Homolytic and Heterolytic Fission

Introduction

In a chemical reaction, the reagents break the bond and forms free radicals to form a new bond. The covalent bond is formed by sharing the electrons of two atoms to form a new molecule. In a chemical reaction, the old bonds are broken, and new ones are formed. Two different methods do this breaking of bonds. The first one is homolytic fission in which the covalent bond is broken, and the electron is taken by both the reagents. In heterolytic fission, the covalent bond is broken in such a way that both the electron of the covalent bond is taken by one reagent and forms anions and cations.

What is Homolytic?

‘Homo’ means the same. In homolytic fission, the chemical bond is broken in such a way that it forms two equal parts, meaning both the atoms carry the same electrons on separation. They form highly reactive free radicals with an equal number of free electrons.

• This type of fission happens in molecules in non-polar bonds.

• The homolytic fission is done under specific conditions under UV light and high temperatures.

• In this process, the energy is either released or absorbed.

• In some reactions, the covalent bonds break under slight heat like in peroxides the covalent bond break on slightly heating the molecule.

Examples

Here are some examples of molecules in which homolytic fission happens.

• One example of homolytic fission is a chlorine-chlorine bond in peroxides. These bonds break on slightly low heat, as they are weak intramolecular bonds. When we heat the chlorine molecule it converts into free radicals of chlorine atoms.

• Another example of homolytic fission is seen in ethylene molecules.

What is Heterolytic Fission?

The term ‘heterolytic’ means unequal and ‘lytic’ means breaking. In heterolytic fission, the bonds are broken in such a way that the one atom in a molecule takes both the electrons of covalent bonding and generates anions and cation. The atom containing a positive charge is called a cation and the one which takes both the electrons bearing a negative charge is called an anion. The process is also known as heterolytic cleavage or heterolysis.

• The heterolytic fission happens at low temperatures.

• For heterolytic fission, there must be a difference in the electronegativity of the atoms.

• The solvent must be polar.

• The bond between the atoms should also be polar.

Heterolytic Fission

Examples

• We can see heterolytic cleavage in methyl alcohol. It breaks into the methyl group with positive cation and hydroxide anion.

• Heterolytic fission in bromomethane the more electronegative bromine takes both the electrons from covalent bonding and generates electronegative carbanion bromine and carbocation methane.

• When heterolytic fission happens in a cyclic compound, they do not form two products, instead, the ring opening is done. Ring-opening in epoxide is done by the heterolysis of 𝐶 − 𝑂 the bond.

How many types of ions are formed by Heterolytic Fission and What are their names?

On hydrolysis of a molecule the molecules break into two parts one is positive and another is negative. The more electronegative atoms pull electrons towards themselves and form carboanion and other forms of carbocation.

• Carbocation − The carbocation carries a positive charge as it does not carry any electrons on cleavage on a covalent bond.

• Carbanion − The more electronegative atom in the molecules pulls the electrons towards itself i.e., carries both the electrons with itself and forms carbanion.

Formation of carbocation and carbanion by Heterolytic Fission

Homolytic Fission of Covalent bond leads to the formation of Free Radical Explain:

A covalent bond is formed by the two same or different molecules by sharing two electrons by each atom. The sharing of electrons helps both the atoms complete their octet. In a chemical reaction when reagents are reacted with different atoms, the covalent bond is broken to form a new product. It is also known as hemolysis.

• In Homolytic fission, breaking is done in such a way that the electrons are equally taken by both the molecules.

• In homo cleavage, the bond must be non-polar, which means the electronegativity between both atoms must be equal.

• It is performed at high temperatures.

• On cleavage both form free radicals.

Formation of free radical by Homolysis

What is the difference between Homolytic and Heterolytic Cleavage?

There are a few differences between homolytic and heterolytic fission −

Homolytic Cleavage	Heterolytic Cleavage
In homolytic cleavage on the breaking of the covalent bond, the electrons are retained by each atom of the molecule.	In heterolytic cleavage on the breaking of the covalent bond, the electrons are taken by a more electronegative atom.
In this process, the free radicals are formed that are highly reactive.	In this process, the carbanion and carbocations are formed.
In this fission both the atoms have the same electronegativity.	In this process, one atom is more electronegative than the other.
It happens at high temperatures.	It happens at a low temperature.
Example − Cl-Cl, cleavage of peroxides, etc	Examples − Bromomethyl, Ethyl Alcohol, cleavage in cyclic compounds, etc.

Conclusion

In the above tutorial, we have studied homolytic and heterolytic fission. In a chemical reaction, the formation and cleavage of a bond depend upon the type of bond. While forming a compound, break a bond and form a new bond. This breaking of bond is done in two ways either homolytic or heterolytic fission. The type of bond and specific condition decides the type of fission. The homolytic fissions are done in a nonpolar compound at high temperatures and polar solvents. The heterolysis cleavage is done based on the ionization of covalent bonds, polarization, and electrophilic properties. The heterolysis of a compound also depends upon the nature of the solvent.

FAQs

1. In biochemistry, the heterolysis of larger molecules is known as?

The heterolysis in larger molecules in biochemistry is known as Catabolism. It happens through the cleavage of internal bonds in the molecules.

2. What happens when protein molecules break by heterolytic fission?

The heterolysis of protein molecules is known as proteomics. By the cleavage of protein molecules, the protein molecules break into smaller units of peptides. The enzymes that facilitate the cleavage of protein molecules are pepsin, trypsin, etc.

3. The factors that affect heterolytic fission are?

The factors that influence heterolytic fission are differences in the electronegativity of the atoms, temperature, polar solvent, type of bond, and ionization of covalent bonds.

4. What happens when heterolytic fission is done in a cyclic compound?

When heterolytic fission happens in a cyclic compound it does not form two separate entities; it simply opens the ring of the cyclic compound.

5. How homolytic fission is symbolized by an arrow?

The homolytic fission is symbolized by a fish hook arrow. It can be seen better in the cleavage of chlorine molecules. In the fission of hydrogen molecules, it is heated at +436 KJ/mol. It breaks into free radicals of two hydrogen atoms with single free electrons.