MolDraw › Chemistry tools › Arrow-Pushing Mechanism Practice
Arrow-Pushing Mechanism Practice
In short: Practise drawing curly arrows on 35 real mechanism steps, from proton transfers and SN2 to aldol, nitration, Diels–Alder and radical chlorination. Tap where the electrons start, then where they go, and the tool checks your arrows, gives hints and shows the product. It is free, works in your web browser and needs no download.
Each step shows the reactants with their lone pairs and charges. Draw every arrow for that step, press Check answer, and you get feedback on each arrow, the product and a short explanation. Filter by level or reaction type. Your progress is saved on this device.
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Tap a lone pair, a bond or a radical to start an arrow.
Dots in pairs are lone pairs; a single dot is an unpaired electron; small grey dots mark carbons in skeletal rings. Your arrows are blue.
Product:
The arrows:
How to use the arrow-pushing practice
- Pick a Level (Intro, Intermediate or Advanced) and, if you like, a Reaction type such as Substitution or Enolates.
- Read the equation. It tells you what the step makes.
- Tap the start of an arrow: a lone pair (the pair of dots), a bond, or an unpaired electron. Tapping an atom with lone pairs also selects a lone pair.
- Tap the end: the atom that receives the electrons, or the bond they move into. The arrow appears in blue and is listed below the drawing.
- Repeat for every arrow in the step. For radical steps, first choose Fish-hook (1 e⁻).
- Press Check answer. Wrong arrows are explained one by one. Remove one with ×, or press Undo or Clear.
- Use Hint if you are stuck, or Show answer to see the correct arrows in orange. Then press Next step.
Scoring: a step solved without help scores 1, a step solved after a hint scores 0.5, and a shown answer scores 0. The coloured step buttons show what you have done.
The rules of curly arrows
- An arrow shows electrons moving, not atoms. A full arrow moves two electrons; a fish-hook moves one.
- Start at the electrons. The tail sits on a lone pair, a π bond or a σ bond. Never start at a positive charge or at an H atom.
- End where the electrons go. On an atom, the pair becomes a lone pair. Between two atoms, it becomes a new bond.
- Keep octets. If a new bond forms to an atom that already has a full octet (like C in CH₃Br or a C=O carbon), another arrow must take a pair away from it in the same step.
- Charge bookkeeping. The atom whose lone pair makes a bond becomes one unit more positive. The atom that gains a lone pair becomes one unit more negative. The total charge never changes.
- Hydrogen moves as H⁺ (a proton) when a base takes it, and as H⁻ (hydride) when it moves with its bonding pair, as from BH₄⁻ or in a hydride shift.
Common arrow patterns
| Pattern | Arrows | Example in the practice set |
|---|---|---|
| Proton transfer | Base lone pair → H; H–A bond → A | HO⁻ + HCl, NH₃ + H₃O⁺ |
| Nucleophilic attack | Lone pair or π bond → electrophilic atom | Water + carbocation (SN1) |
| Loss of a leaving group | C–X bond → X | SN1 ionisation of (CH₃)₃CBr |
| Substitution (SN2) | Nucleophile → C; C–X → X | HO⁻ + CH₃Br |
| Addition to C=O | Nucleophile → C; C=O π → O | CN⁻, BH₄⁻, CH₃MgBr |
| Elimination (E2) | Base → H; C–H → C–C; C–X → X | HO⁻ + CH₃CH₂Br |
| Rearrangement | C–H or C–C bond → neighbouring C⁺ | 1,2-hydride and 1,2-methyl shifts |
| Pericyclic | Three arrows in a ring | Diels–Alder |
| Radical | Fish-hooks, one electron each | Cl₂ homolysis, Cl• + CH₄ |
Worked examples
SN2: HO⁻ + CH₃Br
Arrow 1: lone pair on O⁻ → C. Arrow 2: C–Br bond → Br.
O goes from −1 to 0; Br goes from 0 to −1.
