Heterolysis (chemistry)

In chemistry, heterolysis or heterolytic fission (from Greek ἕτερος, heteros, "different", and λύσις, lusis, "loosening") is the process of cleaving a covalent bond where one previously bonded species takes both original bonding electrons from the other species. During heterolytic bond cleavage of a neutral molecule, a cation and an anion will be generated. Most commonly the more electronegative atom keeps the pair of electrons becoming anionic while the more electropositive atom becomes cationic.

Heterolytic fission almost always happens to single bonds, the process usually produces two fragment species.

The energy required to break the bond is called the heterolytic bond dissociation energy, which is not equivalent to homolytic bond dissociation energy commonly used to represent the energy value of a bond.

One example of the differences in the energies is the energy required to break a Hydrogen-Hydrogen bond H2 → 2 H• ΔH = 104 kcal/mol

H2 → H+ + H− ΔH = 66 kcal/mol (in water)

The discovery and categorization of heterolytic bond fission was clearly dependent on the discovery and categorization of the chemical bond. In 1916, chemist Gilbert N. Lewis developed the concept of the electron-pair bond, in which two atoms may share one to six electrons, thus forming the single electron bond, a single bond, a double bond, or a triple bond. This became the model for a covalent bond. In 1932 Linus Pauling first proposed the concept of electronegativity, which also introduced the idea that electrons in a covalent bond may not be shared evenly between the bonded atoms. However, the ions had been studied before bonds mainly by Svante Arrhenius, who pioneered development of ionic theory and proposed definitions for acids as molecules that produced hydrogen ions, and bases as molecules that produced hydroxide ions.

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