SUBSTITUTION, ADDITION, AND ELIMINATION REACTIONS

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Introduction Substitution reactions Addition reactions Elimination reactions Additional questions	Introduction to organic chemistry Systematic naming of organic compounds	Data Glossary
Learning Outcomes
After studying this section, you will understand what is meant by substitution, addition, and elimination reactions

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.

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 Here, a group of atoms, OH, has been replaced by an iodine atom:

Example 3:

Benzene, C₆H₆, reacts with bromine in the (presence of iron bromide as catalyst) to form bromobenzene, C₆H₅Br. This results in a hydrogen atom being replaced by a bromine atom:

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 5:

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

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 6:

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 7:

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

Additional questions