A LITTLE LIGHTBULB MOMENT
Acid-base chemistry, resonance and electron flow
Curved arrows start at electrons and end at electron-poor sites
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Arrow tails begin on electrons; arrowheads land on electron sinks.
The idea behind the cue
In arrow-pushing, the tail starts at a lone pair or bond and the head points to where that electron pair will end up. The arrow shows electron movement, not atom movement. It should preserve octets when possible.
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Short & simple
Arrow tails begin on electrons; arrowheads land on electron sinks.
Acronym
LPO — Lopsided Pancakes Orbit.
Story
A magician launches tiny electron darts from a crowded bench toward an empty chair. The people never move; only the darts travel. Curved arrows always begin where electrons already are.
Connect the pieces
- L (Lopsided) → Lone pairs: Arrow tails start at a lone pair or bond electron pair.
- P (Pancakes) → Poor sites: Arrow heads point to the electron-poor site where those electrons end up.
- O (Orbit) → Octet aware: Good arrow-pushing tries to preserve octets when possible.
- Lopsided · Lone pairs: Arrow tails start at a lone pair or bond electron pair.
- Pancakes · Poor sites: Arrow heads point to the electron-poor site where those electrons end up.
- Orbit · Octet aware: Good arrow-pushing tries to preserve octets when possible.
Know the limits
For radical mechanisms, single-headed arrows are used for single electrons; this card concerns standard curved arrows for paired electrons.
Further reading · OpenStax Organic Chemistry ↗Check your understanding
Where should the tail of a curved arrow begin in typical organic mechanisms?
A. At the positively charged atom only
B. At the product structure after the change
C. At an electron source such as a lone pair or bond
D. At the atom that will move
Reveal explanation
At an electron source such as a lone pair or bond
Curved arrows originate from electron sources like lone pairs or bonds.