Covalent Bond Fission – Homolytic and Heterolytic (Free Rradicals, Carbocations and Carbanions)

A covalent bond occurs when two different atoms share electrons in the traditional sense. As a result, a single bond, or a sigma bond, consists of two electrons. A chemical reaction occurs when old bonds break down and new ones are formed. In contrast, chemical bonds may be disrupted in several ways. Additionally, the way of breaking a chemical bond is essential in determining the overall outcome of a chemical process. Bond fission is the term used to describe breaking a chemical connection, generally a covalent bond. Heterolytic and homolytic fission are the two primary bond fission forms.

What is homolytic fission?

Homolytic fission, or hemolysis, is a kind of bond fission that separates a particular molecule. An electron is retained in each of the molecule’s original fragments. As a result, two free radicals occur when a neutrally charged molecule undergoes homolytic fission.

Homolytic fission can also be called bond homolysis or homolytic cleavage. These terms originate from the Greek word ‘homo,’ roughly translating to ‘breaking equally.’ The homolytic bond dissociation energy is called the energy necessary to induce homolytic fission in the molecule.

A molecule’s homolytic fission generally necessitates a significant amount of energy. However, in other situations, one can perform homolytic fission with only a tiny amount of heat applied to the molecule. An example is the homolytic breakage of oxygen-oxygen bonds in peroxides. These intramolecular bonds are relatively weaker, it means their bond dissociation energies are lower. As a result, just some quantity of heat energy is necessary to overcome this barrier.

Free radicals

An atom or a group with an unpaired electron is a free radical. They occur during the homolytic fission of covalent bonds. These free radicals have a high level of reactivity. Their tremendous proclivity for pairing up their unpaired electron with another electron from any available source is due to their immense proclivity. Returning to the hybridisation principle, these radicals can take two fundamental structures.

The geometry of free radical is SP2 hybridised  and geometry of free radical is planar in nature.

What is heterolytic fission?

Heterolytic fission, or heterolysis, is bond fission where a covalent bond between two chemical species breaks unevenly. Only one of the chemical species retains the bond pair of electrons. A positive charge will be produced by one of the byproducts of heterolytic fission of a neutrally charged molecule, whereas the other will produce a negative charge.

The chemical species that fails to retain any bound electrons after the bond fission is the cation. It is the positively charged result of the heterolytic fission of a neutral molecule. On the other hand, the negatively charged heterolysis result, or cation, is a chemical entity that maintains both bonded electrons following the bond fission process.

The term ‘heterolysis’ is Greek in origin and approximately translates to ‘breaking unequally.’ Heterolytic cleavage is another name for this process. When a covalent bond undergoes heterolytic fission, the bonded species with the greatest electronegativity retains the bond pair of electrons and obtains a negative charge. However, the electropositive species typically do not retain any electrons and acquire a positive charge.

Carbocation

When the other atom is more electronegative than carbon, carbocations occur by the heterolytic cleavage of a carbon-heteroatom bond, such as a C-O or C-N link. It is rational because the bond’s two electrons must move to the other atom if the cleavage produces a carbocation. It is especially advantageous if the other atom is electronegative. Carbocations are generated under challenging conditions with the aid of superacids invented by George Olah and help stabilise these intermediates significantly enough to be studied. Carbocation formation can be aided by combining Ag+ with alkyl halides as substrates.

The positively charged carbon atom is sp2 hybridised in carbocations, implying it is planar. The carbocation’s three substituents are on a plane, with the unhybridized vacant p orbital perpendicular to them. These intermediates react with electron-rich species and stabilise polar solvents due to their charge. Carbocations, like carbanions, are critical intermediates in most processes.

Carbanions

These intermediates arise due to heterolysis, but the carbon atom retains the electron pair from the bond. As you know, the more electronegative atom maintains the electrons. Thus carbon must be the more electronegative of the two atoms that make up the bond in this situation. Because only a few atoms are less electronegative, the C-H bond is the most frequent bond cleavage that produces carbanions. Because an H+ is created with the carbanion, the molecule is an acid in the Bronsted sense. The ease with which this bond may break and form a carbanion is thus a measure of the compound’s acidity.

Carbanions comprise three groups connected by a lone pair of electrons, giving them their negative charge. As a result, the carbon atom is sp3 hybridised, and the shape is pyramidal. In polar solution, carbanions are also stable (electrostatic stabilisation).

Comparing cleavage of covalent bonds under homolytic and heterolytic Conditions

When the bond dissociation energies for the same types of bonds are compared, it is clear that the heterolytic dissociation energy is much higher than the homolytic dissociation energy. Two ions form when a neutral molecule is hydrolysed: a positive and a negative ion. Separating these opposing charges, in contrast, requires a substantial amount of energy. In the gas phase, bond dissociation adopts a more direct path known as homolysis. In an ionising solvent, however, heterolysis is the preferred method of breakdown.

Conclusion 

To recap, covalent bonds are created by the mutual sharing of electrons between atoms in all organic compounds. These covalent bonds can be broken in two ways: homolytic cleavage (symmetrical splitting) and heterolytic cleavage (asymmetrical splitting). The type of the reagent impacts the cleavage of a bond in the substrate (attacking agent). Free radicals occur when light or other substances known as Radical Initiators are used to activate them. Carbocations react with electron-rich species, while carbanions react with positively charged species (electron-deficient species).