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6.4.2Additional lessons

Heterolytic Fission — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 98 of 98

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Part 1 of 6Heterolytic Fission

6.4.2 Heterolytic Fission

Heterolytic fission tends to occur in polar bonds.

Definition
  • In heterolytic fission both electrons of a covalent bond go to one atom, giving a cation and an anion.
Direction
  • The shared pair goes to the more electronegative atom, which becomes negative.
Curly arrow
  • A full-headed curly arrow starts at the bond and points to the atom receiving both electrons.
Example
  • H–Br → H⁺ + Br⁻, where bromine takes the pair of electrons.

Part 2 of 6Nucleophiles and Electrophiles

Nucleophiles and Electrophiles
Nucleophile
  • An electron-rich species with a lone pair, such as OH⁻, NH₃ and H₂O.
Electrophile
  • An electron-deficient species that accepts a pair of electrons, such as H⁺, NO₂⁺ and carbocations.
Coordinate bond
  • When a nucleophile donates a lone pair to an electrophile, a coordinate (dative) covalent bond forms.
Attraction
  • Nucleophiles are attracted to positive centres, while electrophiles are attracted to electron-rich regions.

Part 3 of 6Using Curly Arrows

1
Identify the bond that breaks and the more electronegative atom.
2
Draw an arrow from the bond to the atom that takes both electrons.
3
Show the charges on the resulting ions to confirm the total charge is unchanged.
4
When a nucleophile attacks, draw an arrow from its lone pair to the electron-deficient atom.
FeatureNucleophileElectrophile
ChargeNegative or lone pairPositive or δ+
ActionDonates an electron pairAccepts an electron pair
ExampleOH⁻H⁺

Part 4 of 6Linking to Reactions

Linking to Reactions
Halogenoalkane
  • Nucleophilic substitution begins with attack on the C–X carbon, with the halide leaving as an ion.
Alkene addition
  • H–Br adds to an alkene when H⁺ is accepted and Br⁻ then attacks the carbocation.
Polarity
  • The more polar the bond, the more likely it breaks heterolytically.
Energy
  • Heterolytic fission requires energy too, but is easier in a polar solvent that stabilises the ions.

Part 5 of 6Worked Example

Question Show the heterolytic fission of the C–Cl bond in chloromethane and identify the cation and anion formed.

  1. Chlorine is more electronegative than carbon, so the C–Cl pair of electrons goes to chlorine.
  2. The chlorine atom becomes Cl⁻, and the carbon group becomes CH₃⁺.
  3. CH₃⁺ is electron-deficient, so it acts as an electrophile.

Answer

  • CH₃Cl → CH₃⁺ + Cl⁻;
    • the carbocation is an electrophile and the chloride ion could act as a nucleophile.

Part 6 of 6Exam Checklist & Tips

6.4.2 – Key Points for the Exam

  • Heterolytic fission Breaking of a bond so that both electrons go to one atom.
  • Electrophile A species that accepts an electron pair.
  • Nucleophile A species that donates an electron pair.
  • Coordinate bond A covalent bond in which one atom supplies both shared electrons.

Common Mistakes

  • Using half-headed arrows for heterolytic fission.
  • Giving the electrons to the less electronegative atom.
  • Confusing electrophile with nucleophile because the names look similar.
  • Thinking the electron pair moves from the electrophile to the nucleophile.

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