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.
Types of nucleophilic substitution reactions
Depending upon substrate, nucleophilic substitution reactions are classified into
- SN1 reactions
- SN2 reactions
- SNI reactions
- 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.
Types of electrophilic substitution reactions
Depending upon the type of substrate, electrophilic substitution reactions are classified as:
- Aliphatic electrophilic substitution reactions
- 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
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
- Electrophilic addition reactions
- Nucleophilic addition reactions
- Free radical addition reactions
- 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 (π) to the sigma bonds (σ).
In the second step, an electron-rich species approaches the positive center (usually a carbocation) and give an addition product.
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.
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
()
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.
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
-
- 𝜶 Elimination reactions
- 𝜷 Elimination reactions
- 𝜸 Elimination reactions
- Conjugate Elimination reactions
- Pyrolytic Elimination reactions
- Extrusion reactions
𝜶 Elimination reactions
In 𝜶 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.
𝜷 Elimination reactions
In 𝜷 elimination reactions, two groups are removed from adjacent carbon atoms. One is from 𝜶 carbon and the second is from 𝞫 carbon. This is the most common type of elimination reaction.
𝜷 elimination reactions can proceed in a single step (E2) as well as in two steps (E1).
Types of 𝞫 Elimination reactions
𝞫 elimination reactions are subdivided into three types
- E1 elimination reactions (unimolecular)
- E2 elimination reactions (bimolecular)
- E1cB elimination reactions
𝜸 Elimination reactions
𝜸 elimination reactions make a three-membered ring structure. Here one atom is removed from 𝜶 carbon and the other is removed from 𝜸 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.
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.
()
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.
()
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.
()
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.
()
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?
𝜶 elimination reactions proceed with the formation of carbene intermediates. In 𝜶 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 “Advanced Organic Chemistry” by John Wiley and Sons Inc. 4th Edition, 2000.
- Textbook of Thomas H. Lowry and Kathleen S. “Mechanism and Theory in Organic Chemistry” New York. 3rd Edition, 1987.
Reference Links
- Types of organic reactions (vedantu.com)
