Types of Organic Reactions and their Examples

The organic compound undergoes chemical changes (transfer of groups) to attain stability. These chemical changes are known as organic reactions. There are eight main types of organic reactions. They are named substitution reaction, addition reaction, elimination reaction, rearrangement reaction, condensation reaction, pericyclic reaction, polymerization reaction, and oxidation-reduction reaction. They are further divided into subtypes.

Types of organic reactions

1. Substitution reactions

Substitution reactions are also known as displacement reactions. In these reactions, one of the atoms or groups is replaced by another atom or group and gives new products.

Types of substitution reactions

Substitution reactions are divided into three types based on types of reactants (substrate):

  • Nucleophilic substitution reactions
  • Electrophilic substitution reactions
  • Free radical substitution reactions

Nucleophilic substitution reactions

In the nucleophilic substitution reactions, the attacking reagent (Nu) having lone pair of electron(s) attacks and forms a new chemical bond by replacing the nucleofuge (leaving group). Nucleophile comes with pair of electrons and leaving group goes away with the electron pair.

In general nucleophile attacks, form a new bond, and the leaving group leaves.

neuclophilic substitution reaction mechanism

Types of nucleophilic substitution reactions

Depending upon substrate, nucleophilic substitution reactions are classified into

  1. SN1 reactions
  2. SN2 reactions
  3. SNI reactions
  4. SNAr reaction

Electrophilic substitution reactions

In electrophilic substitution reactions, electrophiles replace the leaving group. Here electron-deficient species (electrophile) attack an electron-rich substrate and replace the electrofuge (leaving group). In electrophilic substitution reactions often the leaving group is the H+ ion.

Electrophilic substitution reactions

Types of electrophilic substitution reactions

Depending upon the type of substrate, electrophilic substitution reactions are classified as:

  1. Aliphatic electrophilic substitution reactions
  2. Aromatic electrophilic substitution reactions

Free radical substitution reactions

In a free radical substitution reaction, a free radical attack a substrate and replace the leaving group. In such reactions, attacking species and leaving species both are free radicals. They are also named chain reactions. They proceed in the following three steps:

  • Initiation
  • Chain propagation
  • Chain termination

free radical reactions

2. Addition reactions

Addition reactions are given by molecules having double or triple bonds. Two or more molecules combine and give new products.

Types of addition reactions

Addition reactions are classified into four different classes

  1. Electrophilic addition reactions
  2. Nucleophilic addition reactions
  3. Free radical addition reactions
  4. Cyclic addition reactions

Electrophilic addition reactions

In electrophilic addition reactions, an electrophile (positive species) approaches the double or triple bond. In the first step, it converts the pi bond (ฯ€) to the sigma bonds (ฯƒ).

In the second step, an electron-rich species approaches the positive center (usually a carbocation) and give an addition product.

Electrophilic addition reactions

Nucleophilic addition reactions

Nucleophilic addition reactions are the opposite of electrophilic addition reactions. Here a nucleophile converts the pi bond to a sigma bond. So, the first step is the attack of electron-rich species (Nu) on the double or triple bonds of a carbon atom which results in the generation of carbanion. The second step is the combination of carbanion with positive species forming an addition product.

Nucleophilic addition reactions

Free radical addition reactions

In these reactions, free radical converts the pi bond to the sigma bonds. Free radical addition reaction leads to polymerization. As discussed earlier free radical reactions are named as chain reactions and proceed in three steps.

  • Initiation
  • Chain propagation
  • Chain termination

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Cyclic addition reactions

Cyclic addition reactions are the single steps addition reactions (simultaneous addition). In these reactions, the initial attack is not on the one-carbon of double or triple bond but both carbon atoms are simultaneously attacked. This leads to a cyclic transition state.

Cyclic addition reactions

3. Elimination reactions

Elimination reactions are opposite to the addition reactions. Sigma bonds are converted into the pi bond. This happens when two or more atoms or groups of atoms are removed to form a double or triple bond.

Types of elimination reactions

    • ๐œถ Elimination reactions
    • ๐œท Elimination reactions
    • ๐œธ Elimination reactions
    • Conjugate Elimination reactions
    • Pyrolytic Elimination reactions
    • Extrusion reactions

๐œถ Elimination reactions

In ๐œถ elimination reactions both the groups are removed from the same atom. This results in the formation of carbene in the case of carbon and nitrene in the case of nitrogen.

๐œท Elimination reactions

In ๐œท elimination reactions, two groups are removed from adjacent carbon atoms. One is from ๐œถ carbon and the second is from ๐žซ carbon. This is the most common type of elimination reaction.

๐œท elimination reactions can proceed in a single step (E2) as well as in two steps (E1).

๐œท elimination reactions

Types of ๐žซ Elimination reactions

๐žซ elimination reactions are subdivided into three types

  • E1 elimination reactions (unimolecular)
  • E2 elimination reactions (bimolecular)
  • E1cB elimination reactions

๐œธ Elimination reactions

๐œธ elimination reactions make a three-membered ring structure. Here one atom is removed from ๐œถ carbon and the other is removed from ๐œธ carbon. Three-member rings are very unstable due to angle strain.

Extrusion reactions

Extrusion reactions involve the removal of a fragment from within a chain or a ring.

Conjugate Elimination reactions

They involve the removal of atoms from the conjugate system. The conjugate system has alternate single and double bonds.

