Key Definition
A nucleophile is a reactant that forms a bond to its reaction partner (the electrophile) by donating both bonding electrons. Nucleophiles are electron-rich species: they have a lone pair of electrons or a negative charge that they can use to form a new covalent bond.
Recognising Nucleophiles
You need to be able to identify nucleophiles in chemical reactions. There are two main categories.
Negatively charged nucleophiles
Anions carry a full negative charge and have lone pairs readily available for donation.
| Nucleophile | Formula | Lone pairs on attacking atom |
|---|---|---|
| Hydroxide ion | OH⁻ | 3 lone pairs on O |
| Cyanide ion | CN⁻ | 1 lone pair on C |
| Chloride ion | Cl⁻ | 3 lone pairs on Cl |
| Bromide ion | Br⁻ | 3 lone pairs on Br |
Neutral nucleophiles
Neutral molecules can also act as nucleophiles if they have a lone pair available for donation.
| Nucleophile | Formula | Why it acts as a nucleophile |
|---|---|---|
| Water | H₂O | O has 2 lone pairs |
| Ammonia | NH₃ | N has 1 lone pair |
How Nucleophiles React
A nucleophile uses its lone pair to form a new covalent bond with an electron-deficient centre (a carbon atom with a partial positive charge, δ+). The lone pair moves from the nucleophile to the electrophilic carbon.
The Key Feature
All nucleophiles share one thing in common: they have a lone pair of electrons (or a negative charge) that they can donate to form a new bond. The curly arrow always starts from the nucleophile and points towards the electrophilic centre.
Nucleophile Strength
Generally, negatively charged nucleophiles are stronger than neutral ones because they have greater electron density to donate. For example, OH⁻ is a stronger nucleophile than H₂O.
Strong Nucleophiles
- OH⁻ (hydroxide)
- CN⁻ (cyanide)
- NH₂⁻ (amide ion)
Negative charge = higher electron density
Weaker Nucleophiles
- H₂O (water)
- NH₃ (ammonia)
Neutral, but still have available lone pairs
Think About It
Why is ammonia (NH₃) a nucleophile even though it has no charge?
Nitrogen has a lone pair of electrons that is not involved in bonding. This lone pair can be donated to an electron-deficient carbon atom to form a new covalent bond. A full negative charge is not required: the lone pair alone is enough.
Common Exam Mistakes
- Thinking nucleophiles must be negatively charged. Neutral species like H₂O and NH₃ are also nucleophiles.
- Drawing a curly arrow starting from the electrophile. The arrow must start from the nucleophile (where the electrons come from).
- Confusing nucleophiles with electrophiles. Remember: nucleophiles donate electron pairs, electrophiles accept them.
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