Homolytic and heterolytic fission

1.0Introduction

In chemistry, breaking chemical bonds is called bond cleavage or bond fission. Based on the mechanism of bond breaking, bond cleavage is categorized into two types:

1. Homolytic Fission

2. Heterolytic Fission

2.0Homolytic Fission

Homolytic fission occurs when a covalent bond breaks evenly, with each bonded atom receiving one electron from the shared pair. This type of bond cleavage leads to the formation of free radicals, which are highly reactive species containing an unpaired electron.

For example, if a molecule WY undergoes homolytic fission:

WY→W^⋅+Y^⋅

Each fragment gets one electron, forming free radicals (W^· and Y^·).

Characteristics of Homolytic Fission:

• The bond electrons are equally distributed between the two atoms.
• Generates free radicals such as Cl^∙, CH₃^∙, and CH₃CH₂^∙.
• Requires a significant amount of energy to break the bond.
• The energy needed to break a bond in this manner is called bond dissociation energy (BDE).
• Stronger bonds need higher bond dissociation enthalpy for homolytic fission to occur.

Conditions Favoring Homolytic Fission

Homolytic fission typically requires high-energy conditions, including:

1. Low Electronegativity Difference

If the two bonded atoms have similar electronegativities, the bond is more likely to break homolytically rather than heterolytically.

1. Non-Polar Bonds

Non-polar molecules and bonds tend to undergo homolytic fission more easily.

1. High Temperature or UV Radiation
   Exposure to heat (thermal energy) or UV light facilitates the homolytic cleavage of bonds.
2. Electric Discharge or Peroxides
   The presence of electrical energy or peroxide compounds can also promote homolytic fission.

Examples of Homolytic Fission

1. Dissociation of Chlorine Molecules Under UV Light

When chlorine gas is exposed to UV radiation, each atom retains one electron, forming chlorine free radicals.

1. Formation of Alkyl Radicals
   CH₄+Cl^⋅→CH₃^⋅+HCl
2. The chlorine radical abstracts a hydrogen atom from methane, producing a methyl radical (CH₃∙).
3. Formation of Alkoxy Radicals from Peroxides
   When exposed to heat or UV light, organic peroxides undergo homolytic cleavage to generate alkoxy radicals (RO∙).

Homolytic fission is crucial in free radical reactions, including combustion, polymerization, and photochemical processes.

3.0Heterolytic Fission

Heterolytic fission occurs when a covalent bond breaks in such a way that the shared electron pair is entirely transferred to one of the bonded atoms. This results in the formation of two ions:

• The atom that gains both electrons becomes a negatively charged ion (anion).
• The atom that loses electrons becomes a positively charged ion (cation).

Since this process generates charged species, heterolytic fission is also referred to as ionic fission.

Characteristics of Heterolytic Fission:

• The bond electrons are unequally distributed, leading to ion formation.
• Results in the formation of cations and anions.
• The more electronegative atom attracts the shared electron pair.
• Requires heterolytic bond dissociation enthalpy, which is the energy needed to break the bond in this manner.
• Plays a key role in forming carbocations and carbanions in organic reactions.

Conditions Favoring Heterolytic Fission

Heterolytic fission is favoured under the following conditions:

• High Electronegativity Difference
  A large difference in electronegativity between the two bonded atoms causes the more electronegative atom to pull the shared electrons completely toward itself.
• Polar Bonds
  Since polar molecules have uneven charge distribution, they are more likely to undergo heterolytic fission.
• Presence of a Polar Solvent
  Polar solvents like water or ethanol stabilize the ions formed during heterolytic fission, making the process more favourable.
• Low Temperature
  Unlike homolytic fission, which requires high energy, heterolytic fission is often favoured at lower temperatures.

Examples of Heterolytic Fission

1. Dissociation of Hydrochloric Acid (HCl)

 HCl→ H⁺ + Cl^-

• The chlorine atom, being more electronegative, attracts the shared electrons, forming a chloride ion (Cl⁻), while hydrogen becomes a proton (H⁺).

2. Formation of Carbocations in SN₁ Reactions

 CH₃CH,Br→CH,CH₂⁺+Br⁻

• In the SN₁ reaction of bromoalkane, the bond between carbon and bromine undergoes heterolytic cleavage, forming a carbocation (CH₃CH₂⁺) and a bromide ion (Br⁻).

3. Ionization of Water

 H₂O → H⁺ + OH^-

• Water dissociates into hydrogen and hydroxide ions via heterolytic fission.

Heterolytic fission is fundamental to many ionic reactions, including acid-base chemistry, nucleophilic substitutions, and electrophilic reactions.