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Organic chemistry · Mechanism fundamentals

How to draw curved arrows in organic chemistry

A curved arrow records where electrons start and where they go. Read a mechanism as a set of changes to bonds and lone pairs, then redraw the resulting structure.

What does a curved arrow mean?

A full arrowhead represents two electrons. Its tail begins at a lone pair or a bond containing that pair. Its tip points to the atom or bond receiving the electrons. The arrow does not trace the movement of an atom through space.

A half-headed “fishhook” represents one electron and is used for radical steps. If a bond splits equally into two radicals, draw two fishhooks from that bond, one toward each atom. Ordinary polar substitution and elimination mechanisms use full-headed arrows.

Keep curved electron-flow arrows separate from the straight reaction arrow between reactants and products. A reaction arrow summarizes a chemical change; the curved arrows explain its electron bookkeeping.

A reliable drawing routine

  1. Reveal the electrons. Add relevant lone pairs, formal charges, and any hydrogen involved in the step. A negative sign is a clue, but the arrow starts at electrons, not at that sign.
  2. Find a plausible receiver. A nucleophile supplies an electron pair; an electrophile accepts it. Neither needs a full charge: ammonia is a neutral nucleophile, and the carbon bonded to bromine in methyl bromide is electrophilic.
  3. Check what must leave. If making a bond would overfill an atom’s valence shell, another pair must move away in the same step.
  4. Draw the result. Change every indicated bond, update lone pairs and charges, and check the total charge before proceeding.

Example 1: transfer a proton

For a direct proton transfer from HCl to ammonia, the nitrogen lone pair forms an N–H bond. At the same time, the H–Cl bond pair goes to chlorine. Hydrogen cannot retain both bonds. Nitrogen ends with four bonds and a +1 formal charge; chloride has four lone pairs and a −1 charge.

Two electron-pair arrows in a proton transfer Ammonia nitrogen has three hydrogen bonds and one lone pair. An arrow runs from that lone pair to the hydrogen of HCl; a second runs from the H–Cl bond to chlorine. Products are ammonium with four nitrogen–hydrogen bonds and a positive charge, and chloride with four lone pairs and a negative charge. Electron flow in one step HNHH HCl + NHHHH Cl +− + NH₄⁺ + Cl⁻
Both orange arrows belong to one proton-transfer step. In aqueous hydrochloric acid, hydronium is the usual proton donor; HCl here makes the bond bookkeeping explicit.

Example 2: make and break bonds together

In this SN2 step, hydroxide donates a lone pair to the carbon of methyl bromide, and the C–Br pair moves to bromine. These changes occur together. Drawing a separate intermediate with five full bonds to carbon would be incorrect. The transition state has partially formed and broken bonds, not five ordinary bonds.

Concerted SN2 electron flow at methyl carbon Hydroxide has three oxygen lone pairs and a negative charge. One lone pair points to the carbon in methyl bromide, while the carbon–bromine bond points to bromine. Products are neutral methanol and bromide with four lone pairs and a negative charge. HOC HHHBr −+ CH₃OHBr +−
The oxygen becomes neutral; bromine becomes Br⁻. The net charge is −1 on both sides. Methyl carbon is not a stereocenter; see SN1 versus SN2 for inversion at a stereocenter.

Resonance arrows describe one species

For resonance, move only electron pairs while keeping the atomic framework fixed. Join the contributors with a double-headed resonance arrow (↔), not equilibrium arrows. Acetate does not oscillate between the two drawings: its real electron distribution is delocalized.

Electron bookkeeping between acetate resonance contributors The left acetate contributor has a carbonyl oxygen above carbon and a singly bonded negatively charged oxygen to the right. An arrow starts at a lone pair on the negative oxygen and ends at the carbon–oxygen bond. A second arrow starts at the carbonyl pi bond and ends at its oxygen. The right contributor has the same atoms in the same relative positions, with the carbon–oxygen bond orders and oxygen charges exchanged. CH₃COO CH₃COO −− ↔ Same atoms, same net charge, different electron placement
One oxygen lone pair becomes a π bond as the original π pair becomes a lone pair on the other oxygen. Carbon keeps an octet in both contributors.

Finish with a charge and valence check

Use formal charge = valence electrons − nonbonding electrons − half the bonding electrons. Oxygen with two single bonds and two lone pairs is neutral; with one single bond and three lone pairs it is −1. Nitrogen with four single bonds and no lone pair is +1.

Hydrogen accommodates two electrons. Do not give ordinary second-row carbon, nitrogen, or oxygen more than eight in a Lewis structure. Fewer can be legitimate: a three-coordinate carbocation has six. Add the charges of all species, including leaving groups. Electron pairs can be redistributed without changing the total charge of the system.

Curved-arrow FAQ

Can an arrow start at a positive charge?

No. Start at an actual electron pair. A positive atom may receive electrons, but its charge symbol is not an electron source.

Does an arrow ending at a bond make a bond?

It places electrons into that bonding region. For example, a lone pair entering an existing single bond can create the π component of a double bond. Check the other atom’s octet too.

Do valid arrows prove a reaction occurs?

No. Correct bookkeeping is necessary, but reactivity also depends on energy, leaving groups, solvent, geometry, and conditions. Draw a chemically plausible step, not merely a structure with acceptable charges.

Textbook references

For further examples, see OpenStax Organic Chemistry: curved arrows in polar mechanisms, rules for resonance, and formal charges.

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