Nucleophiles, electrophiles and curved arrows
A nucleophile is electron-rich and donates a pair of electrons. An electrophile is electron-poor and accepts a pair. A curved arrow shows a pair of electrons moving, from the nucleophile toward the electrophile.
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Analogy
A nucleophile is a hand reaching out to give. An electrophile is a hand held out to take.
Example
HO⁻ (nucleophile, lone pair)attacks C in CH₃–Cl (electrophile, δ+)Arrow 1: lone pair to carbonArrow 2: C–Cl bond to chlorine
In practice
Nearly every organic mechanism is a story of curved arrows: where electrons start and where they end up.
Practice
Flashcards
- What is a nucleophile?An electron-rich species that donates a pair of electrons to form a bond.
- Where does a curved arrow start?At the electrons: a lone pair or a bond.
Questions people ask
How can I tell which species is the nucleophile?
Look for lone pairs, negative charges or pi bonds, because these are electron-rich. Hydroxide, ammonia and water are nucleophiles. The electrophile is usually a carbon bonded to a more electronegative atom, which makes it partly positive, or a positive ion such as a carbocation.
Where should a curved arrow start and end?
The arrow starts where the electrons are, at a lone pair or a bond, and points to where they go, an atom or a new bond. It never starts at an atom without electrons. A full-headed arrow shows two electrons moving, and a half-headed arrow shows one.