15     Types of chemical reaction

15.1   General types

1        Combination

This is when two or more elements or compounds unite to form a single compound. For example:

2Mg + O₂ ® 2MgO

KCN + S ® KSCN

K₂O + SO₃ ® K₂SO₄

2        Decomposition

This is when a compound breaks up into two or more elements or simpler compounds. For example:

2H₂O₂ ® 2H₂O  + O₂

NH₄NO₃ — heat ® 2H₂O + N₂O

2FeSO₄ — heat ® Fe₂O₃ + SO₃ + SO₂

A reversible decomposition is called a “dissociation”. Examples:

PCl₅  PCl₃ + Cl₂

2HI  H₂ + I₂

NH₄Cl  NH₃ + HCl

3        Displacement

This is when one element replaces another in a compound, or one compound replaces another in a larger compound. For example:

Fe₂O₃ + 2Al ® 2Fe + Al₂O₃

Zn + CuSO₄ — aq ® Cu + ZnSO₄

4HNO₃ + P₄O₁₀ ® 4HPO₃ + 2N₂O₅

4        Double decomposition

This is when two compounds interact by exchange of parts to produce two new compounds. The parts may be atoms or radicals. Examples:

AgNO₃ + NaCl — aq ® AgCl + NaNO₃

PCl₃ + 3AgF ® PF₃ + 3AgCl

SnCl₄ + 4PhMgCl ® SnPh₄ + 4MgCl₂

5        Addition

This is combination viewed from the point of view of one of the reactants, which has further atoms or groups of atoms added to it. For example:

C₂H₄ + Br₂ ® C₂H₄Br₂

2PCl₃ + O₂ ® 2POCl₃

CuSO₄ + 5H₂O ® CuSO₄·5H₂O

In each example, the first reactant is said to have atoms or groups of atoms from the second added to it.

6        Substitution

This is displacement or double decomposition viewed from the point of view of one of the reactants, in which one atom or radical is replaced by another atom or radical. For example:

CH₄ + Cl₂ ® CH₃Cl + HCl

C₆H₆ + HNO₃ ® C₆H₅NO₂ + H₂O

These are double decompositions:

CH₃–H + Cl–Cl ® CH₃–Cl + H–Cl

C₆H₅–H + HO–NO₂ ® C₆H₅–NO₂ + H–OH

7        Insertion

This is an addition reaction in which an atom or group of atoms is inserted between two atoms initially bound together. An example is the reaction

which takes place when R is one of a number of transition-metal radicals, e.g. Mn(CO)₅.

8        Isomerization

This is when a substance changes into another form with different properties but the same molecular formula. For example:

CH₂=CH–CH₂–CH₃ — catalyst ® CH₃–CH=CH–CH₃

red  HgI₂ — heat ® yellow HgI₂

The different forms are called “isomers” (Greek isos, equal), and the phenomenon of the existence of different forms “isomerism”.

A rapidly reversible isomeric change is called a “tautomeric” change. The different forms are called “tautomers”, and the phenomenon “tautomerism”.

An isomeric change in the case of an element is called an “allotropic” change. The different forms are called “allotropes”, and the phenomenon “allotropy”.

9        Polymerization

This is when a substance changes into another substance with the same composition but a much higher molecular mass. For example:

nC₂H₄ — heat, pressure, catalyst  ®  (–CH₂–CH₂–)_n

The product of such a reaction is called a “polymer”, and the starting material the corresponding “monomer” (Greek polus, much; meros, share; monos, alone).

The term polymerization is also used for processes in which a polymer is formed, not from the monomer, but from other reactants of low molecular mass. This usage is somewhat misleading, but is well established.

10      Oligomerization

This is similar to polymerization except that the product contains only a small number of monomer units (Greek oligos, small). For example

3C₂H₂ — heat, catalyst ® C₆H₆

The degree of oligomerization is specified by the numerical prefixes di, tri, etc., as in dimer, trimerize, etc.