R3.4.3 Heterolytic fission

Consider the molecule hydrogen chloride (H-Cl). When this bond undergoes heterolytic fission:

$$\text{H-Cl}\to {\text{H}}^{+}+{\text{Cl}}^{-}$$

Here, the chlorine atom takes both electrons from the bond, becoming a negatively charged chloride ion (${\text{Cl}}^{-}$), while the hydrogen atom becomes a positively charged proton (${\text{H}}^{+}$).

Heterolytic Fission of H-Cl

Let’s break down the heterolytic fission of the H-Cl bond step by step:

• Start with the polar bond:
  • In H-Cl, the chlorine atom is more electronegative than hydrogen, meaning it attracts the shared electrons in the bond more strongly.
  • This creates a partial negative charge (${\delta }^{-}$) on chlorine and a partial positive charge (${\delta }^{+}$) on hydrogen.$${\text{H}}^{\delta }-{\text{Cl}}^{\delta }$$
• Electron pair movement:
  • When the bond breaks, the shared electron pair moves entirely to the chlorine atom.
  • This is represented by a curly arrow starting at the bond and pointing toward the chlorine atom.
• Formation of ions: The result is the formation of a hydrogen cation (${\text{H}}^{+}$) and a chloride anion (${\text{Cl}}^{-}$). $$\text{H-Cl}\to {\text{H}}^{+}+{\text{Cl}}^{-}$$

Heterolytic Fission of Bromomethane $C{H}_{3}Br$

1. Draw the structure of bromomethane.
2. Identify the partial charges: Carbon (${\delta }^{+}$) and bromine (${\delta }^{-}$).
3. Use a curly arrow to show the movement of the electron pair from the C-Br bond to the bromine atom.
4. Products: A methyl carbocation (${\text{CH}}_3^{+}$) and a bromide anion (${\text{Br}}^{-}$).