Curved arrows are how organic chemists write down what the electrons do during a reaction. Structures show where atoms are; arrows show how bonding electrons move to get from reactants to products. Learn to draw them correctly and every mechanism — substitution, addition, elimination, acid–base — becomes the same short vocabulary applied over and over.
1. A curved arrow moves one pair of electrons
The standard curved arrow has two barbs (a full arrowhead) and represents the movement of one electron pair. A single-barbed “fishhook” arrow moves just one electron and is reserved for radical chemistry. Almost every polar mechanism you draw in a first-year course uses only the double-barbed kind.
2. The tail starts at electrons, the head shows their destination
Every arrow answers two questions: where do these electrons come from and where do they go. The tail must start on an actual electron source — a lone pair or the middle of an existing bond. It never starts on an atom nucleus or on a positive charge. The head points to where the new electron pair ends up: onto an atom (as a new lone pair) or into the space between two atoms (as a new bond).
3. Bonds form and break, and charge is conserved
When an arrow points to an atom or between two atoms, a bond forms; when an arrow starts on a bond and points away, that bond breaks. Bookkeeping is automatic if you obey two conservation laws. First, conserve charge: the total charge on the left must equal the total on the right. An atom that donates a lone pair to make a bond becomes one unit more positive; an atom that receives a breaking bond as a lone pair becomes one unit more negative. Second, never exceed an octet on a second-row atom (C, N, O, F). If a new bond would give carbon five bonds, an old bond must break in the same step.
4. Nucleophilic attack is a lone pair forming a bond
The first canonical move: a lone pair on an electron-rich atom swings out to form a new bond to an electron-poor atom. This is the arrow you draw for any nucleophile attacking any electrophile. In an SN2 reaction the two arrows happen at once — the nucleophile forms its bond to carbon while the leaving-group bond breaks, so carbon never exceeds an octet.
5. A bond can become a lone pair, or become another bond
The remaining two moves round out the toolkit. A bond becoming a lone pair is how a leaving group departs (see the carbocation figure above). A bond becoming a new bond is what happens at a π system: the electrons of a double bond can shift to grab a proton or attack an electrophile. Watch this in nucleophilic addition to a carbonyl — a nucleophile attacks the carbon and the C=O π electrons fold up onto oxygen as a new lone pair.
6. Electrons always flow electron-rich to electron-poor
Every correct arrow in a polar mechanism points the same direction in energy: from a region of high electron density to a region of low electron density. Partial charges (δ+ / δ−) and formal charges are your map. Draw the arrow starting where electrons are abundant — a negative charge, a lone pair, a π bond, an electron-rich atom — and ending where they are wanted, at a δ+ carbon, a proton, or a positive center. If you ever find yourself drawing an arrow that pushes electrons toward a negative charge or onto an already-full octet, the arrow is wrong.
7. Summary
A curved arrow is a promise about electrons: two barbs move one pair, the tail sits on a real source (lone pair or bond), and the head shows the pair's destination (a new bond or a new lone pair). Obey the conservation laws — charge in equals charge out, and no second-row atom exceeds an octet — and every step decomposes into three canonical moves: a lone pair forming a bond, a bond becoming a lone pair, and a bond becoming a new bond. Above all, electrons flow from electron-rich to electron-poor. Master this and you can read, check, and write any polar mechanism.
Quiz yourself
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A double-barbed (full arrowhead) arrow shows the movement of one electron pair and is used for polar mechanisms. A single-barbed “fishhook” arrow moves just one electron and is used for radical reactions.
The tail must start on an electron source — a lone pair or the middle of an existing bond. It can never start on an atom nucleus, on a positive charge, or on empty space.
If cyanide only formed a new bond to carbon without the C–Br bond breaking, carbon would have five bonds and exceed its octet. The second arrow — C–Br breaking onto bromide — keeps carbon at an octet, so both happen in one concerted step.
Always from electron-rich to electron-poor. Use formal and partial charges as a map: arrows start at negative charges, lone pairs, and π bonds, and point toward δ+ carbons, protons, and positive centers.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.