There are four types of reactions:

Substitution Reactions

The reactions in which an atom or group of atoms in a molecule is replaced or substituted by different atoms or group of atoms are called substitution reaction. For example,

These reactions can be of two types:

Nucleophilic Substitution

In this type of substitution, atom or group of atoms in the molecule is replaced by a nucleophile. These can be either S_N1 (substitution, nucleophilic, unimolecular) or S_N2 (substitution, nucleophilic, bimolecular) type.

In the S_N1 reaction only one molecule is involved which dissociates spontaneously to generate a carbocation intermediate. The carbocation reacts with the added nuclophile in a fast step to yield the product.

Intermediate

In the S_N2 reaction the nucleophile attacks the substrate from a position 180^o away from the leaving group and that the reaction takes place in a single step without intermediates.

Transition state

Electrophilic Substitution

In this type of substitution, an electrophile attacks the carbanion of the substrate substituting one of the hydrogens or other groups.

Addition Reactions

Reactions in which two molecules react to form a single product are called addition reactions. For example,

Depending upon the type of attacking reagent, the substitution or addition reactions are also classified as free radical, electrophilic or nucleophilic substitution or addition reactions. For example, if the attacking reagent is nucleophile in addition reaction, the reaction is called nucleophilic addition.

Elimination Reactions

The reactions in which a small molecule is removed from adjacent carbon atoms resulting in the formation of additional (multiple) bond between them are called elimination reactions. For example,

Rearrangement Reactions

In these reactions the migration of atoms or groups of atoms takes place from one position to another within the molecule under suitable conditions.

For example,