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6.4.2Additional lessons

Heterolytic Fission — IB Chemistry HL Revision Notes & Topical Questions

5 min read 4 parts Lesson 151 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 4Unequal Bond Breaking

6.4.2 Heterolytic Fission

Polar bonds tend to break in this way, particularly in polar solvents.

Definition
  • In heterolytic fission both electrons of a shared pair go to one atom when a covalent bond breaks.
Products
  • The result is a cation and an anion, such as H–Br → H⁺ + Br⁻.
Curly arrow
  • A full double-headed arrow starts at the bond and points to the atom that takes both electrons.
Direction
  • The pair goes to the more electronegative atom, which forms the anion.

Part 2 of 4Organic Examples

1
In 2-bromo-2-methylpropane the C–Br bond is polar, with bromine more electronegative than carbon.
2
The bond breaks so that the bromine takes both electrons and leaves as the bromide ion Br⁻.
3
The carbon fragment is left with a positive charge, forming a carbocation (CH₃)₃C⁺.
4
Carbocations are electrophiles, since they have an empty orbital that can accept a pair of electrons.
5
The halide ion, or a nucleophile such as OH⁻, can then react with the carbocation.

Part 3 of 4Nucleophiles and Electrophiles

Nucleophiles and Electrophiles
Nucleophile
  • A nucleophile is an electron-pair donor that is attracted to a positive centre, such as OH⁻ or NH₃.
Electrophile
  • An electrophile is an electron-pair acceptor, such as H⁺, NO₂⁺ or a carbocation.
Charge
  • Nucleophiles are usually negative or have lone pairs, while electrophiles are positive or have δ+ atoms.
Mechanism role
  • Many organic mechanisms combine an electrophile and a nucleophile after heterolytic fission.
FeatureHomolyticHeterolytic
ElectronsOne eachBoth to one atom
ProductsRadicalsIons
Typical bondNonpolarPolar

Part 4 of 4Exam Checklist & Tips

6.4.2 – Key Points for the Exam

  • Heterolytic fission Breaking a covalent bond so that one atom takes both bonding electrons.
  • Carbocation A positively charged carbon species formed when a leaving group departs with the bonding pair.
  • Nucleophile A species that donates a pair of electrons.
  • Electrophile A species that accepts a pair of electrons.

Common Mistakes

  • Using a single-headed arrow for heterolytic fission.
  • Starting the curly arrow from the atom instead of from the middle of the bond.
  • Saying electrophiles are electron-rich.
  • Forgetting to show the charges on the ions formed.

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