Definition of an Electrophile
An electrophile is an electron-deficient species (cation or positive end of a dipole) that accepts an electron pair from a nucleophile to form a new covalent bond.
Nucleophiles vs Electrophiles
These are complementary partners. A reaction requires both.
Nucleophiles vs Electrophiles
Nucleophile (R3.4.1)
Electron-pair donor (Lewis base). Electron-rich with available lone pairs (e.g. \(\text{OH}^-, \text{NH}_3, \text{CN}^-\)).
Electrophile (this topic)
Electron-pair acceptor (Lewis acid). Electron-deficient with vacant orbitals (e.g. \(\text{H}^+, \text{NO}_2^+, \text{Br}^{\delta+}\)).
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 Identify Electrophiles
- Positively charged ions: \(\text{H}^+, \text{NO}_2^+, \text{CH}_3^+, \text{Br}^+\)
- Polarised neutral molecules: The \(\delta+\) end of \(\text{H}^{\delta+}-\text{Br}^{\delta-}\) or \(\text{C}^{\delta+}-\text{Cl}^{\delta-}\)
- Polarisable neutral molecules: Non-polar \(\text{Br}_2\) or \(\text{Cl}_2\), where approach to a \(\text{C}=\text{C}\) bond induces a temporary dipole (\(\text{Br}^{\delta+}-\text{Br}^{\delta-}\)).
Induced Dipoles in Halogen Electrophiles
Non-polar bromine (\(\text{Br}_2\)) acts as an electrophile because the high electron density of an alkene's \(\pi\) bond repels electron density in the \(\text{Br}-\text{Br}\) bond as it approaches, inducing an instantaneous dipole (\(\text{Br}^{\delta+}-\text{Br}^{\delta-}\)).
Electrophile Misconceptions
- Direction confusion: Electrophiles accept electrons; nucleophiles donate electrons.
- Neutral electrophiles: Remember that neutral molecules (\(\text{Br}_2, \text{SO}_3, \text{BF}_3\)) can be powerful electrophiles.
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