Heterolytic Fission (DP IB Chemistry): Revision Note

Heterolytic fission

What is heterolytic fission?

• Heterolytic fission is the breaking of a covalent bond in such a way that both bonding electrons are taken by the more electronegative atom.

• This forms:

  • A negative ion after gaining both electrons

  • A positive ion which is left with none

Diagram to show the process of heterolytic fission

• In mechanism diagrams, a curly double-headed arrow is used to show the movement of a pair of electrons

  • The electron pair moves from the bond to one atom

• For example, when a HCl bond breaks heterolytically:

  • The electrons from the H–Cl bond move to the chlorine atom, forming Cl^-

  • This leaves hydrogen as H⁺

• - The resulting ions have distinct roles in organic reactions:

  • The negative ion:

    • Is an electron-rich species

    • Can donate a pair of electrons

    • Is a nucleophile

  • The positive ion:

    • Is electron-deficient

    • Can accept a pair of electrons

    • Is an electrophile

Reaction of nucleophile and electrophile

• A nucleophile can donate a pair of electrons to an electrophile

  • This forms a coordinate covalent bond

• This is the opposite direction to heterolytic bond breaking, and is key to many organic mechanisms (e.g. nucleophilic substitution)

Diagram of a nucleophile donating a pair of electrons to form a coordinate bond