Addition Reactions

What are Addition Reactions?

Addition reactions are a fundamental type of chemical reaction in organic chemistry, particularly involving unsaturated compounds like alkenes.

In these reactions, atoms or groups of atoms are added to the double or triple bonds of molecules, converting them into saturated compounds.

Alkenes, which contain a carbon-carbon double bond ($C=C$), are especially reactive in addition reactions, making them valuable in both chemical and industrial processes.

Addition Reactions of Alkenes

Alkenes readily undergo addition reactions with a variety of substances, including hydrogen ($H₂$), halogens ($Cl₂$, $Br₂$), hydrogen halides ($HCl$, $HBr$), and water ($H₂O$).

Here are the main types of addition reactions that alkenes, like ethene ($C₂H₄$), participate in:

Hydrogenation (Addition of Hydrogen)

• In hydrogenation, hydrogen ($H₂$) is added across the double bond of an alkene, converting it into an alkane.
• Industrial Importance: Hydrogenation is widely used to convert unsaturated vegetable oils into margarine or other solid fats, a process known as the hardening of oils.

Halogenation (Addition of Halogens like Chlorine or Bromine)

• Alkenes react with halogens ($Cl₂$ or $Br₂$) to form dihalides.

• Bromine Test: This reaction can be used as a test for unsaturation (double bonds) because bromine water changes from brown to colourless when it reacts with an alkene.

Hydrohalogenation (Addition of Hydrogen Halides like $HCl$ or $HBr$)

• When an alkene reacts with hydrogen chloride ($HCl$), it forms a haloalkane.
• Ethene reacts with $HCl$ to form chloroethane ($C₂H₅Cl$). Markovnikov's Rule: In asymmetrical alkenes, the hydrogen atom attaches to the carbon with more hydrogen atoms, and the halogen attaches to the other carbon.

Hydration (Addition of Water)

• In the presence of an acid catalyst, alkenes can react with water to form alcohols.

Mechanism of Ionic Addition

In ionic addition reactions, such as the addition of hydrogen chloride ($HCl$), bromine ($Br₂$), or chlorine ($Cl₂$) to ethene, the reaction follows an electrophilic addition mechanism:

Step 1: The alkene's $π$-bond is broken as it reacts with an electrophile (such as $H⁺$or $Br^+$), forming a carbocation intermediate.

Step 2: The carbocation then reacts with a nucleophile (such as $Cl^-$, $Br^-$, or $OH^-$) to form the final product.

Evidence of the Mechanism

The reaction of ethene with bromine water in the presence of sodium chloride provides evidence for this mechanism. This reaction forms:

• 2-bromoethanol ($C₂H₄BrOH$),
• 1-bromo-2-chloroethane ($C₂H₄BrCl$),
• 1,2-dibromoethane ($C₂H₄Br₂$). The formation of multiple products indicates that nucleophiles such as $Br^-$ and $Cl^-$ are involved in the reaction, supporting the idea of a carbocation intermediate.

Industrial Importance of Addition Reactions

Addition reactions involving alkenes are critical in the petrochemical industry for producing a wide variety of industrial products.

Addition of Chlorine, Bromine, Water, and $HCl$ to Ethene

• Chloroethane ($C₂H₅Cl$): Formed by the addition of $HCl$ to ethene, chloroethane is used in the production of plastics, particularly polyvinyl chloride ($PVC$).
• 1,2-Dichloroethane ($C₂H₄Cl₂$): Produced by the addition of chlorine to ethene, it is a key precursor in the manufacture of PVC plastics.
• Ethanol ($C₂H₅OH$): Produced from the hydration of ethene, ethanol is used as a solvent, fuel, and raw material in chemical synthesis.

Hydrogenation of Vegetable Oils

This reaction is used to solidify unsaturated vegetable oils by adding hydrogen across the double bonds, producing solid fats used in margarine and shortening. It also increases the shelf life of products by reducing the number of reactive double bonds.

Polymerisation of Alkenes

In addition to the simple addition reactions discussed, alkenes can also undergo polymerisation to form long chains of repeating units.

Ethene ($C₂H₄$) and propene ($C₃H₆$) are the two main raw materials used in the production of plastics:

• Polyethene (PE): Made from ethene, it is used in plastic bags, bottles, and containers.
• Polypropene (PP): Made from propene, it is used in packaging, textiles, and automotive parts.

Unreactivity of Benzene in Addition Reactions

While alkenes are highly reactive in addition reactions, benzene, an aromatic compound, is relatively unreactive due to the stability of its delocalized $π$-electrons.

Unlike ethene, benzene undergoes substitution reactions rather than addition because breaking its stable π-electron cloud requires a significant amount of energy.

Scope of the Petrochemical Industry

The petrochemical industry relies heavily on addition reactions for the production of synthetic products.

Two major categories of products are:

1. Plastics: Polyethylene and polypropylene, produced from the polymerisation of ethene and propene, are essential in the production of packaging, containers, and textiles.
2. Pharmaceuticals: Addition reactions are used in the synthesis of various drug molecules, including pain relievers and antibiotics.