Homolytic and Heterolytic Fission

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What is Homolytic and Heterolytic Fission?

Most of the chemical reactions involve the existing chemical bond breakage, including forming the new ones. However, chemical bonds can be broken in various methods. Furthermore, the manner where a chemical bond breaks play a vital role in deciding the chemical reaction's entire outcome. The chemical bond (generally a covalent bond) breakage is often referred to as bond fission. The two primary types of bond fission can be given as heterolytic fission and homolytic fission.

What is Homolytic Fission?

Homolytic fission (also called hemolysis, sometimes) is a bond fission type, which involves dissociating a given molecule wherein every original fragment of the molecule retains one single electron. Thus, when a neutrally charged molecule is subjected to the homolytic fission, 2 free radicals are received as the product (because each of the chemical species retains 1 electron from every bond pair).

It should also note that homolytic fission is also called bond homolysis or homolytic cleavage. These are the terms derived from the Greek root 'homo,' which can be roughly translated as 'equal breaking.'

The energy needed to facilitate homolytic fission in a molecule is often called the molecule's homolytic bond dissociation energy. A picture detailing the homolytic fission of a molecule AB, resulting in two free radicals (Ao and Bo) formation, is represented below.

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Typically, a huge amount of energy is needed to spark the molecule's homolytic fission. This is the primary reason this type of bond fission only occurs in some cases, as given in the below list.

  • When a molecule is subjected to ultraviolet radiation (the electromagnetic radiation, which is corresponding to the electromagnetic spectrum's ultraviolet region).

  • When a molecule is subjected to the necessary amount of heat to overcome the required dissociation energy of the bond for the homolytic fission.

  • When the carbon compounds are subjected to extremely high temperatures in the absence of oxygen to facilitate the molecule's pyrolysis.

  • In a few cases, homolytic fission can be achieved by supplying only a lesser amount of heat to the molecule. One similar example is the homolytic cleavage of the oxygen-oxygen bonds in peroxides. These intramolecular bonds are fairly weak, implying they have dissociation energies of the very small bond. Thus, this barrier can be overcome only with a less amount of heat energy.

Define Heterolytic Fission

Let us look at the heterolytic fission definition. Heterolytic fission is also called heterolysis. It is a type of bond fission. A covalent bond between the two chemical species is broken unequally by resulting in the bond pair of electrons that are being retained by one of the chemical species (while the other remaining species does not retain any electrons from the bond pair). Whenever a neutrally charged molecule undergoes the heterolytic fission, one of its products will have a positive charge, whereas the other has a negative charge.

It should be noted that the heterolytic fission's positively charged product of a neutral molecule, in general, known as a cation, is the chemical species, which did not retain any bonded electrons after the bond fission. Whereas, the heterolysis's negatively charged product (which is also known as an anion) can be given as the chemical species that retains both bonded electrons post the bond fission process.

'Heterolysis' is a term with Greek roots and can be roughly translated to 'unequal breaking.' It can also be known as homolytic cleavage. A representation that explains the two ways where a molecule AB can undergo heterolytic fission is given below. In the first case, the bond pair of electrons are retained by B by making it the anion and the cation, A. In the second case, A retains the bond pair and becomes anion; on the other side, B becomes the cation.

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It should also make a note that, when a covalent bond is subjected to heterolytic fission, the bonded species having the greater electronegativity is the one that generally retains the bond pair of electrons and also obtains a negative charge. On the other side, the more electropositive species generally do not retain any electrons and obtain a positive charge.

The energy that is required to cleave a covalent bond through the heterolytic cleavage is often called the heterolytic bond dissociation energy (where homolytic bond dissociation energy is another one). Sometimes, this value can be used to denote the bond energy of a covalent bond. One such example of homolytic fission is observed in the hydrogen chloride molecule, given in the chemical reaction provided below.

H-Cl → H+ + Cl

Here, the chlorine atom retains the electrons' bond pair because its electronegativity is higher than hydrogen. Thus, the formed products are given as the chloride anion and the hydrogen cation.

Did You Know?

The free radicals of the carbon are primarily generated by:

  • Photolysis (the action of light) such as acetone alpha cleavage.

  • Another radical initiator, such as allylic bromination, by NBS (N-Bromosuccinimide).

FAQ (Frequently Asked Questions)

1. Explain the Comparison of Homolytic and Heterolytic Cleavage of the Covalent Bonds.

Answer: The bond dissociation energy for similar bond types can be observed because the dissociation energy of the heterolytic bond is considerably higher than that of the homolytic dissociation for the same bond. But, heterolysis of a neutral molecule yields both positive and a negative ion. However, the separation of these charges, which are quite the opposite, requires a huge amount of energy. In the gaseous phase, the bond dissociation takes place by an easier route, which is called hemolysis. However, heterolysis is the preferred kind of breakage in an ionizing solvent.

2. What is a Bond Fission?

Answer: A covalent bond produces when electrons are shared between the two atoms in a classical sense. Thus, a single bond or sigma bond is made up of two electrons. Now, a chemical reaction occurs when old bonds are broken and new ones are created. Thus, it is not possible to break a single bond because there are plainly two ways to go about bond-breaking, which are:

  • Homolytic fission, and

  • Heterolytic fission.

3. What are Free Radicals?

Answer: These are the neutral intermediates, which are formed due to the homolytic cleavage of a single bond. A few common bonds cleave to produce free radicals in organic chemistry can be given as C-Cl, C-O, C-I, C-Br, C-H, C-C.