Most chemical reactions involve the breakage of existing chemical bonds and the formation of new ones. However, chemical bonds can be broken in multiple ways. Furthermore, the manner in which a chemical bond breaks plays a vital role in deciding the overall outcome of the chemical reaction. The breakage of a chemical bond (usually a covalent bond) is often referred to as bond fission. The two primary types of bond fission are homolytic fission and heterolytic fission.
What is Homolytic Fission?
Homolytic fission (sometimes referred to as homolysis) is a type of bond fission that involves the dissociation of a given molecule wherein one electron is retained by each of the original fragments of the molecule. Therefore, when a neutrally charged molecule is subjected to homolytic fission, two free radicals are obtained as the product (since each of the chemical species retains one electron from the bond pair).
It can be noted that homolytic fission is also known as homolytic cleavage or bond homolysis. These terms are derived from the Greek root ‘homo’, and the term can be roughly translated as ‘equal breaking’.
The energy required to facilitate homolytic fission in a molecule is often referred to as the homolytic bond dissociation energy of the molecule. An illustration detailing the homolytic fission of a molecule AB, resulting in the formation of two free radicals (Ao and Bo) is provided below.
Typically, a large amount of energy is required to spark the homolytic fission of a molecule. This is the reason why this type of bond fission only occurs in some cases, as listed below.
- When the molecule is subjected to ultraviolet radiation (the electromagnetic radiation corresponding to the ultraviolet region of the electromagnetic spectrum)
- When the molecule is subjected to the required amount of heat in order to overcome the required bond dissociation energy for the homolytic fission
- When carbon compounds are subjected to extremely high temperatures in the absence of oxygen in order to facilitate the pyrolysis of the molecule
In some cases, homolytic fission can be achieved by supplying only a small amount of heat to the molecule. One such example is the homolytic cleavage of the oxygen-oxygen bonds in peroxides. These intramolecular bonds are fairly weak, implying that they have very small bond dissociation energies. Therefore, this barrier can be overcome with only a small amount of heat energy.
What is Heterolytic Fission?
Heterolytic fission, also known as heterolysis, is a type of bond fission in which a covalent bond between two chemical species is broken in an unequal manner, resulting in the bond pair of electrons being retained by one of the chemical species (while the other species does not retain any of the electrons from the bond pair). When a neutrally charged molecule undergoes heterolytic fission, one of the products will have a positive charge whereas the other product will have a negative charge.
It can be noted that the positively charged product of the heterolytic fission of a neutral molecule, usually called the cation, is the chemical species that did not retain any of the bonded electrons post the bond fission. On the other hand, the negatively charged product of the heterolysis (also known as the anion) is the chemical species that retains both the bonded electrons after the bond fission process.
The term ‘heterolysis’ has Greek roots and can be roughly translated as ‘unequal breaking’. It is also referred to as homolytic cleavage. An illustration detailing the two ways in which a molecule AB can undergo heterolytic fission is provided below. In the first scenario, the bond pair of electrons is retained by B, making it the anion and A the cation. In the second scenario, A retains the bond pair and becomes the anion whereas B becomes the cation.
It can also be noted that when a covalent bond is subjected to heterolytic fission, the bonded species with the greater electronegativity is the one that usually retains the bond pair of electrons and obtains a negative charge. On the other hand, the more electropositive species usually does not retain any electrons and obtains a positive charge.
The energy required to cleave a covalent bond via heterolytic cleavage is often referred to as the heterolytic bond dissociation energy (not to be confused with homolytic bond dissociation energy). This value is sometimes used to denote the bond energy of a covalent bond. An example of homolytic fission can be observed in the hydrogen chloride molecule, as illustrated in the chemical reaction provided below.
H-Cl → H+ + Cl–
Here, the chlorine atom retains the bond pair of electrons because its electronegativity is higher than that of hydrogen. Therefore, the products formed are the chloride anion and the hydrogen cation.
Comparing Homolytic and Heterolytic Cleavage of Covalent Bonds
The bond dissociation energy for the same types of bond, it can be observed that the heterlytic bond dissociation energy is considerably higher than the homolytic dissociation for the same bond. Heterolysis of a neutral molecule yields a positive and a negative ion. However, separation of these charges which are opposite requires a great amount of energy. In the gas phase bond dissociation occurs by an easier route, namely homolysis. However, in an ionizing solvent heterolysis is the preferred kind of breakage.
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