Types of organic reactions
A large variety of organic reactions may be placed in the major categories mentioned below
- Rearrangement reaction
- Substitution reaction.
- Addition reaction.
- Elimination reaction.
- Condensation reaction.
- Pericyclic reaction.
- Polymerization reaction.
Let us discuss Rearrangement reactions first in detail.
Define Molecular Rearrangement reaction.
The reactions involving the migration of an atom ( like H) or group (like methyl, ethyl, etc) from one atom to another (i.e. change in the bonding sequence within a molecule) are known as molecular rearrangements or rearrangement reactions.
A rearrangement reaction is an organic reaction in which an atom, ion, group of atoms, or chemical unit migrates from one carbon atom to another carbon atom in the same or different molecule resulting in a structural isomer of the original molecule. The reaction often includes the breaking and/or making of carbon-carbon sigma bonds.
Rearrangements occur to create more stable carbocations or carbanions. Carbocation rearrangements happen very readily and often occur in many organic chemistry reactions.
These rearrangements are roughly classified on the basis of the nature of the migrating group/atom,
i. Nucleophilic or Anionotropic: migrating group migrates with its electron pair.
ii. Electrophilic or cationotropic: migrating group migrates without its electron pair.
iii. Free radical migrating group: migrates with only one electron.
But most commonly found are Nucleophilic or Anionotropic.
Mechanism of Nucleophilic rearrangement.
These reactions consist of three steps;
a) The First step is the generation of the electron-deficient center of the molecule. As the migrating group migrates with electron pair, the migration terminus must have an incomplete octet. This can be obtained in two ways, i. Through carbocation: Carbocations can be formed in various ways. The most common is dehydration of alcohol. This step is similar to that of the SN1 or E1 reaction.
b) Migration: Migrating group migrates to the electron-deficient center with its electron pair creating the new electron-deficient center.
c) In the third step, the newly formed electron-deficient center acquires octet either by accepting a nucleophile or excluding proton. It is observed in many cases that either two or all three steps take place simultaneously. As seen in many cases SN1 type of the first step is commonly followed by rearrangement to give a more stable species.
It is observed in many cases that either two or all three steps take place simultaneously. As seen in many cases SN1 type of the first step is commonly followed by rearrangement to give a more stable carbocation. It is proved by the fact that the rate of reaction increases with the ionizing power of the solvent and it is unaffected by the concentration of the base. It has been shown that the rate of migration increases with the degree of electron deficiency of migration terminus.
Types Rearrangement Reactions
Rearrangements are divided into two types
- intramolecular and
- intermolecular processes.
Intramolecular and intermolecular processes
In the intramolecular process, the group that migrates is not completely detached from the system in which rearrangement is taking place. In contrast, in the intermolecular process, the migrating group is first detached and later re-attached at another site.
Some general but important trends in the migrating aptitude of different groups. Aryl groups exhibit a higher propensity for migration than that alkyl groups. The migratory aptitude of hydrogen is unpredictable. Hence, the mixture of migrated products is obtained. In the case of aryl groups, those with electron-donating substituents at the meta or para positions migrates preferentially over those containing substituents at the ortho position. Aryl group containing electron-withdrawing groups shows reduced migratory aptitude.
Examples of Rearrangement Reaction- There are several types of rearrangement reactions in organic chemistry. Some of the well-known rearrangement reactions in chemistry are
- Beckmann Rearrangement
- Hoffman Rearrangement.
- Pinacol–pinacolone rearrangement
- Benzilic acid rearrangement etc..
You can click on the names mentioned above to know the principle and the mechanism of these reactions in detail. All above reactions are explained broadly to understand the concept easily.
Practical examples of Rearrangement Reaction
- But-1-ene is the expected dehydration product in this reaction. However, but-2-ene is formed as a result of a rearrangement reaction. As a result of rearrangement, the less stable primary carbocation has been converted to the more stable secondary carbocation.
2. Wohler’s synthesis of urea from ammonium cyanate is also an example of a rearrangement reaction.
a. In which reaction(s) migrating group migrates to electron-deficient nitrogen atom?
In Beckmann rearrangement migrating group migrates to the electron-deficient nitrogen atom.
b. In which reactions rearrangement of the carbon skeleton of the substrate is possible?
Rearrangement of the carbon skeleton of the substrate is possible only in the reactions which involve the formation of a carbocation.
c. Which types of isomers are formed in rearrangement reactions?
Structural isomers because products formed have the same molecular formula, but their atoms have different arrangements or bonds.
d. What are addition reactions?
A reaction in which two or more molecules combine with another to form a larger molecule with no additional products is called an addition reaction. Example polymerization.