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Difference Between Homolytic and Heterolytic Cleavage

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Last updated date: 17th Apr 2024
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Unlocking the Potential of Concentration Chemistry

In the majority of chemical reactions, existing chemical bonds are broken and new ones are created. Chemical bonds can be broken in a variety of ways, though. The way a chemical bond breaks also has a significant impact on how the chemical reaction will turn out in the end. Bond fission is a common term for the dissolution of a chemical bond, most frequently a covalent bond. Homolytic fission and heterolytic fission are the two main forms of bond fission. Let us discuss what is homolytic and heterolytic cleavage are in greater detail:

What is Homolytic Cleavage: An In-Depth Analysis

When a particular molecule is dissociated through homolytic fission (also known as hemolysis), one electron is preserved by each of the original components of the molecule. Due to each chemical species retaining one electron from the bond pair, when a neutrally charged molecule undergoes homolytic fission, two free radicals are produced as the end result. 


In other words, when one electron is transferred from one atom to another, a covalent bond is broken, a process known as homolytic cleavage. This means, it occurs when two atoms equally share electrons. This particular sort of fission can produce two distinct molecules or a new molecule where there are more than two atoms.


A covalent bond breaks symmetrically during homolytic cleavage, leaving one electron per connected atom. It is indicated by a fish hook arrow with a half-head. This kind of cleavage takes place at high temperatures or when UV light is present in a molecule that has non-polar covalent bonds made between atoms with equal electronegativity. In these compounds, free radicals are created when bonds are broken. They live for a brief time and are quite receptive. An environment that is rich in electrons, which can encourage the cleavage of covalent bonds, also causes homolytic cleavage. This kind of cleavage is frequently observed in molecules with a high electron density, such as those with several double bonds or lone pairs of electrons. Alkyl free radicals are produced in organic processes via homolytic fission of C-C bonds.


Homolytic Cleavage

Homolytic Cleavage


Heterolytic Cleavage: A Rigorous Study

A type of bond fission known as heterolysis, also known as heterolytic fission, occurs when a covalent bond between two chemical species is broken unevenly, causing one of the chemical species to retain the bond pair's electrons. One of the products of heterolytic fission has a positive charge(carbocation), whereas the other has a negative charge(carboanion). This is an unsymmetrical bond fission.


To put it in a more simplified manner, heterolytic cleavage occurs when a covalent bond breaks asymmetrically, leaving one of the linked atoms with the bond pair of electrons. This reaction produces both an anion and a cation. A curving arrow pointing in the direction of the more electronegative atom denotes the cleavage. 


Heterolytic cleavage of methyl chloride

Heterolytic cleavage of methyl chloride


Bond Dissociation Energy: Demystifying the Energy Concept 

Calculating the strength of a chemical bond between two species can be done using bond dissociation enthalpy, also known as bond dissociation energy. Although the homolytic bond dissociation energy of a covalent bond is measured at absolute zero (0K)as the enthalpy change, it is typically defined as the enthalpy change of the homolytic fission of the bond at absolute zero (0K)


The concept of bond dissociation enthalpy  has the following important components:


  • It is the minimum amount of energy needed to disintegrate a chemical link between two species.

  • It is a method of assessing the strength of a chemical bond.

  • In diatomic molecules, it has a value equal to the bond energy. For example Cl-Cl bond. The amount of energy required to break this bond is nothing but the bond energy itself. This is approximately 59 kcalmol-1.

  • Weak bond dissociation energies is an important characteristic exhibited by covalent bonding between atoms or molecules.


Characteristics of Homolytic and Heterolytic Cleavage: 

The electrons of covalent bond are evenly dispersed among its constituent atoms during the homolytic cleavage. One of the two atoms undergoes heterolytic cleavage, receiving both electrons, and typically acquiring a negative charge. Without receiving any electrons, the other atom normally acquires a positive charge. During homolytic cleavage, free radicals, which are entities carrying an unpaired electron, are created. Ions, or species with a positive or negative charge, are created as a result of heterolytic cleavage.


