What are Alkenes? 

Alkenes are organic compounds that contain one double bond between carbon-carbon atoms. They are a series of unsaturated compounds with a general molecular formula, C_nH_2n. 

Bonds 

The molecule exists as a three-dimensional figure and the angle formed around the double bond of alkene is about 120 degrees. The hybridisation of alkenes is sp². The double bond consists of a sigma bond and one pi bond. Due to the presence of this pi bond, alkenes are a little unstable and the electrons are loosely held and this makes alkenes more reactive than their corresponding alkanes.  

Nomenclature 

A common suffix is used while naming the molecules so that it easily falls under the same homologous series. The suffix used is “–ene” and the prefix used is decided depending on the number of carbon atoms. Naming is done according to the rules of International Union of Pure and Applied Chemistry (IUPAC). Some of the names have been given in the table shown below.

Formulae 

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

Below shown are different ways of representing formula of butane. 

• Molecular formula : It shows the exact number of each type of atoms present in the molecule. It is written in the format of C_nH_2n.  Example: C₄H₈. 

• Empirical formula : It shows the simplest whole number ratio of the atoms present in the molecule.  Example: CH₂. 

• Condensed formula : It shows the atoms arranged in the order as it appears in the molecule’s arrangement with minimal or no bonds. Example: CH₂=CHCH₂CH₃. 

• Structural formula : It is a 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 : In this, hydrogen atoms are omitted and the carbon atoms are represented by lines. It is the most common way and easiest way of representation. 

Isomerism   

The atoms in a molecule can be rearranged to form different structures with the same molecular formula; they are termed as isomers. Alkenes show structural as well as geometrical isomerism. 

In long-chain alkenes, the double bond can be placed at various different positions, giving rise to structural isomers. The following example shows various structural isomers of pentene. 

Alkenes show geometrical isomerism as the double bond restricts the rotation of atoms/groups around it, giving rise to different geometries. They are of two types: 

Cis isomer: In this, two identical atoms/groups lie on the same side of the double bond. 

Trans isomer: In this, two identical atoms/groups lie on the opposite side of the double bond.  

Due to different arrangements, they also differ in their physical properties and chemical properties. 

Preparation of Alkenes 

They can be produced by various different reactions. 

• During dehydration of alcohol. 
• During cracking of large alkane molecules into alkenes using catalysts and required conditions. 
• Partial reduction of alkynes. 
• Elimination of hydrogen halide from halogenoalkanes. 
• During decarboxylation (removal of carbon dioxide from carboxylic acid) 

Physical Properties 

 Solubility 

Alkenes are soluble in organic solvents but not in water. This is because it is considered as a non-polar molecule, that is, there is a very little difference in the electronegativity between atoms. 

Alkene’s van der waal forces do not release enough energy to break the hydrogen bonds in water but it is enough to break the forces in organic solvents. 

Boiling point 

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

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

 Melting point 

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

States of matter 

First three alkenes in the nomenclature series are gases, the next fourteen are liquids and the higher ones are solids. 

Reactions of Alkenes 

Combustion 

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

Addition Reaction 

As the name suggests, addition in this context means combining. When an alkene reacts with a compound, there are chances the atoms get added across the double bond to form a single molecule. There are several possible reagents used for such reactions. Some are listed below with conditions.   

• Hydrogen atoms from an alkane (140°C and nickel catalyst). 
• Water/ steam to form alcohol (330°C, 60 atm pressure, and phosphoric acid). 
• Hydrogen halides from halogenoalkanes. 
• Halogen form vicinal dihalides. 

Polymerisation  

A large number of molecules of alkenes can be added together to form a long chain of alkane. This is called a polymer and the process is called polymerization. The repeating units called a monomer. In this case, the monomers are the alkene molecules. 

Identification of Alkenes 

Alkenes can be identified by a small, very common test, called the bromine water test. When bromine water is added to a container containing an alkene, the orangish colour of the bromine water fades to form a colourless solution. This is because an addition reaction takes place to form dibromoalkane which is a colourless compound. This cannot be observed in alkanes as it lacks a double bond. 

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. 
• While drawing repeating units for addition polymerization, students often forget to bend the molecule. 

Context and Applications 

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry.