Covalent Bond Fission – Homolytic and Heterolytic

INTRODUCTION:

The majority of chemical reactions entail the breaking of existing chemical bonds as well as the production of new chemical bonds. Chemical bonds, on the other hand, can be broken in numerous ways. Also important in determining the ultimate outcome of a chemical reaction is the method in which a chemical bond is broken during the reaction. Bond fission is a term used to describe the breaking of a chemical bond (typically a covalent bond) when it occurs. Homolytic fission and heterolytic fission are the two basic types of bond fission that can occur.

What is Homolytic Fission 

Homolytic fission(also known as Homolysis)  is a type of bond fission in which one electron is preserved by each of the original pieces of a particular molecule throughout the dissociation of the molecule. It is one of the most often observed types of bond fission.. When an electrically neutrally charged molecule is subjected to homolytic fission, two free radicals are produced as a byproduct of the reaction . (As each of the chemical species retains one electron from the bond pair).

Observe that homolytic fission is also referred to as homolytic cleavage or bond homolysis, to name a few terms. It is believed that these phrases stem from the Greek word homo, and that the concept may be approximately rendered as equal breaking.

Molecular homolytic fission is facilitated by the release of energy from the homolytic bond dissociation energy of the molecule, which is measured in joules. In the following section, you will find a diagram that illustrates the homolytic fission of a molecule AB, which results in the creation of two free radicals (A˙ and B˙).

Homolytic Fission 

In most cases, a significant amount of energy is required to initiate the homolytic fission of a molecule in solution. In order to avoid confusion, we will only discuss this form of bond fission in the context of the circumstances indicated below.

When the molecule is exposed to ultraviolet rays, it changes (the electromagnetic radiation corresponding to the ultraviolet region of the electromagnetic spectrum)

In order to overcome the requisite bond dissociation energy for homolytic fission, a molecule must be subjected to a sufficient quantity of heat in order to achieve that result.

In order to enhance the pyrolysis of a molecule, carbon compounds are treated to extremely high temperatures in an oxygen-free environment in order to accelerate the process.

In some situations, homolytic fission can be performed by giving only a little amount of heat to the molecule during the fission process. In peroxides, for example, the homolytic cleavage of the oxygen-oxygen bonds is an example of this. In this case, the intramolecular bonds are rather weak, meaning that their bond dissociation energies are extremely low. It is therefore possible to break through this barrier using only a modest quantity of heat energy.

What is Heterolytic Fission 

Histolytic fission, also called as  heterolysis, is a bond fission where a covalent bond between two chemical species is broken in an uneven way, resulting in the bond pair of electrons being retained by one of the chemical species and the bond pair of electrons being released by the other chemical species (Whereas the other species does not retain any of such electrons from the bond pair). One of the products of heterolytic fission of a neutrally charged molecule has a positive charge, whereas the other product has a negative charge when the molecule undergoes heterolytic fission.

Notably, the positively charged outcome of heterolytic fission of a neutral molecule, which is commonly referred to as the cation, is a chemical species in which no bound electrons were retained as a result of bond fission. However, the negatively charged result of the heterolysis (also known as the anion) is the chemical species that retains both bonded electrons after the bond fission process, as opposed to the positively charged product of the heterolysis.

“Heterolysis” is a term that has its origins in Greek and can be loosely translated as “unequal breakdown.” It is referred to as homolytic cleavage in some circles. In the following section, you will see an image illustrating the two possible methods in which the molecule AB can undergo heterolytic fission. In the first situation, B retains the bond pair of electrons, resulting in it being designated as the anion and A as the cation. According to the second scenario, A keeps the bond pair and is transformed into the anion, whilst B is transformed into the cation.

Heterolytic Fission

The fact that, when a covalent bond is subjected to heterolytic fission, the bound species with the larger electronegativity is usually the one that retains the bond pair of electrons and receives a negative charge can also be noticed. The more electropositive species, on the other hand, is frequently unable to retain any electrons and hence acquires a positive electrical charge.

Homolytic bond dissociation energy is the energy necessary to break a covalent bond using heterolytic cleavage, which is a term that is frequently employed (not to be confused with homolytic bond dissociation energy). When referring to the bond energy of a covalent bond, this value is sometimes used to denote the value. In the hydrogen chloride molecule, an example of homolytic fission can be observed, as indicated in the chemical reaction provided below.

H-Cl → H+ + Cl–

Because chlorine’s electronegativity is stronger than that of hydrogen, the chlorine atom retains the bond pair of electrons formed with the hydrogen atom. Therefore, the chloride anion and hydrogen cation are generated as byproducts of the reaction.

Comparing the Cleavage of Covalent Bonds by Homolytic and Heterolytic Cleavage

Using the same sorts of bonds, it is possible to observe that the heterolytic bond dissociation energy is significantly larger than the homolytic bond dissociation energy for the same type of binding. The heterolysis of a neutral molecule results in the formation of both a positive and a negative ion. The separation of these charges, which are stringently  opposed, demands a significant amount of energy. In the gas phase, bond dissociation happens via a more straightforward mechanism known as homolysis. Heterolysis, on the other hand, is the favoured type of breakage in an ionising solvent.

CONCLUSION:

So to Conclude When bonding atoms have different electronegativity differences and there are polar solvents present at low temperatures, heterolytic fission is more likely to occur. In homolytic fission, a covalent link is broken in such a way that each of the bound atoms receives one of the electrons that were previously shared between them.