Alkanes – Homologous series and isomerism

Hydrocarbons are “organic compounds,” which consist of carbon and hydrogen atoms in different numbers. There are three main types of hydrocarbons: 

• alkanes
• alkenes
• alkynes

Among these three types, alkanes are called “saturated hydrocarbons,” while the rest are “unsaturated hydrocarbons.” 

The first hydrocarbon created using only one carbon unit is called “methane.” The formula of methane is CH4. It has one carbon atom and four hydrogen atoms connected by a single bond.

A single bond is formed when one electron of carbon and another electron of hydrogen are shared between the two atoms, respectively. 

When we take two methane molecules that we want to join together, it is only possible when we have a bond between two carbon atoms because a hydrogen atom can only handle two electrons that are satisfied by being bonded to carbon atoms once. So, the resultant would be called ethane.

So as we can see, there is a relationship between the number of carbon and hydrogen atoms whenever there is a bond between the carbon atoms, which will be explored further. These are called saturated hydrocarbons because they contain the maximum hydrogen atoms bonded to carbon atoms. These types of organic compounds are also called “alkanes.”

In alkenes, there are double bonds between carbon atoms. The smallest possible alkene is ethene. 

Alkynes are organic compounds with triple bonds between two carbon atoms.

Introduction to the homologous series in alkanes 

As we increase the number of atoms, we realize that different compounds are formed, and each compound has a relationship with the previous compound concerning the number of carbon and hydrogen atoms. 

If we add a  molecule CH2, we can jump to the next alkane. All alkanes can be represented by CnH2n+2. The table above is a small part of the homologous series of alkanes. 

Since they all have similar chemical properties and are in the same group (alkanes), they are considered part of a homologous series. 

Homologous series of alkanes have the same formula, CnH2n+2. We can work on infinite compounds with this same formula, and they will all be saturated hydrocarbons. 

There are two types of alkanes:

• Branched alkanes
• Cycloalkanes

Cycloalkanes are alkanes wherein all the carbon atoms are cyclically linked. The smallest cycloalkane is called “cyclopropane.” It is the least stable type of alkane as it has very low stability due to a significant deviation angle and bond strain. Even cycloalkanes form a homologous series and have the formula CnH2n. 

The general formula of different homologous series can be similar to those of the alkenes and cycloalkanes, but since they are different in structures and functions, they belong to other homologous series. 

Homologous series have some of the following characteristics: 

• They should have a similar method of preparation.
• They have the same general formula, functional group, and chemical properties.

If all of these characteristics are satisfied, it can only be considered a part of the same homologous series. 

Types of homologous series

There are different homologous series of alkanes, alkenes, and alkynes.

Alkanes

Alkanes are saturated hydrocarbons with the formula CnH2n+2There is a single bond between the carbon atoms. They are initially colourless and odourless gases which later on turn into liquids. They have weak forces of attraction, and they are nonpolar. 

Alkenes

Alkenes are unsaturated hydrocarbons with the formula CnH2n. There is a double bond between the carbon atoms. They are initially colourless and odourless gases which later on turn into liquids. They have weak forces of attraction, and they are nonpolar. 

Alkynes

Alkynes are unsaturated hydrocarbons with the formulaCnH2n-2. There is a triple bond between the carbon atoms. They are initially colourless and odourless gases which later on turn into liquids. They have weak forces of attraction, and they are nonpolar. 

Within similar homologous series, compounds with different structural arrangements of atoms in space can be found. These compounds are called “isomerism.”

Introduction to isomerism

Hydrocarbons with four or more carbon atoms can be represented using similar molecular formulas but different arrangements of the atoms. Accordingly, there are changes in the properties of the respective compounds. 

The lowest possible isomer can have four carbon atoms. They both have the same formula, C4H10, butane, and  2-Methylpropane (isobutane). 

The two types of Isomerism in alkanes: 

• structural
• stereo

Structural isomerism

Structural isomerism happens when the arrangement of atoms is different in space. There are four types of isomers:

• chain
• position
• functional group
• tautomers

Chain isomerism

Chain isomerism can arise when different branches are possible within carbon atoms. Here, the formula of each molecule is  C 4H10. Accordingly, there are some changes in the properties of each compound in a different form of branching. 

e.g. Butane and 2-Methylpropane are chain isomers.

The isomers can exist in long chains and branched forms. Whenever all the carbon atoms are arranged in a straight line, it is called “linear isomers.” Linear isomers tend to have a lower boiling point due to the more significant gaps between each molecule and a weaker force than branched isomers. 

Branched isomers exist when any carbon-hydrogen group is placed as a branch from another carbon atom. These have higher boiling points than their linear counterparts. Beware of making the mistake of writing the same compound in a different form.

Sometimes, we may think that the two structures above are isomers, but when we straighten the last carbon bond, we realize they are the same. In the above figure, we can see they both are butane. This is not the case with isomers. 

The other types of isomers do not apply to alkanes as no functional group can be involved in this homologous series. However, a different kind of isomerism happens here, and that is stereoisomers, which will be studied in detail later on. Chain isomers can also exist in alkenes and alkynes.

Stereoisomerism

The isomerism, which is caused by the non-similar arrangements of functional groups or atoms that belong to an atom in space. It is of two types i.e., enantiomers and diastereomers.

Conclusion 

In conclusion, we can say that we have understood what homologous series are, the homologous series of alkanes, the list of alkanes in the homologous series, and the different isomers possible in alkanes. We also briefly got an idea about other homologous series like alkenes and alkynes, and we understood some essential characteristics and types of homologous series and isomers.