Key Definition
An electrophile is a reactant that forms a bond to its reaction partner (the nucleophile) by accepting both bonding electrons from that partner. Electrophiles are electron-deficient species: they have a positive charge, a partial positive charge, or an empty orbital.
Nucleophiles vs Electrophiles
These are complementary partners. A reaction requires both.
Nucleophile (R3.4.1)
Donates an electron pair. Electron-rich. Has a lone pair or negative charge.
Electrophile (this lesson)
Accepts an electron pair. Electron-deficient. Has a positive or δ+ charge, or an empty orbital.
Recognising Electrophiles
You need to be able to identify electrophiles in chemical reactions. There are two main categories.
Positively charged electrophiles
| Electrophile | Formula | Why it is electron-deficient |
|---|---|---|
| Hydrogen ion | H⁺ | No electrons at all, empty 1s orbital |
| Nitronium ion | NO₂⁺ | Positive charge on nitrogen |
| Carbocation | R₃C⁺ | Carbon has only 6 electrons (incomplete octet) |
Neutral electrophiles
Neutral molecules can act as electrophiles if they have a partial positive charge (δ+) on one atom due to bond polarity.
| Electrophile | Formula | Which atom is δ+? |
|---|---|---|
| Hydrogen halides | HBr, HCl | H is δ+ (halogen pulls electron density) |
| Bromine (polarised) | Br₂ | Becomes polarised near an electron-rich centre |
| Halogenoalkanes | R-X | Carbon bonded to halogen is δ+ |
How to Spot an Electrophile
Look for species that are electron-deficient. Ask yourself: does this species have a positive charge, a partial positive charge (δ+), or an incomplete octet? If yes, it is likely an electrophile. The curly arrow will always point towards the electrophile from the nucleophile.
Think About It
Why can Br₂ act as an electrophile even though it has no overall charge and no permanent dipole?
When Br₂ approaches an electron-rich region (e.g. the C=C double bond of an alkene), the electrons in the pi bond repel the electrons in the Br-Br bond. This creates a temporary (induced) dipole, making the nearest Br atom δ+ and the other δ-. The δ+ end can then accept an electron pair from the alkene.
Common Exam Mistakes
- Saying an electrophile "donates" electrons. Electrophiles accept electron pairs.
- Forgetting that neutral molecules can be electrophiles if they have a δ+ region.
- Confusing nucleophile and electrophile. Think: nucleophile = nucleus-loving = attracted to positive, electrophile = electron-loving = attracted to negative/electron-rich.
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