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Homolytic And Heterolytic Fission: Explained With Conditions & Examples
IMPORTANT LINKS
Organic Chemistry
Homolytic and Heterolytic fission are two ways in which chemical bonds can break. In homolytic fission, the bond splits evenly, and each atom gets one electron, forming neutral but highly reactive particles called free radicals. This usually happens in non-polar molecules under high temperature or UV light. In heterolytic fission, the bond breaks unevenly, with one atom taking both electrons, leading to the formation of charged particles: one positive (cation) and one negative (anion). This type of bond breaking is common in polar molecules, especially in the presence of polar solvents or low temperatures. Understanding these processes helps explain how chemical reactions start and proceed.
Homolytic Fission
The process of dividing a molecule into two parts by transferring a single electron with the same electron spin from one atom to another, this is known as homolytic fission. Homolytic fission includes the distribution of only one bond electron to each part of the molecule. This results in the formation of reactive radical molecules like Cl, CH3, CH3CH2. The energy utilized in the process of homolytic fission is known as bond dissociation energy (BDE). It is referred to as the per mole of enthalpy required to break a particular bond molecule. Stronger bonds require more bond dissociation enthalpy for homolytic fission.
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Along with the Homolytics and Heterolytics fission, learn Antibiotics classification here.
Conditions for Homolytic Fission
Hololytic fission usually happens under certain conditions, such as:
- High temperature: Heat provides the energy needed to break bonds evenly.
- Presence of UV light: Ultraviolet light can excite molecules and cause bonds to split into radicals.
- Non-polar solvents: These don’t stabilize charged particles, so they support radical formation.
- Low or no polarity in the bond: Bonds between atoms with similar electronegativity (like Cl-Cl) are more likely to break homolytically.
These conditions help split the bond so that each atom takes one electron, forming free radicals.
Along with homolytic and heterolytic fission, learn about Homonuclear Diatomic Molecules here.
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Homolytic Fission Example
Some of the most common examples of homolytic fission are discussed as follows:
- Homolysis of chlorine molecules in the presence of UV radiation or heat.
- Formation of an alkyl radical in the presence of chlorine radical.
- Formation of alkoxy radical from peroxides in the presence of heat or UV radiation.
Along with Homolytic and Heterolytic Fission, learn about the Difference between atomic mass and atomic weight here.
Heterolytic Fission
Heterolytic fission is when a chemical bond breaks and both shared electrons go to one atom instead of being split equally. As a result:
- One part of the molecule gains both electrons and becomes a negative ion(anion).
- The other part loses the electrons and becomes a positive ion (cation). This process is also known as ionic fission.
Heterolytic fission helps explain the formation of carbocations (positively charged carbon ions) and carbanions (negatively charged carbon ions).
The energy needed to break a bond in this way is called heterolytic bond dissociation enthalpy.
Conditions for Heterolytic Fission
Heterolytic fission happens more easily under the following conditions:
- Big difference in electronegativity: When two atoms in a bond have very different electronegatives, the more electronegative atoms pulls the shared electrons towards itself.
- Presence of a polar bond: Molecules with polar bonds (where one side is more negative) are more likely to break unevenly.
- Low temperature or polar solvents: These conditions support the uneven breaking of bonds, making it easier for one atom to take both bonding electrons.
Along with Homolytic and Heterolytic Fission, learn about Difference between Evaporation and Distillation here.
Heterolytic Fission Examples
The most commonly observed examples of heterolytic fission are as follows:
- In hydrochloric acid, the bond is broken via heterolytic fission.
- The chlorine atom is highly electronegative.
- It attracts the electron pair towards itself.
- Formation of carbocation during the SN1 reaction of bromoalkane
Along with the difference between homolytic and heterolytic fission learn about Electrophilic Substitution Reaction here.
|
Feature |
Homolytic Fission |
Heterolytic Fission |
|
Definition |
Bond breaks evenly, each atom gets one electron |
Bond breaks unevenly, one atom gets both bonding electrons |
|
Type of particles formed |
Two neutral radicals (highly reactive) |
One positive ion (cation) and one negative ion (anion) |
|
Electron sharing |
Electrons are split equally |
Electrons go to the more electronegative atom |
|
Common in |
Non-polar molecules |
Polar molecules |
|
Conditions |
High temperature, UV light or heat |
Polar solvents, low temperature |
|
Example |
Cl–Cl → Cl• + Cl• |
H–Br → H⁺ + Br⁻ |
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FAQs
What is homolytic fission? Give an example.
Homolytic fission is known as the process of dividing a molecule into two parts by transferring a single electron with the same electron spin from one atom to another. The breaking of chlorine molecules in the presence of heat or UV light follows homolytic fission.
What is heterolytic bond fission?
In heterolytic fission, the bond is broken unequally by transferring the whole shared pair of electrons to one entity of the compound which results in the formation of a cation and an anion. For example, bond fission of hydrochloric acid (HCl)
Which has higher bond dissociation enthalpy- homolytic or heterolytic?
The homolytic bond dissociation enthalpy is higher than the homolytic bond dissociation energy.
Which type of fission is easier in the process- homolytic or heterolytic?
Homolytic fission does not require a higher amount of energy. Hence, it is easier in the process to accomplish.
What is the homolysis of a covalent bond?
Homolysis of covalent bonds refers to the homolytic bond cleavage which results in the formation of free radicals.