CH₃OH + Br⁻, two arrows, one step
E2: HO⁻ + CH₃CH₂Br
Arrow 1: O⁻ lone pair → β-H. Arrow 2: C–H bond → C–C bond. Arrow 3: C–Br bond → Br.
The C–H pair becomes the new π bond.
CH₂=CH₂ + H₂O + Br⁻
Cyanide adds to acetone
Arrow 1: C lone pair of ⁻C≡N → carbonyl C. Arrow 2: C=O π bond → O.
Only the π bond breaks; the σ bond stays.
Tetrahedral alkoxide (CH₃)₂C(CN)O⁻
Enolate formation
Arrow 1: HO⁻ → α-H. Arrow 2: C–H → C–C. Arrow 3: C=O π → O.
The negative charge ends on oxygen.
CH₂=C(CH₃)O⁻ + H₂O
Diels–Alder
Three arrows round the ring: C1=C2 → C2–C3, C3=C4 → C5, C5=C6 → C1.
Two σ bonds form, one π bond moves.
Cyclohexene, no charges
Radical H abstraction
Fish-hooks: Cl• electron → H; one C–H electron → H; the other C–H electron → C.
One radical in, one radical out.
H–Cl + •CH₃
Common mistakes
- Arrows pointing the wrong way. Drawing from C to the nucleophile, or from H⁺ to a base. The tail is always on the electrons.
- Starting at a positive charge or an empty orbital. A carbocation receives electrons; it cannot give them.
- Five-bond carbon. Attacking a C=O carbon or CH₃Br without moving a pair away gives carbon ten electrons.
- Breaking the σ bond of C=O. In addition to a carbonyl, only the π bond moves onto oxygen.
- Ending an E2 arrow on the atom. The C–H electrons go into the C–C bond to make the π bond, so the arrow ends on the bond.
- Full arrows in radical steps. Homolysis splits a bond evenly: two fish-hooks, one electron to each atom.
- Forgetting charges. After the arrows, recount: the nucleophile’s atom becomes more positive and the leaving group’s atom more negative.
Drawing your own mechanisms? Add lone pairs, charges, curly arrows and fish-hooks in MolDraw, a free online chemical structure editor, and export a PNG or SVG for notes and slides.
Open MolDraw editorFrequently asked questions
Where does a curly arrow start and end?
A curly arrow starts at a pair of electrons: a lone pair, a π bond or a σ bond. It ends where that pair goes: on an atom, where it becomes a lone pair, or between two atoms, where it becomes a new bond. Arrows never start at a positive charge or at a hydrogen atom.
What is the difference between a curly arrow and a fish-hook arrow?
A full curly arrow, with a two-sided head, moves two electrons. A fish-hook arrow, with a one-sided head, moves one electron. Fish-hooks are used for radical reactions, such as the homolysis of Cl₂ in light and the steps of radical halogenation.
How many arrows does an SN2 reaction need?
Two. One arrow goes from a lone pair on the nucleophile to the carbon. The second goes from the carbon–leaving group bond to the leaving group. Both happen in one concerted step. An E2 elimination needs three arrows.
Does the order of the arrows matter?
No. All the arrows of one step happen together, so the practice tool accepts them in any order. What matters is that each arrow starts and ends in the right place and that the product follows from them.
How do I work out the charges after drawing the arrows?
An atom whose lone pair forms a new bond becomes one unit more positive. An atom that receives a bonding pair as a lone pair becomes one unit more negative. Check with formal charge = valence electrons − non-bonding electrons − number of bonds. The total charge must be the same on both sides.
Is my progress saved?
Yes. The steps you solve are saved in this browser on this device, so you can come back later. Press Reset progress to start again. Nothing is sent to a server.
Which reactions are included?
35 single steps at three levels: proton transfers, SN1 and SN2, E1 and E2, cyanide, hydride and Grignard additions to C=O, hemiacetal and imine formation, enolates, aldol and Michael additions, HBr and Br₂ additions to alkenes, nitration and Friedel–Crafts acylation, ester hydrolysis, the Diels–Alder reaction, radical chlorination and carbocation shifts.
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