Definition of a Nucleophile
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 with a lone pair of electrons or a negative charge.
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 Essential Feature of Nucleophiles
All nucleophiles share one essential feature: they possess at least one non-bonding lone pair of electrons (or a negative charge) that can be donated to an electron-deficient centre (\(\delta+\) carbon). Curly arrows always start from the lone pair/negative charge and point 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 vs Weaker Nucleophiles
Stronger (Anionic) Nucleophiles
- Hydroxide ion (\(\text{OH}^-\))
- Cyanide ion (\(\text{CN}^-\))
- Amide ion (\(\text{NH}_2^-\))
Full negative charge = higher electron density
Weaker (Neutral) Nucleophiles
- Water (\(\text{H}_2\text{O}\)) - lone pairs on O
- Ammonia (\(\text{NH}_3\)) - lone pair on N
Neutral, but possess available lone pairs
Neutral Nucleophiles: Ammonia
Why is ammonia (\(\text{NH}_3\)) a nucleophile even though it has no charge?
Nitrogen has a non-bonding lone pair of electrons. 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: an available lone pair is sufficient.
Common Nucleophile Errors
- Negative charge misconception: Thinking nucleophiles must be negatively charged (remember \(\text{H}_2\text{O}\) and \(\text{NH}_3\)).
- Curly arrow direction: Drawing arrows starting from the electrophile instead of from the lone pair of the nucleophile.
- Lone pair omissions: Always show the lone pair on the nucleophilic atom in mechanism diagrams!
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