Half-headed arrows are used in the curved arrow formalism to represent the motion of individual electrons. Full-headed arrows show the motion of electron pairs. 


The bound species with the higher electronegativity is often the one that retains the bond pair of electrons and acquires a negative charge when a covalent bond is subjected to heterolytic fission. The more electropositive species, on the other hand, typically does not hold onto any electrons and develops a positive charge.


The heterolytic bond dissociation energy is frequently used to describe the energy needed to break a covalent bond by heterolytic cleavage. This number is occasionally used to represent a covalent bond's bond energy. 


Differentiate Between Homolytic and Heterolytic Cleavage: 


S.No

Category

Homolytic Cleavage

Heterolytic Cleavage


Definition

Homolytic cleavage is the breaking of a covalent bond in symmetrical fashion to share two electrons equally to two atoms or molecules. 

Heterolytic cleavage is the breaking of bond in an asymmetrical fashion where the more electronegative atom/molecule(carboanion) gets electrons and the electropositive atom/molecule(carbocation) gets no electron. 


Product

Usually form two symmetrical fragments.

Generally forms two asymmetrical fragments.


Bond electrons

Gives one bond electron each to one fragment as a result forming free radicals which are unstable.

Gives two bond electrons to the more electronegative fragment.


Type of energy

The energy required to form homolytic cleavage is called homolytic bond dissociation energy 

The energy for a heterolytic cleavage is called heterolytic bond dissociation energy.


Curved arrow formalism

Half headed arrows are used to show a homolytic bond cleavage. This shows electrons are shared between two fragments

Full headed arrow is utilized in heterolytic bond cleavage to show that a more electronegative fragment gets the electrons.


Summary

Numerous chemical reactions are caused by homolytic cleavage, which is a significant component of chemistry. It is a procedure that results in the production of free radicals by breaking the link between two atoms. Small molecules frequently undergo this sort of fission, whereas large molecules frequently undergo heterolytic fission. Homolytic and heterolytic cleavage examples are discussed in detail. Examples of homolytic cleavage include the splitting of water into hydrogen and oxygen radicals, the dissolution of chlorine gas into water to produce hydrochloric acid, and the reaction of sodium metal with water to produce sodium hydroxide and hydrogen gas.


Examples of heterolytic fission include heterolytic cleavage of methyl chloride or tert-butyl bromide in the presence of water. One can better comprehend how these reactions take place if they can differentiate between homolytic and heterolytic cleavage.

FAQs on Difference Between Homolytic and Heterolytic Cleavage

1. What is homolytic and heterolytic cleavage?

In moving a single electron from one atom to another, a covalent bond can be broken by a process known as homolytic cleavage using external conditions and freed radical initiators. Homolytic cleavage occurs when two atoms share electrons evenly, in other words, this fission can produce two same molecules or a new, larger molecule with more atoms. Whereas in a heterolytic cleavage, when two atoms A and B form a covalent bond, heterolytic cleavage happens when one of the connected atoms takes away both of the shared pair of electrons from the covalent bond.

2. What are free radical initiators in a homolytic cleavage?

The substances that encourage homolytic cleavage in the compound are free radical initiators. Azobisisobutyronitile (AIBN), a free radical initiator, and peroxides like benzoyl peroxide are used in polymerization processes where homolytic cleavage is required. Free radicals with unpaired electrons are neutral and unstable, thus they seek an electron to stabilise themselves. Chemical species known as radical initiators swiftly generate free radicals when exposed to light and heat. Halogens (Cl2), acetophenone, benzophenone, etc. are a few examples. The energy required to break a bond is sufficiently present under these benign circumstances.

3. Why is a polar solvent required for heterolytic cleavage?

Polar solvents tend to have a very high dielectric constant(Dielectric constant denotes the capacity to reduce the electrostatic forces between charges) which result in the formation of A+ and B- ions effectively in the solvent. Also it is usually noted that in polar solvents, enthalpy of solvation due to high dielectric constant of the solvent gains predominance. By stabilising the generated charges in the solvent, the enthalpy of solvation makes it easier for charges to form. Enthalpy of solvation is the amount of energy released when a solute dissolves in solvent.