Difference Between Homolytic and Heterolytic Bond Dissociation Energy

Key Difference – Homolytic vs Heterolytic Bond Dissociation Energy
 

Bond dissociation energy is a measure of the strength of a chemical bond. A bond can be dissociated in a homolytic manner or a heterolytic manner. Bond dissociation energy is defined as the standard enthalpy change when a chemical bond is cleaved via homolysis. Homolytic bond dissociation energy is the amount of energy required to dissociate a chemical bond via hemolysis whereas heterolytic bond dissociation energy is the amount of energy required to clave a chemical bond via heterolysis. The value of homolytic bond dissociation energy is different from that of heterolytic bond dissociation energy for the same compound. This is the key difference between homolytic and heterolytic bond dissociation energy.

CONTENTS

1. Overview and Key Difference
2. What is Homolytic Bond Dissociation Energy
3. What is Heterolytic Bond Dissociation Energy
4. Side by Side Comparison – Homolytic vs Heterolytic Bond Dissociation Energy in Tabular Form
5. Summary

What is Homolytic Bond Dissociation Energy?

Homolytic bond dissociation energy is the amount of energy required to dissociate a chemical bond via hemolysis. Hemolysis of a chemical bond is the symmetric cleavage of the bond forming two radicals, not two ions. Here, the bond electrons between the atoms are divided into two halves and are taken by the two atoms. For example, the homolytic cleavage of a sigma bond forms two radicals having one unpaired electron per each radical.

The bond dissociation energy is defined as the amount of energy required to cleave a chemical bond via hemolysis at standard conditions. The homolytic bond dissociation energy determines whether a chemical bond is strong or weak. If the value of homolytic bond dissociation energy is higher, it indicates that a high amount of energy should be provided in order to cleave that bond; hence, it is a strong bond.

What is Heterolytic Bond Dissociation Energy?

Heterolytic bond dissociation energy is the amount of energy required to clave a chemical bond via heterolysis. Heterolysis is the cleavage of a chemical bond in asymmetric manner. Heterolysis forms cations and anions. This is because, in heterolysis, the bond electron pair is taken by the electronegative atom (it is converted into the anion) whereas the other atom takes no electrons (it forms the cation).

When compared with the homolysis of a molecule, the heterolysis of the same molecule is a different value from that of homolysis. This means the homolytic bond dissociation energy of a compound is different from the heterolytic bond dissociation energy of the same molecule.

Ex: Let us consider the cleavage of the H-H bond in the hydrogen molecule.

Homolytic bond dissociation: H₂      →    H●  +   H●   (bond dissociation energy is 104 kcal/mol)

Heterolytic bond dissociation: H₂     →   H⁺   +   H^–  (bond dissociation energy is 66 kcal/mol)

What is the Difference Between Homolytic and Heterolytic Bond Dissociation Energy?

Homolytic vs Heterolytic Bond Dissociation Energy
Homolytic bond dissociation energy is the amount of energy required to dissociate a chemical bond via hemolysis.	Heterolytic bond dissociation energy is the amount of energy required to clave a chemical bond via heterolysis.
Product
Homolytic bond dissociation energy is associated with the formation of radicals by the cleavage of chemical bonds.	Heterolytic bond dissociation energy is associated with the formation of cations and anions by the cleavage of chemical bonds.

Summary – Homolytic vs Heterolytic Bond Dissociation Energy

Bond dissociation energy is the energy required to cleave a chemical bond via homolysis at standard conditions. There are two types of bond cleavages as homolysis and heterolysis. Homolytic bond cleavage forms radicals whereas heterolytic bond cleavage forms cations and anions. The key difference between homolytic and heterolytic bond dissociation energy is that the value of homolytic bond dissociation energy is different from that of the heterolytic bond dissociation energy for the same compound.

Reference:

1. “Bond-Dissociation energy.” Wikipedia, Wikimedia Foundation, 28 Feb. 2018, Available here.
2. “Heterolysis (Chemistry).” Wikipedia, Wikimedia Foundation, 19 Feb. 2018, Available here.
3. “Bond Dissociation Energies = Homolytic Cleavage.” Master Organic Chemistry RSS, Available here.

Image Courtesy:

1. “Homolysis (Chemistry)” By Jürgen Martens – Jürgen Martens (Public Domain) via Commons Wikimedia
2. “Heterolysis (Chemistry)” By [Jürgen Martens – Jürgen Martens (CC BY-SA 3.0) via Commons Wikimedia