Pyrolytic elimination reactions

As the name pyrolytic eliminations show that these elimination take place due to heating. There are several compounds that undergo elimination on heating when no other reagent is present. They are often in the gas phase. Such reactions are called pyrolytic elimination reactions.

4. Rearrangement reactions

The reactions in which an atom or group of atoms moves from one atom to another atom within a molecule are called rearrangement reactions. They result in the formation of structural isomers of original molecules. Connectivities of groups changes within the molecule but most of the migrations are from one atom to an adjacent atom and are called 1,2 shifts. However, there can be some migrations over a long distance.

Rearrangement reactions

Types of rearrangement reactions

  • Nucleophilic rearrangement reactions
  • Electrophilic rearrangement reactions
  • Free radical rearrangement reactions

Nucleophilic rearrangement reactions

Nucleophilic rearrangement reactions are the most common rearrangement reactions and are also named as anionotropic rearrangements. In these rearrangements, the migrating group is a nucleophile. It will move with its electron pair.

Electrophilic rearrangement reactions

Nucleophilic rearrangement reactions are also named as cationotropic rearrangements or prototropic rearrangements. In these rearrangements, the migrating group is an electrophile. It will move without its electron pair.

Free radical rearrangement reactions

Free radical rearrangement reactions are not common. In these rearrangements, the migrating group is a free radical. It will move with just one electron.

5. Condensation reactions

Condensation reactions are the type of organic reaction that involves the combination of two molecules to form a single product by the loss of small molecules. In most cases, a water molecule is removed during condensation reactions. In the case of water molecules, these reactions are also called dehydration reactions.

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There are certain other small molecules that can be removed during condensation reactions, i.e ammonia, ethanol, acetic acid, etc. Well known condensation reactions are:

  • Aldol condensation
  • Claisen condensation
  • Deckman condensation
  • Knoevenagel condensation

6. Pericyclic reactions

These are the type of organic reactions that requires light or heat to proceed and have a concerted mechanism. The bond breaking and bond, formation takes place simultaneously.

These reactions have a single transition state with cyclic geometry and are also known as nonionic reactions as no intermediate is formed during the chemical reaction. They are highly stereospecific.

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Types of pericyclic reactions

Pericyclic reactions are classified into 7 different types:

  • Cycloaddition reactions
  • Electrocyclic reactions
  • Sigmatropic reactions
  • Group transfer reactions
  • Ene reactions
  • Cheletropic reactions
  • Dyotropic reactions

7. Polymerization reactions

These are the organic reactions that involve the formation of larger molecules called a polymer. Polymers consist of a large number of small repeating units called monomers. These repeating units combine chemically to produce a polymer.

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Polymerization reactions are also called as chain reactions. They proceed in three steps:

  • Initiation
  • Chain propagation
  • Chain termination

Types of polymerization reactions

These are classified into two types

  • Addition polymerization
  • Condensation polymerization

8. Oxidation-reduction reactions

Oxidation-reduction reactions are also called redox reactions. They involve the loss or gain of electrons as a result of which the oxidation state of species changes. Oxidation and reduction take place simultaneously. One of the molecules gets oxidized while the other gets reduced at the same time.

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Although oxidation and reduction are concurrent, still we classify them separately depending on whether an organic compound is reduced or oxidized.

Concepts Berg

What is meant by organic reactions?

The organic reaction is the process that involves the conversion of organic reactant to product.

Is sublimation an organic reaction?

No, sublimation is not an organic reaction, rather it is a purification technique in which solid objects are directly converted to gas form without passing through a liquid stage.

What are examples of organic solvents?

Commonly used organic solvents are:

  • Acetone
  • Acetonitrile
  • Ethanol
  • Methanol
  • Tetrahydrofuran (THF)
  • Dimethyl Formamide (DMF)
  • Dimethylsulfoxide (DMSO)
  • Glacial Acetic acid
  • Toluene
  • Ammonia
  • Dichloromethane (DCM)
  • Benzene

What are some examples of strong organic acids?

Sulfonic acids are among the strongest organic acids: Trifluoromethanesulfonic acid, benzene sulfonic acid, methanesulfonic acid, toulenesulfonic acid, etc.

What are the different types of hydrocarbons?

Hydrocarbons are classified into two main branches which are further classified into subclasses:

Open chain (acyclic compounds)

  • Straight chain
  • Branched-chain

Closed chain (cyclic compounds)

  • Homocyclic compounds
    • Alicyclic compounds
    • Aromatic compounds
  • Heterocyclic compounds
    • Saturated compounds
    • Unsaturated compounds

Is tautomerism a rearrangement reaction?

Yes, tautomerism is a type of electrophilic rearrangement reaction.

Can alkyne give nucleophilic addition reactions?

Yes, alkynes can give nucleophilic addition reactions because of greater acid character due to sp hybridization.

What reactions occur by a carbene intermediate?

๐œถ elimination reactions proceed with the formation of carbene intermediates. In ๐œถ elimination reactions both the groups are removed from the same carbon atom. This results in the formation of carbene.

References Books

  • Textbook of Jerry March โ€œAdvanced Organic Chemistryโ€ by John Wiley and Sons Inc. 4th Edition, 2000.
  • Textbook of Thomas H. Lowry and Kathleen S. โ€œMechanism and Theory in Organic Chemistryโ€ New York. 3rd Edition, 1987.

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