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The bulk of chemical reactions entail breaking and forming new chemical bonds. On the other hand, chemical bonds can be broken in a variety of ways. Furthermore, the manner in which a chemical bond breaks is critical in determining the ultimate outcome of a chemical reaction. The shattering of a chemical link is referred to as bond fission (typically a covalent bond). The two basic types of bond fission are homolytic and heterolytic.
Homolytic fission
The two electrons in a cleaved covalent link are split evenly amongst the products in homolytic cleavage, or homolysis. This is also known as radical fission or homolytic fission. A bond’s bond-dissociation energy is the amount of energy necessary to homolytically cleave the bond. One measure of binding strength is the enthalpy change.
The energy necessary for homolytic dissociation of a sigma bond is the triplet excitation energy, although because of the repulsion between electrons in the triplet state, the actual excitation energy may be larger than the bond dissociation energy.
A radical mechanism is used in homolytic fission. A homolytic cleavage occurs when the Cl-Cl bond is disrupted, allowing each Cl atom to take one electron:
A half-headed arrow indicates homolytic cleavage (fish hooks). Two fish hook arrows are used to depict how the relationship is broken in this illustration. The first arrow moves from the middle of the bond to the first atom, while the second arrow moves from the middle of the bond to the second atom. As a result, each atom receives one electron, resulting in the formation of radical species.
Radicals are unpaired electron-containing reactive intermediates that react fast to generate stable compounds.
The Cl radical created in the first step, for example, interacts swiftly with ethane to produce a hydrogen and a new radical:
The radical is finally trapped/quenched by another radical, resulting in the formation of a neutral molecule.
The energy of homolytic bond cleavage:
Energy is required for both homolytic and heterolytic cleavages. The heat of reaction, also known as the enthalpy of the reaction, is a measure of how hot a reaction is. Keep in mind that enthalpy is the heat produced at normal pressure. Bond dissociation energies describe the enthalpy of a homolytic cleavage. Because homolysis is an endothermic process, these are always positive quantities. For further information on the relationship between exothermic and endothermic processes, as well as the indicators of enthalpy change, read this post about energy changes in chemical reactions.
Bond formation, on the other hand, is an exothermic process in which energy is always released.
The hydrogen molecule (H2), for example, is created when two free hydrogen atoms come into close proximity.
A 436 kJ mol-1 potential energy loss in the form of heat is connected with this activity. Breaking the link and forming two hydrogen atoms will require the same amount of energy (homolytic cleavage).
As a result, the H-H bond strength is 436 kJ/mol, and the energy required to break it is known as the bond dissociation energy. On the other hand, when H2 is generated, we are talking about the heat of formation, and the only difference between the two is the sign.
The bonds between all elements, like the H-H bond, are characterised by a unique bond dissociation energy (bond strength).
A large amount of energy is normally required for homolytic fission of a molecule. This is why, as explained below, this type of bond fission only occurs in a few situations.
When a molecule is exposed to UV light, it undergoes photosynthesis (the electromagnetic radiation corresponding to the ultraviolet region of the electromagnetic spectrum)
• When a sufficient quantity of heat is delivered to a molecule to overcome the bond dissociation energy required for homolytic fission,
Pyrolysis is the process of heating carbon compounds to extremely high temperatures in the absence of oxygen in order to enable pyrolysis.
In some cases, homolytic fission can be achieved by applying a small amount of heat to the molecule. One example is the homolytic breaking of oxygen-oxygen bonds in peroxides. The bond dissociation energies of these intramolecular bonds are quite low, indicating that they are weak. As a result, only a little amount of heat energy is needed to break through this barrier.
Dissociation of Cl2
A typical example is the dissociation of molecular chlorine (Cl2). This happens when Cl2 is heated to a high temperature or when it is exposed to harsh light.
This is known as “homolytic” bond cleavage because the electron pair distribution in the products is quite even. To depict the motion of one electron, we use the “fishhook” (a curving arrow with only one “barb” on the arrowhead). When the link in question is not polar and there is no electrophile or nucleophile present to induce heterolytic patterns, homolytic bond cleavages occur.
Conclusion
The major difference between homolytic and heterolytic fission is that homolytic fission provides one bond electron to each fragment, whereas heterolytic fission provides two bond electrons to one fragment and none to the other. Fission is defined as the breaking of a covalent chemical link.
