Nomenclature of Saturated Hydrocarbons in Organic Chemistry

What do you notice when you squeeze a wet sponge that is full of water? Or, when you wring a piece of cloth soaked in water? Water pours out in a gush, or in droplets, right? Such a sponge or piece of fabric is said to be saturated with water. Something that is saturated with water contains an optimum amount of it and cannot hold more. Similarly, organic hydrocarbons that are saturated are called saturated hydrocarbons.

Before you move on to learning about the various rules of nomenclature of saturated hydrocarbons, let us take a look at what these hydrocarbons are.

Saturated Hydrocarbons: What are They?

Hydrocarbons (organic compounds made up of carbon, hydrogen) containing single bonds between consecutive carbon atoms are known as saturated hydrocarbons. Moreover, these are the simplest type of hydrocarbons. They are called so, mainly owing to the fact that each carbon atom in such a hydrocarbon is bonded to as many atoms of hydrogen as possible.

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In the figure above, we can see how each carbon atom in ethane is bonded to three atoms of hydrogen, each dash representing a single bond. These bonds are covalent, that is, two atoms share a pair of valence electrons.

Pop Quiz 1

1. Which of these is the chemical formula of ethane?

1. C2H5

2. C2H6

3. C3H5

Naming Hydrocarbons

All saturated hydrocarbons go by the generic name of alkanes. Each alkane has a name that always ends in -ane. Furthermore, the nomenclature of saturated hydrocarbons is done using a specific codex, which we will soon see. 

Additionally, it is essential to remember that methane is the smallest alkane (CH4). It has only one carbon atom. Likewise, ethane has two carbon atoms and is the second smallest alkane.

Each of these alkanes has different chemical and physical properties, such as boiling and melting points, as the table below will illustrate. The melting points and boiling points of alkanes are determined by the number of atoms of carbon that each of them has. Therefore, more the number of carbon atoms in an alkane, higher are the melting and boiling point. 

Revise Your Concepts: Before you start with how to name saturated hydrocarbons, revise what you learnt in the nomenclature of hydrocarbons class 11, as this will help give you a sense of clarity and a quick recap.

Pop Quiz 2

1. Which of the alkanes shown above in the table have larger melting and boiling points?

1. Methane

2. Hexane

3. Ethane

4. Propane

Let us take a deeper look at what nomenclature is and how such rules are formed. 

Nomenclature of Saturated Hydrocarbons

So what is nomenclature in chemistry? It is, virtually a set of rules or regulations that need to be followed in order to name hydrocarbons. To put it simply, it is a process that assigns the correct scientific name to an organic compound.

As you learnt above, saturated hydrocarbons consist of carbon and hydrogen atoms bonded by single covalent bonds. Saturate hydrocarbons contain the optimum number of hydrogen atoms bonded to each carbon atom, for instance, alkanes. The first segment of the name of an alkane indicates the number of carbon atoms in each molecule of the alkane. 

There are several rules governing the nomenclature of saturated hydrocarbons. These rules are:

1. Longest Chain Rule: Saturated hydrocarbons always contain a parent chain, and this parent chain is the longest chain containing carbon atoms. The parent chain can be straight or present in any other shape.

2. Locants: An alkane, for instance, ethane is numbered, by starting from the end of the chain. This end denotes the lowest number of the carbon atom that is bonded to a substituent group.

3. Multiple Occurrences of Same Substituent: If a saturated hydrocarbon on a parent chain happens to contain a substituent group more than once, then the prefixes, di, tri, poly and so on, are used to indicate the number of times the substitute is present.

4. Presence of Different Substituents: If a hydrocarbon contains more than one type of substituent, then they are named following alphabetical order. 

5. Presence of Different Substituents at Similar Positions: If a hydrocarbon contains more than one type of substituent group, but these are present in the same positions, from the two opposite ends, then the naming is done a little differently. These groups are named such that the substituent group, which comes first alphabetically, is assigned the lowest number.

6. Complex Substituents: Some substituent groups may often have a branched structure. In that case, the substituent group is named as a substitute alkyl. Likewise, the carbon atom, present on this substituent group, is numbered by 1. The name of a substituent group is indicated in brackets.

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1. In the above figure, note that the straight-chain consists of 9 carbon atoms. 

2. The 5th carbon atom (from both the ends of the chain) is bonded to a substituent group. 

3. This group further has three chains. The first two carbons of this substituent group have one carbon atom attached.

4. If you look closely at this, it appears to be another new parent chain, with one carbon atom attached to each substituent group.

5. The parent chain must be numbered first.

6. There are nine carbon atoms in this parent chain. The substituent group lies in the fifth position. 

7. Furthermore, there are three substituent carbon atoms. Among these, two substituents have additional carbon atoms.

8. The longest main chain is the one with the three carbon atoms. Thus we name it propane.

9. Similarly, we see that there are methyl groups at the 1st and 2nd positions.

10. Thus, it is named as 1-2 Dimethyl propane. Since it is a substituent group, the correct name must be 1-2 Dimethyl propyl.

11. Coupling the substituent group and the parent chain, we get 5-(1-2-Dimethyl Propyl). Since the parent chain contains nine carbons, it is denoted by nonane. Therefore, the final accurate name of this particular hydrocarbon will be 5-(1-2-Dimethyl Propyl)nonane.

Similar, if not exactly the same, rules are followed for the nomenclature of unsaturated hydrocarbons. You will soon learn more about these in upcoming chapters.

Some Important Pointers

1. Names of complex substituent groups must always be written within brackets.

2. When you decide the alphabetical order of the substituents, you must name the complex substituent by starting with the first letter of its name.

3. If simple substituents are present, you do not need to consider multiplying prefixes. Their names are alphabetized and then multiplying prefixes are placed, wherever required. 

4. If names of two or more complex substituent groups have the same words, then you must give priority to the substituent with the lower locant.

For example, in the following image, 1-methylbutyl is preferred, as it has a lower locant compared to 3-methylbutyl.

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5. If the same substituents groups occur multiple times, then they are indicated by the multiplying prefixes bis, tris, tetrakis and so on.

Alkane Shapes

Hydrocarbon compounds are mostly represented with the help of structural formulae. This is because molecules usually have varying shapes, and this leads to confusion. On the other hand, structural formulae clearly show all atoms are arranged. Saturated hydrocarbons can exist in many forms, such as straight-chain alkanes, branched-chain alkanes and cyclic alkanes. Follow the rules of nomenclature of saturated hydrocarbons to decipher how such alkanes can be named.

This was all about saturated hydrocarbons. For more on this chapter and the IUPAC nomenclature of organic chemistry, refer to our detailed notes on IUPAC nomenclature, reference notes, and solved papers, available on the Vedantu app. It offers easier access to these detailed materials.