Nucleophiles and electrophiles are the main electron partners in many organic reactions. A nucleophile is electron rich and donates an electron pair, while an electrophile is electron poor and accepts an electron pair. Learning to identify them helps students predict where bonds form, where bonds break, and how a reaction mechanism moves forward.
This idea matters because most acid base reactions, substitutions, additions, and many carbonyl reactions are controlled by electron density.
Key Facts
- Nucleophile means nucleus-loving: it donates an electron pair to an electron-poor atom.
- Electrophile means electron-loving: it accepts an electron pair from a nucleophile.
- Curved arrows in mechanisms show electron pair movement, not atom movement.
- A common bond-forming step is Nu: + E+ -> Nu-E.
- Negative charge, lone pairs, and pi bonds often make a species nucleophilic.
- Positive charge, partial positive charge, empty orbitals, and polar bonds often make a species electrophilic.
Vocabulary
- Nucleophile
- A nucleophile is an electron-rich species that donates an electron pair to form a bond.
- Electrophile
- An electrophile is an electron-poor species that accepts an electron pair to form a bond.
- Curved arrow
- A curved arrow is a mechanism symbol that shows the movement of an electron pair from a donor site to an acceptor site.
- Partial charge
- A partial charge is a small imbalance of electron density in a polar bond, often written as delta positive or delta negative.
- Leaving group
- A leaving group is an atom or group that departs with an electron pair during a reaction.
Common Mistakes to Avoid
- Drawing curved arrows from the electrophile to the nucleophile is wrong because arrows begin at electron-rich sites and point toward electron-poor sites.
- Assuming every negatively charged species is a strong nucleophile is wrong because stability, solvent, size, and resonance can reduce nucleophilicity.
- Ignoring partial positive atoms is wrong because many important electrophiles are neutral molecules with polarized bonds, such as carbonyl carbons.
- Confusing nucleophilicity with basicity is wrong because nucleophilicity measures attack on an atom, while basicity measures attraction to a proton at equilibrium.
Practice Questions
- 1 In the reaction HO- + CH3Br -> CH3OH + Br-, identify the nucleophile, electrophile, and leaving group.
- 2 For acetaldehyde, CH3CHO, identify the electrophilic atom and explain the role of the C=O bond polarity.
- 3 A student draws a curved arrow from the carbonyl carbon to the lone pair on NH3 during addition of ammonia to a carbonyl. Explain why this arrow is incorrect and describe the correct electron movement.