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.

Commas are used to separate numbers in iupac nomenclatue : 2,2,3-Trimethylbutane

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 :

Find the longest chain in the following compound :

IUPAC nomenclature scenario : When principal chain can be chosen in two ways

Solution 1 : The longest chain contains 5 carbon :

Principal chain should contain more side chains

Solution 2 : But, there is one more way of getting the longest chain

Principal chain should contain more side chains

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.

principal chain contains more side chains : 3-Ethyl-2-methylpentane

3-Ethyl-2-methylpentane

Case 2 : Di, tri, tetra etc., are not considered while deciding the alphabetical order of alkyl groups.

in iupac nomenclature prefixes di tri are not considered while naming alphabetically : 4-Ethyle-2,3-dimethylheptane

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.

iupac name : 2-methylbutane

You may be wondering, why numbering cannot be done like this :

iupac name : 2-methylbutane

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.

iupac name : 2-methylbutane
iupac name : 2-methylbutane
iupac name : 2-methylbutane

However, no other way is simpler than this one :

iupac name : 2-methylbutane

Hence, this way of writing structures is preferred.


More examples on saturated hydrocarbons
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