Curved arrow notation is the language chemists use to show how electrons move during a reaction mechanism. It matters because bonds form and break through electron movement, not by atoms randomly rearranging. A well-drawn curved arrow lets you predict products, charges, and the order of steps in many organic reactions.
For example, hydroxide can attack chloromethane by donating an electron pair to carbon while the C-Cl bond electrons move to chlorine.
Key Facts
- A full curved arrow with a double barb shows movement of an electron pair.
- A fishhook arrow with a single barb shows movement of one electron, often in radical mechanisms.
- Arrows start at an electron source such as a lone pair, negative charge, or bond.
- Arrows end at an electron acceptor such as an atom, bond-forming position, or antibonding site.
- In SN2 attack: Nu:- + R-Cl -> R-Nu + Cl:-, one arrow goes from Nu:- to carbon and one goes from the C-Cl bond to Cl.
- Formal charge = valence electrons - nonbonding electrons - 1/2 bonding electrons.
Vocabulary
- Curved arrow
- A symbol used in reaction mechanisms to show the direction of electron movement.
- Nucleophile
- An electron-rich species that donates an electron pair to form a new bond.
- Electrophile
- An electron-poor species that accepts an electron pair to form a new bond.
- Leaving group
- An atom or group that takes electrons and departs from a molecule during a reaction step.
- Formal charge
- The assigned charge on an atom based on its valence electrons, lone pairs, and shared bonding electrons.
Common Mistakes to Avoid
- Starting an arrow at an atom instead of electrons is wrong because curved arrows show electron movement, so the tail must begin at a lone pair, bond, or negative charge.
- Pointing the arrow from electrophile to nucleophile is wrong because nucleophiles donate electrons and electrophiles accept them.
- Forgetting the leaving group arrow in substitution reactions is wrong because the bond to the leaving group must break to avoid exceeding carbon's valence.
- Using a full curved arrow for a radical step is wrong because radical mechanisms move one electron at a time and require single-barb fishhook arrows.
Practice Questions
- 1 Draw the curved arrows for HO:- + CH3Cl -> CH3OH + Cl:-. Identify the electron source and electron acceptor for each arrow.
- 2 In the step Br:- + CH3I -> CH3Br + I:-, assign formal charges to bromine and iodine before and after the reaction.
- 3 A student draws an arrow from the carbon of CH3Cl to the oxygen of HO:- during nucleophilic attack. Explain what is wrong with the arrow direction and how to correct it.