Homolytic and Heterolytic Fission

Science is the study of the physical world, and natural laws, their unbiased observation and systematic experimentation. It is also one of the most interesting subjects in a student’s syllabus. While being taught as a single subject in the junior classes, it is later branched into three individual subjects namely physics, chemistry, and biology in the senior classes. The field of science yields a wide variety of career choices. The detailed study of each of the three subjects changes the perspective of how the world we see around us works.

Let’s briefly know what physics, chemistry & biology is about. While physics deals with matter and energy and how they interact with sound, electricity, heat & light, Biology is the study of all living things and Chemistry is the study of properties, their composition and their structures. 

Chemistry

Chemistry is further bifurcated into two parts - Organic Chemistry and Inorganic chemistry. As the name suggests organic chemistry is the study of organic compounds which mostly includes carbon and hydrogen, it may also include n-number of elements such as oxygen, halogens or nitrogen. Inorganic chemistry is the science that studies inorganic compounds like water, sodium chloride, etc.

Organic Chemistry

Organic chemistry studies the sharing, breaking (or cleaving) and therefore forming chemical bonds of various compounds and elements with mostly carbon & hydrogen. The study of chemical bonds is of mainly three types given below -

1. Covalent Bond

2. Ionic Bond – Ionic bond is the interaction of two oppositely charged ions or two atoms with very different electronegativities.

3. Hydrogen Bond – As the name suggests, a hydrogen bond or the H-Bond is the interaction of a hydrogen (h) atom located between a pair of other atoms having a high number of electrons.

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 plays 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.

Covalent Bond

The covalent bond is a chemical bond that shows the sharing of electron pairs between atoms. In order to do that, it breaks the existing bonds and forms a new one. The general term which is referred for the breakage is known as bond fission. Bond fission is divided into two parts - Homolytic Fission and Heterolytic Fission.

Homolytic Fission

Definition- Homolytic fission which is also known as hemolysis is the type of bond fission wherein the process of sharing of atoms or molecules the fragments retain one of the originally bonded electrons.

Heterolytic Fission

Definition- Heterolytic fission is the type of bond fission where during the process of sharing the fragment obtains both original bonding compounds of the other fragment.

Now let’s understand about these two types of 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 smaller amount of heat energy.

What is 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 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).

This is all about homolytic and heterolytic fission defined and explained. Understand the conceptual difference between these two fissions along with proper examples to develop your conceptual foundation properly in this topic.