What is an Addition Reaction?

An addition reaction occurs when an unsaturated hydrocarbon reacts with another chemical to produce a single product. Unsaturated hydrocarbons are known for their addition reactions, which include the addition of hydrogen, chlorine, or bromine. 

All unsaturated hydrocarbons with a double or triple bond provide additional reactions as well. In other words, all alkenes and alkynes provide additional reactions. Let’s look at an addition reaction that involves adding hydrogen to unsaturated hydrocarbons with carbon-carbon double bonds.

Addition reaction of ethene with hydrogen

When ethene is heated in the presence of a nickel catalyst, it combines with hydrogen to produce ethene.

CH₂=CH₂      +        H₂            →             CH₃-CH₃          (in the presence of Ni catalyst)
(Ethene)            (Hydrogen)             (Ethane)

In this reaction, one H atom is added to each C atom of the ethene, causing the double bond in the ethene to open up and create a single bond. One molecule of hydrogen is added to an unsaturated hydrocarbon with a double bond to make a saturated hydrocarbon with a single bond, which is called ethane. In general, unsaturated hydrocarbons add hydrogen to produce saturated hydrocarbons in the presence of a catalyst such as nickel (Ni). The process of adding hydrogen to an unsaturated hydrocarbon to form a saturated hydrocarbon is known as hydrogenation. The hydrogenation process takes place in the presence of nickel metals, which act as catalysts.

• Only the addition reaction of hydrogen with unsaturated hydrocarbons has been discussed. Other elements, such as chlorine (Cl₂) and bromine (Br₂), can also add to unsaturated compounds such as alkenes and alkynes. 
• Since bromine is employed as a test for unsaturated chemicals, it is particularly significant. Bromine is employed in the form of bromine water for this purpose. Bromine water is a solution of bromine in water.
• The presence of bromine in bromine water gives it a red-brown colour. When bromine water is added to an unsaturated compound, bromine is added, and the red-brown colour of bromine water is discharged, leaving the unsaturated molecule colourless. 
• As a result, if an organic chemical decolorizes bromine water, it is an unsaturated compound with a double or triple bond. 
• All unsaturated molecules, such as alkenes and alkynes, decolourize bromine water, but saturated compounds, such as alkanes, do not. 
• Since ethene and ethyne are unsaturated chemicals, they decolourize bromine water, but methane and ethane are saturated hydrocarbons and do not decolourize bromine water. 

Test to distinguish between cooking oil and butter.

The bromine water test can spot the difference between cooking oil and butter chemically. In separate test tubes, mix bromine water with a little cooking oil and butter.

• Bromine water decolorizes when cooked in oil, indicating that it is an unsaturated compound.
• Since butter does not discolor bromine water, it is a saturated compound.

Hydrocarbons are the most abundant carbon compounds such as alkanes, alkenes, and alkynes. We’ll now discuss some of the chemical properties of hydrocarbons, which are carbon compounds. Combustion reactions, substitution reactions, and addition reactions are the chemical properties that will be discussed here. All types of hydrocarbons (saturated and unsaturated) can be used in combustion reactions, but only saturated hydrocarbons (or alkanes) can be used in substitution reactions, and only unsaturated hydrocarbons can be used in addition reactions (alkenes and alkynes). These reactions are discussed further below.