Bond Fission & Reagents (Electrophiles/Nucleophiles)

CompleteFundamentals of Chemistry

How bonds break (homolytic vs heterolytic) and how reagents attack (electrophiles vs nucleophiles) sets up every mechanism.

Full notes

Definition: Homolytic fission: the shared pair splits equally, each atom keeps one electron → free radicals (initiated by heat or light). Heterolytic fission: one atom keeps the pair → cation + anion (favoured in polar solvents, by electronegativity difference).

1. Drawing electron movement

Use curved arrows: a full (double-headed) arrow shows movement of an electron PAIR; a half-headed (fish-hook) arrow shows movement of a SINGLE electron (used in radical steps). Arrows always point from the electron source to its destination.

2. Reagents

Electrophile (electron-loving): electron-deficient species seeking electrons — H⁺, NO₂⁺, Br⁺, AlCl₃, BF₃.
Nucleophile (nucleus-loving): electron-rich species donating electrons — OH⁻, CN⁻, NH₃, H₂O, R⁻.
Carbanion is a nucleophile; carbocation is an electrophile; radicals can be either.

3. Types of organic reactions

Substitution (A and B substitute)
Addition (π bond opens, adds)
Elimination (small molecule lost)
Rearrangement (atoms reshuffle; e.g., 1,2-shift)

Feature	Homolytic	Heterolytic
Products	Free radicals	Ions
Requires	Heat/light	Polar solvent / electrostatics
Arrow	Half-headed	Double-headed
Example	Cl₂ → 2Cl•	CH₃–Cl → CH₃⁺ + Cl⁻

Note: Exam point: arrows — half-headed for radicals, double-headed for pairs; always from electron source to sink.

Exam notes

Homolytic → radicals (single-electron arrows). Heterolytic → ions (pair arrows). Electrophiles = e⁻ deficient; nucleophiles = e⁻ rich.

Important points

All mechanisms are simply curved-arrow bookkeeping of electrons.

Common mistakes

Drawing double-headed arrows for radical steps.

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