Question
Explain what an electrophile and a nucleophile are. Give two examples of each and identify whether the following are electrophiles or nucleophiles: H⁺, OH⁻, CN⁻, BF₃, NH₃.
Solution — Step by Step
“Electro” = electron, “phile” = lover. An electrophile is an electron-lover — it wants electrons. “Nucleo” = nucleus (which is positive), so a nucleophile is a nucleus-lover — it wants to attack positive centers by donating electrons.
This word-splitting trick alone will help you answer any definition question in 5 seconds flat.
An electrophile is electron-deficient — it either carries a positive charge or has an atom with an incomplete octet. It accepts an electron pair during a reaction.
Examples: H⁺ (bare proton, no electrons at all) and NO₂⁺ (nitronium ion, used in nitration of benzene).
A nucleophile is electron-rich — it has a lone pair or a negative charge it’s willing to donate. It attacks electron-deficient centers.
Examples: OH⁻ (hydroxide, has lone pairs + negative charge) and CN⁻ (cyanide, lone pair on carbon).
Work through each one:
| Species | Electron-rich or deficient? | Type |
|---|---|---|
| H⁺ | No electrons at all | Electrophile |
| OH⁻ | Negative charge + lone pairs | Nucleophile |
| CN⁻ | Negative charge + lone pair on C | Nucleophile |
| BF₃ | B has only 6 electrons (incomplete octet) | Electrophile |
| NH₃ | Lone pair on N, neutral but electron-donating | Nucleophile |
Answer: H⁺ and BF₃ are electrophiles. OH⁻, CN⁻, and NH₃ are nucleophiles.
Why This Works
Organic reactions are fundamentally about electron flow. Something electron-deficient pulls electrons toward itself; something electron-rich pushes electrons away. Every substitution and addition reaction in organic chemistry is just a nucleophile attacking an electrophile.
The BF₃ case trips up many students because it carries no charge — but charge is not the only criterion. Boron in BF₃ has only 6 electrons around it (3 bonds × 2 = 6), so it desperately wants two more. It will accept a lone pair from, say, NH₃ to form a coordinate bond — classic Lewis acid behaviour.
NH₃ is the mirror image: neutral, but nitrogen has a lone pair sitting available. That lone pair is what makes it a nucleophile. This is why ammonia reacts with alkyl halides (nucleophilic substitution) and coordinates to metal ions.
Alternative Method — Lewis Acid/Base Framework
In NCERT and JEE questions, electrophiles are often called Lewis acids and nucleophiles are called Lewis bases. The concepts are identical — Lewis acids accept electron pairs, Lewis bases donate them. If you see “Lewis acid” in a question, think electrophile automatically.
Lewis acid = electrophile: accepts an electron pair.
Lewis base = nucleophile: donates an electron pair.
So BF₃ (Lewis acid) + NH₃ (Lewis base) → BF₃·NH₃ is the same logic as electrophile + nucleophile → product. Knowing this equivalence means one framework covers both inorganic coordination chemistry and organic reaction mechanisms.
Common Mistake
Students write “nucleophile = negatively charged species.” That’s wrong. Charge is not required.
NH₃, H₂O, alcohols (R–OH), and even alkenes are nucleophiles despite being neutral. What matters is the availability of electrons — a lone pair or a π bond. If you define nucleophile purely by charge, you’ll get SN2 mechanism questions wrong when the nucleophile is a neutral solvent molecule like water or ethanol.