What a curly arrow actually says
A curly arrow is not decoration and it is not a summary. It is a precise claim about two electrons: where they started and where they ended up. Everything below assumes those electrons live in orbitals that are either filled or empty — if that sentence is not yet physical to you, the rung below builds it.
The tail sits on the electrons — a lone pair, or the middle of a bond. The head points at where those electrons are going — an atom, or the space between two atoms where a new bond will form. Nothing else about the drawing carries meaning.
- Electron source — where the tail sits
- Electron sink — where the head points
- Structure · unchanged framework
Push and pull: the whole of polar chemistry
Two of the twelve moves do most of the work. A pair of electrons pushes in, and a leaving group pulls a pair out. Every polar reaction you have heard of is just these two, arranged in time.
This is the single most compressive idea on this page. SN1, SN2, E1, E2, carbonyl addition and carbonyl substitution are not six mechanisms to memorise. They are one pair of moves with six schedules.
| Schedule | What happens | You already call it |
|---|---|---|
| Push only | A pair arrives, nothing leaves. The molecule gets bigger. | Addition to an aldehyde or ketone |
| Push, then pull | A pair arrives and waits in an intermediate, then a group leaves. | Ester and amide substitution |
| Push and pull together | One step. The arriving pair shoves the leaving group out. | SN2 |
| Pull, then push | The group leaves first, making a carbocation. A pair fills it. | SN1 |
| Push from a C–H bond | The pushing pair comes from a neighbouring bond, so a pi bond forms. | E2 (together) or E1 (after) |
| Pull only | A group leaves and nothing replaces it yet. | Ionisation — the start of many things |
Notice what actually varies: where the pushing pair comes from (a nucleophile, a lone pair, or a C–H bond) and when it arrives (before, during, or after the leaving group departs). That is the whole decision tree.
The twelve moves
Grouped by which engine they belong to. Position carries the grouping here, not colour — four families is too many to hue-code honestly.
Polar moves — five, and they carry most of the book
An acid gives H to a base. The fastest step in chemistry, and usually reversible. When a mechanism looks stuck, a proton has probably moved.
A pair pushes into an electron-poor atom. Three common targets: saturated carbon, a carbonyl carbon, a conjugated alkene. Same arrow each time.
A group departs taking the bonding pair. It leaves willingly only if the resulting anion is stable — which is a pKa question.
The pushing pair comes from a C–H bond next door, so a pi bond forms as the leaving group goes. Needs the two to line up in space.
A neighbouring bond slides across into an empty orbital, carrying its atom with it. This is why carbocations rarely stay where you put them.
Radical moves — three, using half-arrows
A weak bond splits evenly, one electron each way. Started by heat, light, or a peroxide. This is how a chain begins.
A radical adds to a pi bond and a new radical appears further along. The chain propagates rather than terminating.
A radical steals a whole atom, usually hydrogen or halogen, leaving a new radical behind. Selectivity here follows bond strength.
Concerted move — one, and it has no intermediate at all
Arrows chase each other round a closed ring and all bonds change at once. No charges, no intermediate. Whether it works is decided by orbital symmetry, not by rich-and-poor. Diels–Alder is the famous one.
Metal moves — three, with different bookkeeping
A metal inserts itself into a bond, taking both fragments as ligands. Count electrons on the metal, not octets on carbon.
The middle step. A second partner is loaded onto the metal, or an alkene slots into an existing metal–carbon bond.
Two ligands on the metal join and leave as one molecule. The metal is handed back unchanged, ready to go round again — this is catalysis.
Worked example: four moves make a named reaction
Base-mediated ester hydrolysis — the reaction that turns fat into soap. Textbooks give it a name and a page. It is moves 2, 3 and 1, in that order, and nothing else.
- Electron-rich — stable end point
- Electron-poor — the target
- Leaving group departing
- Neutral intermediate
The last step is worth pausing on. A carboxylic acid is far more acidic than an alcohol, so the proton transfer runs downhill and cannot come back. That is why this reaction needs a full equivalent of base rather than a catalytic trace. The mechanism explains the recipe — which is the point of learning it this way.
Grok check
Recall questions are the wrong test. These are prediction questions: if the page worked, you can answer them about reactions nobody has taught you.
- An ester has a much better leaving group than an amide. Using move 3 alone, predict which one hydrolyses faster — and why boiling an amide takes hours.
- A tertiary alkyl bromide reacts with a weak nucleophile in water. Which schedule from the push-and-pull table applies, and what does that predict about the stereochemistry of the product?
- Why can an arrow never start at H+, and where must the tail sit instead when an acid protonates a ketone?
- Chlorination of an alkane needs light to start. Which of the twelve moves is the light performing, and why does the reaction then continue in the dark?
- A palladium cross-coupling turns over hundreds of times on one metal atom. Which move returns the catalyst to its starting state?
Struggling with one of these points at a specific gap, not a general one. Questions 1 and 3 want pKa; question 2 wants energy and geometry; questions 4 and 5 want the other engines. Each has a page coming — see the route on the map.