Bond Dissociation Energy

About

Bond Dissociation Energy (BDE) is the amount of energy required to break a specific chemical bond in a molecule, resulting in two isolated atoms or fragments, each with an unpaired electron (radicals). BDE is an important thermodynamic property that reflects the strength of a chemical bond.

Key Concepts of Bond Dissociation Energy (BDE):

1. Definition:

   • BDE is the energy required to homolytically break a bond, meaning that the bond is broken such that each atom retains one electron from the bond, leading to the formation of radicals. The reaction is represented as: $\text{AB} \rightarrow \text{A}^{\bullet} + \text{B}^{\bullet}$ where $\text{A}^{\bullet}$ and $\text{B}^{\bullet}$ are radical species.

2. Units:

   • BDE is usually measured in kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol), which indicates the amount of energy per mole of bonds broken.

3. Bond Strength:

   • A higher BDE indicates a stronger bond, requiring more energy to break, while a lower BDE suggests a weaker bond that requires less energy to break.

   • For example, a C-H bond typically has a BDE of around 413 kJ/mol, while a C=C double bond has a BDE of about 614 kJ/mol, indicating that double bonds are stronger than single bonds.

4. Homolytic vs. Heterolytic Bond Dissociation:

   • Homolytic dissociation (used in defining BDE) breaks a bond evenly, leading to radicals.

   • Heterolytic dissociation results in one atom taking both bonding electrons, leading to ion formation, and requires a different energy value called heterolytic bond dissociation energy.

5. Factors Affecting BDE:

   • Bond Type: Single, double, and triple bonds have different BDEs, with triple bonds being the strongest and single bonds being the weakest.

   • Atom Size and Electronegativity: Larger atoms or atoms with lower electronegativity tend to form weaker bonds, leading to lower BDEs. For example, a C-I bond has a lower BDE compared to a C-F bond.

   • Molecular Environment: The chemical environment, such as nearby atoms, steric effects, and the presence of resonance or delocalization, can affect bond strength. Resonance often lowers BDE, making bonds easier to break.

   • Bond Polarity: Polar bonds tend to have higher BDEs due to the additional electrostatic attraction between atoms.

6. Measurement:

   • BDE can be determined experimentally using techniques like spectroscopy, calorimetry, or from theoretical calculations using quantum chemistry methods (e.g., Density Functional Theory, DFT).

7. Applications:

   • Reaction Mechanism Insights: BDE is used to understand reaction mechanisms, particularly in free radical reactions, where bond breaking and formation are central processes.

   • Stability of Molecules: Molecules with high BDEs are generally more stable, while those with low BDEs are more reactive. For example, the high BDE of the C-H bond in methane explains its stability.

   • Thermodynamics of Chemical Reactions: BDE helps calculate reaction enthalpies, which are key for determining whether a reaction is endothermic or exothermic.

   • Design of Drugs and Materials: Understanding BDEs allows chemists to design molecules with specific reactivity or stability characteristics, which is critical in pharmaceuticals and materials science.

Example:

For the homolytic dissociation of molecular hydrogen:

$\text{H}_2 \rightarrow 2\text{H}^{\bullet}$

The bond dissociation energy of H-H is approximately 435 kJ/mol. This means 435 kJ of energy is needed to break one mole of H-H bonds to form two hydrogen radicals.

Summary:

Bond Dissociation Energy is a crucial concept in understanding the strength and stability of chemical bonds. It provides valuable insights into molecular behavior, reactivity, and the energetics of chemical reactions.

Method

The BDE is predicted using the machine learning model BDE-db2:

SV SS, Kim Y, Kim S, John PC, Paton RS. Expansion of bond dissociation prediction with machine learning to medicinally and environmentally relevant chemical space. Digital Discovery. 2023;2(6):1900-10.

Find

The BDE can be found under the Bond section in the property tree.