Preparation of Hydrocarbons – Alkenes

Hydrocarbons can be described as organic compounds consisting of two major kinds of atoms. These include carbons and hydrogen. Generally, these are colourless gases with nearly no odour. Based on their types, hydrocarbons may possess simple or complex structures. These are classified into four major parts: alkenes, alkanes, alkynes, and aromatics. 

By understanding hydrocarbons, we can look into the chemical properties of several important organic compounds, functional groups, and their preparations. Today, here, we will read about the preparation of hydrocarbons and others in detail. So, without any further ado, let’s get started! 

What are hydrocarbons? 

Hydrocarbons belong to organic compounds made up of essential atoms called hydrogen and carbons. In everyday life, hydrocarbons play a significant role, and their study provides a deep insight into the properties and preparation of its functional groups. 

Hydrocarbons are mainly divided into six categories. These include Saturated hydrocarbons, Unsaturated hydrocarbons, Cycloalkanes, Aromatic hydrocarbons, Aliphatic hydrocarbons, and Alicyclic hydrocarbons. 

Several hydrocarbons, such as propane and butane, are used for commercial fuel purposes, such as LPG or Liquefied Petroleum Gas. Benzene, another important aromatic hydrocarbon, is a raw material for making synthetic drugs. Another hydrocarbon called carotene is an organic pigment commonly found in carrots. 

What are Alkenes? 

Alkenes come from the family of hydrocarbons. Alkenes at least have a single, double bond between the two adjacent carbon atoms – the chemical formula for alkenes CnH2n. There are several methods through which alkenes can be prepared. Some of the most popular methods are discussed here. 

General properties of Alkenes:

Here are some common properties of alkenes: 

• Physical state – In alkenes, the members with two or four carbon atoms are called gases; 5-17 are referred to as liquids, whereas 18+ are known as solids at room temperature. Also, alkenes burn in the air and form a smoky flame. 

• Solubility – Alkenes are soluble in organic solvents, whereas they are insoluble in water. Some examples are benzene and so on. 

• Density – They are lighter than the water. 

• Boiling point – as the chain length or molecular mass increases, alkenes’ boiling points also increase. It shows that intermolecular attractions tend to attain strength as the molecule size increases. 

Preparation of Hydrocarbon – Alkenes in Labs 

Alkenes from alkynes:

To prepare alkenes, alkynes are used quite frequently. Alkyne to alkene conversion is common and is carried out by reducing alkynes with hydrogen when palladised charcoal is present. 

The charcoal used in the process is moderately deactivated through the sulphur and quinoline compounds that ultimately form Alkenes. The alkenes achieved from the reaction above have the cis geometry. The alkynes undergo the reduction process with the sodium in the liquid ammonia to form the trans alkenes. 

Alkenes from alkyl halides:

Alkenes are obtained if the alkyl halides are heated with the alcoholic potash. The alcoholic potash is achieved when potassium hydroxide is dissolved in the alcohol. Here, dehydrohalogenation occurs, which means the single molecule of the halogen acid is eliminated. The reaction rate is mostly dependent on the alkyl group. However, the halogen group’s nature is attached. 

Here is the preparation of Alkenes from alkyl halides:

where X is Cl, Br or I.

Alkenes from vicinal dihalides:

Vicinal Halides are the dihalides where the two adjacent carbon atoms are closely connected with two halogens. Every time such dihalides come in direct contact with the zinc metal, the halogen molecules are lost, forming alkenes. The formation of Alkenes from vicinal dihalides is referred to as dehalogenation. 

Here is the preparation of Alkenes from vicinal dihalides:

Alkenes from alcohols:

When alcohol comes in contact with concentrated sulphuric acid, it forms alkenes because of the elimination of water molecules. Since there are no water molecules here, it is referred to as acidic dehydration of the alcohol. Here, sulphuric acid is the dehydrating agent.

Uses of Alkenes:

The following are the uses of alkenes such as propene, ethene, etc. Let’s have a look:

• Used in manufacturing plastics such as polythene to make buckets, bags, bowls, etc. 

• Used in the manufacturing of polystyrene, which is used to make refrigerator parts, car batteries, etc. 

• Used to make Ethane-1,2-diol for the use in motor car radiators. 

• Used to make synthetic fibre terylene and ethanol. 

• Used to make anti-knock. 

• Used to make plastic to produce packaging material and ropes. 

• Used in making acrylic fibres. 

What are Alkanes? 

Alkanes can be described as an organic compound that possesses single-bonded hydrogen and carbon compounds. CnH2n+2 is the formula of Alkanes. It is further divided into three major groups: cycloalkanes, branched alkanes, and chain alkanes.  

Conclusion:

With this, we come to an end of the topic of preparation of hydrocarbons – alkenes. Alkenes come from the family of hydrocarbons. Alkenes at least have a single, double bond between the two adjacent carbon atoms – the chemical formula for alkenes CnH2n. There are several methods through which alkenes can be prepared. Further, we talked about alkenes’ properties and the preparation of alkenes in detail. 

Several methods are considered for preparing alkenes, such as alkenes from alkanes, alkenes from vicinal dihalides, alkenes from alkyl halides, and alkenes from alcohols which we discussed here. We hope this study material on the preparation of hydrocarbons – alkenes has helped you better understand the preparation of alkenes.