Heterolytic Fission: Uneven Bond Breaking and Ion Formation
What Is Heterolytic Fission?
Definition
Heterolytic fission
In chemistry, heterolytic fission refers to the breaking of a covalent bond where both bonding electrons are transferred to one of the two bonded atoms.
This process results in the formation of two ions:
- A cation (positively charged ion) that loses the electron pair.
- An anion (negatively charged ion) that gains the electron pair.
Example
- Consider the molecule hydrogen chloride (H-Cl).
- When this bond undergoes heterolytic fission: $$
\text{H-Cl} \rightarrow \text{H}^+ + \text{Cl}^-
$$ - 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}^+ $).
Note
Heterolytic fission typically occurs in polar covalent bonds where there is a significant difference in electronegativity between the two bonded atoms.
How Does Heterolytic Fission Happen?
To understand the process, we need to examine the movement of electrons.
- Chemists use curly arrows to represent the flow of electron pairs during bond-breaking and bond-making processes.
- A double-barbed curly arrow shows the movement of an electron pair.
Example
Heterolytic Fission of H-Cl
Let’s break down the heterolytic fission of the H-Cl bond step by step:
- Start with the polar bond:
- In H-Cl, the chlorine atom is more electronegative than hydrogen, meaning it attracts the shared electrons in the bond more strongly.
- This creates a partial negative charge ($ \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}^- $). $$\text{H-Cl} \rightarrow \text{H}^+ + \text{Cl}^-$$
Example
Heterolytic Fission of Bromomethane $CH_3Br$
- Draw the structure of bromomethane.
- Identify the partial charges: Carbon ($ \delta^+ $) and bromine ($ \delta^- $).
- Use a curly arrow to show the movement of the electron pair from the C-Br bond to the bromine atom.
- Products: A methyl carbocation ($ \text{CH}_3^+ $) and a bromide anion ($ \text{Br}^- $).
Common Mistake
- Forgetting to use a double-barbed curly arrow to show the movement of an electron pair: Always use a double-barbed arrow when dealing with heterolytic fission, as both electrons in the bond move together.
- Misidentifying the products of heterolytic fission: Ensure that the more electronegative atom becomes the anion (gains the electrons), while the less electronegative atom becomes the cation (loses the electrons).
Self review
- What type of bond is most likely to undergo heterolytic fission?
- In the heterolytic fission of $H-Cl$, which atom becomes the anion?
- Draw the curly arrow mechanism for the heterolytic fission of $CH_3Cl$.
