A chemical reaction is a process that always results in the conversion of reactants into product or products. The substance or substances initially involved in a chemical reaction are called reactants. A type of a chemical reaction is usually characterized by the type of chemical change, and it yields one or more products which are, in general, different from the reactants.

Generally speaking, chemical reactions encompass changes that strictly involve the motion of electrons in the forming and breaking of chemical bonds. Chemical equations are often used to describe the chemical transformations of elementary particles that occur during the reaction.

Chemical changes are a result of chemical reactions. All chemical reactions involve a change in substances and a change in energy. However, neither matter nor energy is created or destroyed in a chemical reaction. There are so many chemical reactions that it is helpful to classify them into different types including the widely used terms for describing common reactions.

Combination reaction or synthesis reaction: it is a reaction in which 2 or more chemical elements or compounds unite to form a more complex product.

Example: N2 + 3 H2 ' 2 NH3

Isomerisation reaction: is a reaction in which a chemical compound undergoes a structural rearrangement without any change in its net atomic composition.

Example: trans-2-butene and cis-2-butene are isomers.

Chemical decomposition reaction or analysis: is a reaction in which a compound is decomposed into smaller compounds or elements:

Example: 2 H2O ' 2 H2 + O2

Single displacement or substitution: this type of reaction is characterized by an element being displaced out of a compound by a more reactive element.

Example: 2 Na(s) + 2 HCl(aq) ' 2 NaCl(aq) + H2(g)

Metathesis or Double displacement reaction: represents a reaction in which two compounds exchange ions or bonds to form different compounds

Examples: NaCl(aq) + AgNO3(aq) ' NaNO3(aq) + AgCl(s)

Acid-base reactions: broadly these reactions are characterized as reactions between an acid and a base, can have different definitions depending on the acid-base concept employed. Some of the most common are:

Arrhenius definition: Acids dissociate in water releasing H3O+ ions; bases dissociate in water releasing OH- ions.

Brønsted-Lowry definition: Acids are proton (H+) donors; bases are proton acceptors.

Lewis definition: Acids are electron-pair acceptors; bases are electron-pair donors.
Example: HCl(aq) + NaOH(aq) ' NaCl(aq) + H2O(l)

Redox reactions: are reactions in which changes in oxidation numbers of atoms in involved species occur. Those reactions can often be interpreted as transfer of electrons between different molecular sites or species.

Example: 2 S2O32(aq) + I2(aq) ' S4O62(aq) + 2 I(aq)
In this case, I2 is reduced to I- and S2O32- (thiosulfate anion) is oxidized to S4O62-.

Combustion reaction: it is a kind of redox reaction in which any combustible substance combines with an oxidizing element, usually oxygen, to generate heat and form oxidized products.

Example: C3H8 + 5 O2 ' 3 CO2 + 4 H2O

Other types of chemical reactions include organic reactions which are found in organic chemistry.
Organic reactions compose a wide variety of reactions involving compounds which have carbon as the main element in their molecular structure. In opposition to inorganic reactions, organic chemistry reactions are classified in large part by the types of the functional groups that exist within each compound. In this case the reactions are described by showing the mechanisms through which the changes take place.

Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are listed bellow:

- Addition reactions
- Elimination reactions
- Substitution reactions
- Redox reactions
- Rearrangement reactions

- Pericyclic reactions

The general form of the SN2 mechanism for example is as follows:

Where nuc: = nucleophile
X = leaving group (usually halide or tosylate, mesylate)

Example of hydroxide ion that acts as the nucleophile and bromine is the leaving group

This results in the inversion of the configuration because of the backside attack of the nucleophile.

The solvent type, the electrophile and the leaving group, all play an important role in this type of reaction:

Solvents: protic solvents such as water and alcohols stabilize the nucleophile so much that it will not react with substrate. Therefore, the use of a good polar aprotic solvent such as ethers and ketones and halogenated hydrocarbons is required.

Nucleophiles: A good nucleophile is required since it is involved in the rate determining step. A weak nucleophile will not efficiently attack the substrate.

Leaving groups: A good leaving group is required, such as a halide or a tosylate, since it is involved in the rate determining step (better leaving group for halogens: I>Br>Cl>F)
In organic synthesis, organic reactions are used in the construction of new organic molecules. The production of many man-made chemicals such as drugs, plastics, food additives, fabrics depend on organic reactions.

Among these, the oldest organic reactions are combustion of organic fuels and saponification of fats to make soap. Modern and advance organic chemistry starts with synthesis of terpenes, carbohydrates, proteins, steroids and polymerization reactions in the eighteen century. In the history of the Nobel Prize in Chemistry, awards have been given for the invention of specific organic reactions such as the Grignard reaction in 1912, the Diels-Alder reaction in 1950, the Wittig reaction in 1979 and olefin metathesis in 2005.