Mechanism arrow pushing

The mechanism arrow-pushing puzzle drills electron-flow fluency. The student is shown a substrate and asked to draw the curly arrows that push electrons from nucleophile to electrophile. Lives at /play/mechanism/[exerciseId]. Twelve V3 exercises ship in the catalog, spanning concerted, multi-step ionic, radical, organometallic, and pericyclic mechanisms.

What the exercise tests

Arrow-pushing isolates one piece of mechanism literacy: knowing where electrons go in each step. A correct answer requires the student to identify:

• the source of every electron pair (a lone pair, a bond, or an implicit C-H bond),
• the target that pair lands on (an atom or a bond), and
• the electron count (two-electron / heterolytic vs one-electron / homolytic radical).

Multi-step exercises add step ordering: the catalytic cycle of a Suzuki coupling has to be drawn oxidative addition → transmetalation → reductive elimination, in that order, or the grade drops.

Authoring shape

Each exercise is a MechanismExercise record. The two key fields are the substrate SMILES and a hand-authored array of canonical curly arrows.

Sources

The renderer recognizes four ArrowSource kinds:

• lone-pair-on-atom for a non-bonding pair on a heteroatom (the hydroxide oxygen attacking in SN2).
• bond for a sigma or pi bond between two atom indices (order-insensitive).
• atom for an atom-centered electron source (currently reserved; V3 catalog does not author this kind).
• implicit-h-bond for the C-H bond on a heavy atom that has an implicit hydrogen. Names the C-H pair without authoring an explicit-H substrate, which RDKit silently strips. Used by radical hydrogen abstraction, acid-base deprotonation, and Heck beta-hydride elimination.

Targets

Targets are simpler: an atom (the electrons land on a single atom) or a bond (the electrons land on a bond between two atom indices).

Electron count

electronCount: 2 (or omitted) draws the textbook full-headed curly arrow used for two-electron heterolytic flow. electronCount: 1 draws a half-headed fishhook for radical / single-electron flow.

Multi-step exercises

When exercise.steps is populated, each step carries its own substrate, optional reagent SMILES for two-molecule layouts, and its own canonical arrows. The student walks the steps in order; the renderer advances to the next intermediate after each correct submission.

Two flags control behavior:

• enforceArrowOrder: true requires arrows within a step to be drawn in canonical order. Order-only-wrong arrows earn 50% credit instead of 0%. Useful when the sequence is pedagogically meaningful (e.g., the Suzuki catalytic cycle).
• resonanceArrowSets allows alternative valid arrow patterns to earn full credit. The scorer matches the student's submission against whichever set scores highest, then reports which set was used in the comparison modal.

Renderer features

Everything below is keyboard-accessible. The toolbar buttons mirror the keyboard shortcuts.

• Bond click: starts an arrow from that bond's electron pair.
• Atom click: starts an arrow from a lone pair on that atom.
• Implicit-H selection: keyboard-only in V3. Tab to the heavy atom, press Enter, and the renderer treats the implicit C-H bond as the source.
• Drag-sculpt control points: every drawn arrow has a small invisible handle at its Bezier control point. Drag to reshape the curve around crowded atoms; double-click to reset.
• Undo / redo: Ctrl/Cmd-Z and Ctrl/Cmd-Shift-Z, capped at 50 entries per step. Reset Step pushes the current state onto the undo stack first, so an accidental reset is recoverable.
• Did you mean...: a spurious arrow whose source / target sits close to a canonical arrow surfaces a "Did you mean..." hint in the comparison modal. Clicking the hint swaps the spurious arrow for the canonical one in the renderer state.
• localStorage persistence: arrows and undo / redo stacks auto-save (debounced) under a key scoped to the exercise id, so a refresh doesn't wipe progress.

V3 catalog at a glance

Twelve exercises ship in V3, grouped by mechanism class.

Concerted ionic

• sn2-methyl-bromide (V1 SPIKE). Hydroxide + methyl bromide. Teaches: backside attack and the simultaneous arrow pair.

Multi-step ionic

• sn1-t-butyl-chloride. Two-step: ionization then nucleophilic capture. Teaches: carbocation intermediates and Markovnikov selectivity.
• e2-bromocyclohexane. Concerted antiperiplanar elimination. Teaches: stereoelectronic alignment.
• addition-hcl-to-alkene. Two-step: protonation then chloride capture. Teaches: Markovnikov regio, carbocation stability.

Acyl / carbonyl

• wittig-benzaldehyde. Three-step betaine / oxaphosphetane / cleavage. Teaches: 4-membered ring intermediates.
• pcc-oxidation-1-propanol. Two-step Cr(VI) oxidation. Teaches: chromate ester intermediates and beta-hydride elimination from a metal center.
• nabh4-reduction-acetone. Two-step hydride delivery + workup. Teaches: nucleophilic hydride sources.

Electrophilic aromatic substitution

• eas-nitration-benzene. Three-step explicit (nitronium attack → arenium / Wheland → deprotonation). Teaches: the Wheland intermediate and proton transfer back to recover aromaticity.

Radical

• radical-chlorination-methane. Three-step chain (initiation, propagation 1, propagation 2). Every arrow is a fishhook. Teaches: homolytic flow and the chain-carrier handoff.

Organometallic

• suzuki-coupling-biaryl. Three-step Pd catalytic cycle: oxidative addition → transmetalation (with a bystander hydroxide) → reductive elimination. Teaches: two-electron flow on a transition metal and the closed catalytic cycle.
• heck-reaction-styrene. Three-step Pd cycle: oxidative addition → migratory insertion → beta-hydride elimination. Teaches: alkene insertion and Pd-H formation.

Pericyclic

• diels-alder-butadiene-ethylene. Concerted [4+2] with three curly arrows that visibly form a circular flow around the forming ring (V3 wave 5 cyclic-flow rendering). Teaches: HOMO-LUMO overlap and concerted ring formation.
• photochem-2plus2-ethylene-ethylene. Concerted [2+2] under photochemical conditions. Four fishhook arrows because the excited-state pathway is radical-flavored. Teaches: Woodward- Hoffmann symmetry and excited-state geometry.

Comparison modal

After submitting, the student sees a side-by-side canonical-vs-yours panel for each step, the per-arrow ledger with plain-English diagnostic copy on every entry, and the explanation paragraph that teaches the chemistry independent of arrow position.

What to read next

• Mechanism instructor guide: how to author a new mechanism exercise.
• Education puzzles: the other five play exercise types.
• CE modules and certificates: how to bundle mechanism exercises into a certificate-bearing module.