bonddissociation

Bond dissociation, sometimes written bonddissociation in plain text, is the process by which a chemical bond is cleaved to produce two fragments. In many contexts this is described in terms of bond dissociation energy (BDE), the enthalpy change required to break one mole of bonds in a molecule in the gas phase to yield neutral radicals.

Bond cleavage can occur by homolysis, in which each fragment retains one electron from the broken bond, generating two radicals, or by heterolysis, in which both electrons are transferred to one fragment, yielding an anion and a cation. The pathway influences the chemical reactivity of the products and is influenced by the environment and temperature.

Practically, BDE is a key quantity that characterizes bond strength. It is determined experimentally or computed. In simpler terms, it is the energy difference between the reactant and the products at large separation. The magnitude of BDE depends on the bond order, the electronic structure of the fragments, resonance stabilization, hyperconjugation, and strain in the molecule.

Several factors can lower or raise BDEs: substitution that stabilizes radicals, conjugation, heteroatom effects, solvent and phase, and geometric strain in rings or rigid frameworks. BDEs in gas phase can differ from those in solution.

Measurement and calculation: BDEs can be obtained from thermochemical cycles, spectroscopic methods, or mass spectrometry, and are also calculated with quantum chemistry methods such as density functional theory. Knowledge of BDEs informs reaction mechanisms, radical chemistry, photochemistry, and materials science.