CY251 Assignment 1
CY251 Assignment
A covalent bond is formed when electrons are shared between two atoms .
There are two ways to break a bond,The triplet and singlet excitation energies of a sigma bond can be used to
determine if a bond will follow the homolytic or heterolytic pathway.
Homolytic fission
Homolytic fission is where each atom of the bond keeps an electron each resulting in species called free
radicals that is, neutral species that carry an unpaired electron
As the bond breaks to give two similar species each keeping an electron this form of bond breaking is called
Homolytic Fission.
Homolytic fission is also known as homolytic cleavage or bond homolysis.
These terms are derived from the Greek root ‘homo’, and the term can be roughly translated as ‘equal
breaking’.
This type of bond cleavage happens under certain specific conditions like ultraviolet rays, high temperatures
or high temperatures in the absence of oxygen to facilitate pyrolysis.
Most bonds homolyse at temperatures above 200°C.in some cases pressure can induce the formation of
radicals
These conditions excite electrons to the next highest molecular orbital, thus creating a Singly Occupied
Molecular Orbital (or SOMO.
The energy required for homolytic fission in a molecule is called homolytic bond dissociation energy.
This enthalpy change is one measure of bond strength.
Typically, a large amount of energy is required to spark the homolytic fission of a molecule.
If the radical species is better able to stabilize the free radical, the energy of the Singly Occupied Molecular
Orbital will be lowered, as will the bond dissociation energy.
Radical is another highly reactive reaction intermediate, because of the lack of octet.
The larger the electron cloud, the better an atom can stabilize the radical
hybridizations minimizing s-character increase the stability of radicals, and decreases the bond dissociation
energy
Radicals can be stabilized by the donation of negative charge from resonance, or in other words, [electron
delocalization].
Carbon radicals are stabilized by hyperconjugation, meaning that more substituted carbons are more stable,
and hence have lower BDEs.
Homolysis occurs mainly for non-polar bonds
Homolytic fission usually happens in cases where the difference in electronegativities of the atoms involved in
bond formation is not significantly high.
The triplet excitation energy of a sigma bond is the energy required for homolytic dissociation, but the actual
excitation energy may be higher than the bond dissociation energy due to the repulsion between electrons in
the triplet state.
Homolytic fission is depicted by drawing two fish-hook arrows across the bond, pointing towards the two
atoms involved in the bond formation.
