How to push arrows in organic chemistry mechanisms
A curved arrow shows where a pair of electrons moves, never where an atom moves. It must start on electrons — a lone pair or the middle of a bond — and end where those electrons land: on an atom, or between two atoms that are forming a bond. Electrons always flow from electron-rich to electron-poor, and no second-row atom may ever exceed eight valence electrons.
What an arrow actually means
This is the single idea that makes mechanisms learnable: curved arrows track electrons, not atoms. When a proton moves from an acid to a base, the arrow does not go from the proton to the base. It goes from the base's lone pair to the proton, because the electrons are what move — the proton is just a bare nucleus being captured.
Students who draw arrows from atoms get stuck on every non-obvious mechanism, because they are tracking the wrong thing. Students who draw arrows from electrons can reason their way through reactions they have never seen.
Two arrowheads, two meanings. A full arrowhead moves a pair of electrons; that is almost everything you will draw. A half arrowhead (a "fishhook") moves a single electron, and you only need it for radical chemistry — free radical halogenation, radical initiators, anti-Markovnikov HBr addition with peroxides.
The three rules
- Start on electrons. The tail of every arrow sits on a lone pair, on a negative charge, or in the middle of a bond. If your arrow starts on a nucleus, on an atom symbol, or on a positive charge, it is wrong.
- End where the electrons are going. Point the head at an atom (making a lone pair or an anion) or between two atoms (making a bond).
- Respect the octet. Carbon, nitrogen, oxygen and fluorine can never hold more than eight valence electrons. If your arrow would give carbon a fifth bond, another arrow must leave that carbon at the same time.
The four legal moves
Almost every mechanism in a two-semester course is a sequence of these four.
| Move | From | To | Seen in |
|---|---|---|---|
| Lone pair → bond | A lone pair on N, O, S or a carbanion | Between that atom and an electrophilic carbon or H | Nucleophilic attack, protonation, deprotonation |
| Bond → bond | A π bond or a σ bond | Between a different pair of atoms | SN2, alkene attacking an electrophile, hydride shifts |
| Bond → lone pair | A bond | Onto one of its own atoms | Leaving group departure, carbonyl π collapsing onto oxygen |
| Half arrow (single electron) | One electron of a bond or a radical | To form a new bond or a new radical | Radical initiation, propagation, termination |
Where do the electrons go? Follow the charge.
Electron-rich attacks electron-poor. Before you draw anything, label the map:
- Electron-rich (arrow tails live here): negative charges, lone pairs on N/O/S, π bonds of alkenes and arenes, C–Mg and C–Li bonds.
- Electron-poor (arrowheads point here): positive charges, carbonyl carbons, carbons bearing good leaving groups, protons on strong acids, the terminal atom of a polarised halogen.
If you can label both lists on the structure, the arrows almost draw themselves.
Worked mechanisms, arrow by arrow
Protonating an alcohol
One arrow, from a lone pair on the alcohol oxygen to the proton of H₃O⁺. A second arrow from the O–H bond of H₃O⁺ onto its own oxygen, so that oxygen does not exceed its octet. Result: a protonated alcohol with a positive oxygen, and a neutral water molecule. Notice that the reason for the second arrow is purely the octet rule.
SN2 on bromoethane with hydroxide
Two arrows drawn simultaneously, because the step is concerted. Arrow one: from a lone pair on hydroxide to the carbon bearing bromine. Arrow two: from the C–Br bond onto bromine, giving bromide. Carbon never holds five bonds because the two arrows happen at once — which is exactly why the transition state has the nucleophile and leaving group at 180°, and why the configuration inverts.
Forming a carbocation (the E1/SN1 first step)
One arrow, from the C–Br bond onto bromine. Nothing attacks; the bond simply breaks heterolytically with both electrons going to the more electronegative atom. You are left with a planar, positively charged carbon that has only six valence electrons and is desperate for two more.
E2 on 2-bromobutane with ethoxide
Three arrows at once. From a lone pair on ethoxide to the β-hydrogen; from the C–H bond into the space between the α and β carbons, forming the π bond; from the C–Br bond onto bromine. Every atom keeps its octet at every instant, and the requirement that the C–H and C–Br be anti-periplanar falls straight out of needing those orbitals to overlap.
Mistakes graders always circle
- An arrow starting at H⁺. A proton has no electrons to give. The arrow must come to it.
- An arrow starting on a positive charge. Positive means electron-deficient. Arrows point there, they do not leave from there.
- Pentavalent carbon. If an arrow arrives at a carbon that already has four bonds, an arrow must leave it in the same step.
- Arrows that move atoms. Drawing an arrow from a methyl group to show it "moving" is tracking the wrong object. In a methyl shift, the arrow starts at the C–C bond.
- Losing track of charge. Total charge is conserved across every step. If your products have a different net charge from your reactants, an arrow is missing.
- Skipping proton transfers. Acid-catalysed mechanisms are mostly proton transfers. Each one needs its own arrow, and full marks usually require them all.
How to practise this
Arrow pushing is a motor skill, not a fact. Redraw the same five mechanisms — protonation, SN2, carbocation formation, E2, carbonyl addition — until the arrows come without thinking, then extend to combinations. Cover the mechanism in your notes, draw it, then check each arrow against three questions: does it start on electrons, does it end somewhere sensible, and does every atom still have a legal octet?
How Organic Chemistry AI helps here
Each solve is split into numbered steps that say what attacks what, which bond breaks and where the charge ends up — that is the arrow-pushing reasoning written out in words, which is exactly what you need when you are trying to work out why an arrow goes where it goes. The bundled mechanism library holds 106 named reactions you can browse by type, and the tutor will take a follow-up question about any single step you did not follow.
Free radical bromination in the mechanism library — the half-arrows that move single electrons are the notation you only need for radical chemistry.
Frequently asked
When do I use a fishhook half-arrow instead of a full arrow?
Only in radical chemistry, where bonds break homolytically and one electron goes to each fragment. Free radical halogenation, peroxide-initiated HBr addition and polymerisation initiation are the common cases in a first-year course. Everything else moves electron pairs.
Do I have to draw every proton transfer?
In an acid-catalysed mechanism, yes. Mark schemes usually award a point per correct arrow, and proton transfers make up a large share of the arrows in ester hydrolysis, acetal formation and acid-catalysed hydration. Skipping them is the most common way to lose marks on a mechanism you otherwise understand.
How do I know which resonance structure to push from?
Push from the one that makes the electron flow obvious, then check that your product is the same regardless. For a carbonyl, the neutral structure shows why the carbon is electrophilic through polarity; the charge-separated structure (C⁺–O⁻) shows it directly. Both are the same molecule, so both give the same mechanism.
Read a mechanism one step at a time
Photograph the reaction, and get the steps written out — what attacks what, and where the charge goes.
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