Curved Arrow Conventions in Mechanisms
Curved Arrow Conventions in Mechanisms
Curved arrows are the language of mechanisms: they show where electron pairs move. Arrows start at an electron source (lone pair or bond) and point to the destination (atom or bond) that receives those electrons. Keeping charges and octets consistent is key.
Rules for Drawing Curved Arrows
- Arrows = electron pairs: Full-headed arrows move two electrons; fishhooks move one (radicals).
- Start at electron source: Lone pair or bond that will donate electrons—never at a positive charge.
- Point to electron sink: The atom/bond that forms or receives the electrons; when a bond breaks, point the arrow to the atom that takes the pair.
- Check charges/octets: After arrow-pushing, formal charges and octets must make sense.
Example applying the curved-arrow rules: arrow starts at an electron source and ends at the electron sink.
Example: Proton Transfer (Acetic Acid + Water)
- Arrow from water lone pair to the acidic H of acetic acid (new O–H bond).
- Arrow from the acetic acid O–H bond back to its oxygen (lone pair formed).
Result: acetate (CH₃COO⁻) + hydronium (H₃O⁺); arrows track electron flow.
Summary
- Start arrows at electron-rich sites; end them where electrons go.
- Use full heads for two-electron steps (most beginner mechanisms).
- Track resulting charges/octets to confirm the mechanism step is valid.