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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) (unsymmetrical splitting). The type of the reagent has an impact on the cleavage of a bond in the substrate (attacking agent).
Homolytic cleavage:
Homolytic cleavage occurs when a covalent bond breaks symmetrically, preserving one electron for each of the linked atoms. A half-headed arrow denotes it (fish hook arrow). In a molecule possessing non-polar covalent bonds formed between atoms of similar electronegativity, this sort of cleavage happens at high temperatures or in the presence of UV radiation. The breakage of bonds in such compounds produces free radicals. They have a short life span and are extremely reactive. Free radical initiators are the chemicals that promote holmolytic cleavage in the substrate. In polymerisation reactions, free radical initiators such as Azobisisobutyronitile (AIBN) and peroxides such as benzoyl peroxide are utilised.
Because a free radical with an unpaired electron is neutral and unstable, it will want to obtain an electron in order to become stable. Homolytic fission of C-C bonds produces alkyl free radicals in organic processes. The following is a list of alkyl free radical stability:
˙C(CH3)3 > ˙CH(CH3)2>˙CH2CH3 >˙CH3
Heterolytic cleavage:
When a covalent bond breaks asymmetrically, leaving one of the linked atoms with the bond pair of electrons, this is known as heterolytic cleavage. A cation and an anion are formed as a result of this reaction. The anion is formed by the most electronegative of the two connected atoms, while the cation is formed by the other. The cleavage is indicated by a curved arrow heading towards the more electronegative atom.
Because bromine is more electronegative than carbon, the C-Br bond in tert-butyl bromide is polar. Bromine attracts the bonding electrons of the C-Br bond more than carbon. During hydrolysis, the C-Br undergoes heterolytic cleavage to create a tert-butyl cation.
Consider the formation of carbanion from the cleavage of a carbon-hydrogen (C-H) bond in aldehydes or ketones. We know that carbon is more electronegative than hydrogen, so the heterolytic cleavage of C-H bonds produces carbanion (carbon bears a negative charge). In aldol condensation, for example, the OH- ion extracts a -hydrogen from the aldehyde, resulting in the creation of the carbanion indicated below.
Bond dissociation energy:
Bond dissociation enthalpy can be used to calculate the strength of a chemical bond between two species. Despite the fact that it is often measured as the enthalpy change of the homolytic fission of the bond at absolute zero (0K), the bond dissociation energy of a chemical bond is commonly defined as the enthalpy change of the homolytic fission of the bond at absolute zero (0K) (298K).
The following are some key aspects of the bond dissociation enthalpy concept:
It is the amount of energy that must be provided to break a chemical bond between two species.
It’s a way of figuring out how strong a chemical link is.
Its value in diatomic molecules is equal to the bond energy value.
Silicon and fluorine are thought to have the highest bond dissociation enthalpy.
Covalent bonds between atoms or molecules are said to have weak bond dissociation energies.
The bond dissociation enthalpy of a Cl2 is depicted in the diagram below.
The bond dissociation enthalpy is equivalent to the bond energy in diatomic molecules, as previously stated. Because bond energy is the average of all bond dissociation enthalpies of all bonds of the same type in a molecule, this is the case.
The weakest and the strongest chemical bonds:
The concept of bond dissociation enthalpy can be used to find the weakest and strongest chemical bonds. As previously mentioned, the strongest chemical link is between silicon and fluorine.
In a silicon tetrafluoride molecule, the bond dissociation energy required to break the first silicon-fluorine bond is predicted to be 166 kcal/mol. The large disparity in electronegativities between silicon and fluorine atoms, as well as the fact that fluorine is the most electronegative element in the periodic table, can explain this.
Moving on to neutral compounds, the carbon-oxygen bond in carbon monoxide has the strongest bond strength, with a bond dissociation energy of 257 kcal/mol. The bond dissociation energy of the carbon-carbon bond in ethyne is also relatively high, at around 160 kcal/mol.
Thus, in the study of thermochemistry, the idea of bond dissociation enthalpy has shown to be extremely valuable.
Gas phase bond dissociation:
Experimental and computational methods both struggle to get accurate bond dissociation energy for big compounds. The former methods are impeded by a variety of physical and practical restrictions in gas-phase measurement techniques, whilst the latter methods necessitate the use of several approximations, the influence of which on accuracy is not always obvious. When internal benchmarks aren’t available, one hopes that theory and experiment can complement each other. However, a recent study discovered a significant difference between mass spectrometrically recorded gas-phase bond dissociation energies and the corresponding amounts estimated using density functional theory (DFT)-D3 and DLPNO-CCSD(T) approaches. The issue must be rectified as these computational methods are increasingly applied to massive molecular systems.
Conclusion:
When the dissociation energies of identical types of bonds are compared, it can be seen that the heterolytic dissociation energy is much greater than the homolytic dissociation energy. Two ions are created when a neutral molecule is heterolyzed: a positive and a negative ion. Separating these opposing charges, on the other hand, requires a substantial amount of energy. In the gas phase, bond dissociation is facilitated by homolysis. In an ionising solvent, however, heterolysis is the preferred kind of breaking.
