A nucleophile donates an electron pair to form a covalent bond, whereas an electrophile accepts that electron pair. This core distinction in Reactivity 3.4 applies at both IB Chemistry SL and HL.
The Reasoning
A nucleophile has an available electron pair, often as a lone pair or an electron-rich bond. It is attracted to a region of low electron density and forms a bond by donating both bonding electrons.
An electrophile is electron-deficient and can accept an electron pair. It is attracted to a region of high electron density.
| Feature | Nucleophile | Electrophile |
|---|---|---|
| Role | Electron-pair donor | Electron-pair acceptor |
| Typical character | Electron-rich | Electron-deficient |
| Common examples | , , | , when polarized, the carbon in a halogenoalkane |
| Reaction partner | Attacks an electrophile | Is attacked by a nucleophile |
For example, in the reaction between hydroxide ions and a halogenoalkane, is the nucleophile. Its lone pair is donated to the partially positive carbon atom, which acts as the electrophilic centre. A new carbontooxygen bond forms while the carbontohalogen bond breaks, producing a leaving group.
In an alkene, the carbon carbon double bond has high electron density and acts as a nucleophilic region. During electrophilic addition, it donates an electron pair to an electrophile such as .
A common misconception is that nucleophiles must be negative and electrophiles must be positive. Charge is a useful clue, but it is not essential: is a neutral nucleophile, while a atom can be an electrophilic centre without having a full positive charge.
Exam Technique
For an explain question, identify the electron-rich and electron-deficient regions and state the direction of electron-pair movement. In mechanisms, draw curly arrows starting at the lone pair or bond supplying the electrons and ending where the new bond forms.