1. Introduction

Substitution, addition, and elimination reactions are of great importance in a major branch of chemistry known as Organic Chemistry, which covers the chemistry of compounds of carbon. These reactions, which generally involve covalently bonded molecules, are also found, to a much more limited extent, with other compounds.

2. Substitution reactions

A substitution reaction is a reaction in which an atom (or group of atoms) in a molecule is replaced by another atom or group of atoms:

Example 1

The gas ethane, CH3CH3 reacts with bromine vapour in the presence of light to form bromoethane, CH3CH2Br and hydrogen bromide, HBr. In the process, a hydrogen atom in ethane has been substituted for a bromine atom:

Example 2

Ethanol, CH3CH2OH, reacts with hydrogen iodide, HI, to form iodoethane and water. Here, a group of atoms, OH, has been replaced by an iodine atom:

3. Addition reactions

An addition reaction is a reaction whereby a molecule reacts with another molecule having one or more multiple covalent bonds so as to form a molecule whose molecular mass is the sum of the molecular masses of the reacting molecules:

Example 3

Ethene, CH2=CH2 has a double bond joining the two carbon atoms. This substance can add a hydrogen molecule (in the presence of platinum as catalyst) to form ethane, CH3CH3:

Example 4

Ethyne, C2H2 has a triple bond joining the two carbon atoms. Hydrogen bromide adds onto this triple bond to form 1,1-dibromoethane, CH3CHBr2:

4. Elimination reactions

An elimination reaction is a reaction whereby a multiple covalent bond is formed in a molecule by the removal of another, usually smaller molecule:

Example 5

Ethanol, CH3CH2OH, when treated with concentrated sulphuric acid, H2SO4, loses 2 hydrogen atoms and one oxygen atom, forming ethene, CH2=CH2 and water (the atoms that have been eliminated are shown in red):

Example 6

Bromoethene, CH2=CHBr, when treated with potassium hydroxide dissolved in ethanol, loses one hydrogen atom and one bromine atom, forming ethyne, C2H2 (the atoms that have been eliminated are shown in red):

5. Additional questions