What are Alkanes? 

Alkanes are the simplest organic compounds that contain only single bonds between carbon – carbon atoms and carbon – hydrogen bonds. This means they are a series of saturated compounds with a general molecular formula, C_nH_2n+2

Some examples of alkanes can be ethane (C₂H₆), pentane (C₅H₁₂) and hexane (C₆H₁₄). 

Bonds  

The molecules exist as three-dimensional figures in real life, just like any other substance. Therefore, there is an angle formed between the bonds and in alkanes the bond angle is 109.5 degrees. Also, it has a sp³ hybridisation.  

Nomenclature 

Alkanes use a common suffix while naming so that it is easily grouped in a homologous series. The suffix is “-ane”.  

Most commonly we use a set of IUPAC rules to name the alkanes. Some of the names are given as below:  

Formulae 

The same molecule has various forms of expressing itself highlighting different key features. Each way is known as a formula.   

(Using butane as an example in the following):  

• Molecular formula  

It shows the exact number of each type of atoms present. It is written in the format C_nH_2n+2. 

Example: C₄H₁₀. 

• Empirical formula  

It shows the simplest whole number ratio of the atoms present.  

Example: C₂H₅. 

• Condensed formula  

The atoms (symbols) are listed separately in the order as it appears in the molecule’s arrangement.  

Example: CH₃CH₂CH₂CH₃. 

• Displayed formula  

It is a graphical representation of the molecule showing the atoms and bonds present in the molecule. Bonds are represented as lines, and atoms, by their symbols. 

• Skeletal formula  

It mostly displays only the functional group. Hydrogen atoms are omitted and the carbon atoms are represented by lines. It is the most common way and easiest way of representation.  

Cycloalkanes 

As the name suggests, in this, the carbon atoms are arranged in the form of a ring. The bonds between carbon atoms are still single bonds only.  

If carefully observed, it can be seen that it does not obey the general molecular formula C_nH_2n+2. It has a new formula C_nH_2n. The smallest existing cycloalkane is cyclopropane. 

Isomerism   

Even though the number of atoms is the same, there are various ways in which we can arrange them. The atoms in a molecule can be rearranged to form different structures that are termed as isomers.  

Alkanes show structural isomerism (compounds have the same molecular formula but different structural formula). For example, pentane has three isomers: CH₃CH₂CH₂CH₂CH₃, CH₃CH(CH₃)CH₂CH₃ and CH₃C(CH₃)(CH₃)CH₃. 

Preparation 

Crude oil is the main source of alkanes. However, they can be produced in various different reactions:  

• During hydrogenation of unsaturated hydrocarbons.  
• During cracking of larger alkanes. 
• Reduction of alkyl halides. 
• Alkyl halides on reaction with sodium metal in dry ethereal.  
• During decarboxylation (removal of carbon dioxide from carboxylic acid).  

Physical Properties  

Physical properties  

Solubility  

Alkanes are soluble in organic solvents but not in water. This is because it is considered as a non-polar molecule (very little difference in the electronegativity of the atoms). This is because alkane’s Van Der Waals force doesn’t release enough energy to break the hydrogen bonds in water but it is enough to break the forces in organic solvents. Also, alkanes themselves are considered as good solvents for other compounds.  

Boiling Point  

The boiling point increases as the molecular weight of the compound increases. This is because the Van Der Waals forces also increase with the increase in the molecular size. A greater force between molecules indicates the needed for more energy to overcome it.   

Straight chain alkanes are generally observed to be having a greater boiling point compared to its isomers. 

Melting point  

Follows the same pattern as its boiling points; increases as the molecular weight increases.  

Reactions of Alkanes 

Combustion  

Just like any other fuel, even hydrocarbons (alkanes) can be heated in the presence of oxygen to form carbon dioxide and water as products and most importantly energy (heat).  

The general equation for alkanes is: 

CnH2n+2 + ( 3n+1 2 ) O2 → nCO2 + (n+1)H2O

The larger the alkane, greater is the amount of energy it releases.  

However, at times when there is insufficient oxygen present during heating, the compound undergoes incomplete combustion. During this process, a harmful compound carbon monoxide is released which affects the human respiratory system.  

Substitution Reaction  

When an alkane reacts with a compound, there are chances that a stronger and more powerful atom will replace the current hydrogen atoms one at a time.  

A halogen atom breaks to form halide ions. One halide ion replaces a hydrogen atom of the alkane molecule and the other combines with the replaced hydrogen atom. This reaction takes place in the presence of sunlight or UV light.  

Cracking  

Lots of the large alkanes are not very useful as compared to the ones with lower molecular formula so, the demand for smaller alkanes is higher. For the same reason, a process is carried out to convert the larger alkanes to smaller, more useful alkanes; the process is known as cracking. It takes place under the conditions: Heat (550 degree Celsius), catalysts like aluminium oxide and silicon, carbon dioxide and no oxygen.   

Higher alkane → smaller alkane + alkene   

Example:

C₁₀H₂₂ → C₄H₁₀ + C₆H₁₂ 

Common mistakes  

• Students often forget to balance the equations.  
• Remember that bending a chain does not make it an isomer; it has to be branched.  
• Remember the conditions for each reaction correctly.  

Context and Applications 

This topic is useful for undergraduate and postgraduate courses, especially for Bachelors and Masters in Chemistry.