THE CONCEPT
Arrows Track Electrons, Not Atoms
Every reaction mechanism you draw in organic chemistry is a sequence of curved arrows, and each arrow represents the movement of a pair of electrons (or, for a radical mechanism, a single electron using a half-headed 'fishhook' arrow) from one location to another. Crucially, arrows track electrons, never atoms — an arrow never represents an atom physically moving from one place to another, even though the electron movement it depicts often results in an atom ending up newly bonded (or newly unbonded) somewhere.
Because electrons always flow from a place with excess electron density to a place that's electron-deficient and can accept them, every properly drawn arrow follows the exact same underlying logic: it starts where electron density already exists (a lone pair, or an existing bond) and points toward wherever that electron density is going (a new bond forming, or an atom that will end up with the electrons after a bond breaks).
💡 Memory Trick
The hub's trick is the single rule everything else follows from: arrow pushing — electrons flow from nucleophile to electrophile, always from electron-rich to electron-poor. Pin the two ends of every arrow down explicitly before drawing it: the tail sits at the electron source (a lone pair on an atom, or an existing bond), and the head sits at the destination (a new bond forming, or an atom that ends up holding the electrons after a bond breaks). The hub's blunt closing instruction is worth internalizing directly: never draw arrows backwards — from an electron-poor site toward an electron-rich one is never a valid mechanistic step.
THE THREE MOST COMMON ARROW-PUSHING PATTERNS
Recognizing the Same Few Moves Everywhere
Nearly every mechanism in this entire course is built from a small handful of recurring arrow patterns, and recognizing them explicitly makes unfamiliar mechanisms far less intimidating. Lone pair to new bond: a nucleophile's lone pair becomes a new sigma bond to an electrophilic atom (this is the very first step of essentially every nucleophilic addition or substitution mechanism). Bond to new bond (or lone pair): an existing sigma or pi bond's electrons shift to form a different bond, or collapse onto an adjacent atom as a lone pair (this is exactly what happens to the carbonyl pi bond when a nucleophile attacks a carbonyl carbon).
Bond to leaving group: a sigma bond's electrons leave entirely with a departing atom or group, becoming that group's own lone pair once it's left (this is exactly what happens when a leaving group departs in an SN1 ionization, or when a leaving group is displaced in an SN2 or E2 step). Learning to recognize these three recurring patterns, rather than treating every new mechanism as an entirely unfamiliar puzzle, is what makes mechanism-drawing feel systematic rather than like memorizing hundreds of disconnected examples.
🧪 Lab Application
You're asked to draw the complete curved-arrow mechanism for hydroxide attacking a ketone's carbonyl carbon, and need to place every arrow correctly.
1
Identify the first electron source. Hydroxide's lone pair on oxygen is the electron-rich starting point for the first arrow.
2
Draw the first arrow's tail and head correctly. The tail starts at hydroxide's lone pair; the head points to the electrophilic carbonyl carbon, showing a new O-C bond forming.
3
Identify the second electron movement required. Since carbon can't exceed four bonds, the carbonyl pi bond's electrons must move somewhere as the new bond forms.
4
Draw the second arrow's tail and head correctly. The tail starts at the C=O pi bond; the head points to the oxygen atom, showing those electrons collapsing fully onto oxygen as a new lone pair, generating the tetrahedral alkoxide intermediate.
📌 Exam Application
Mechanism-drawing questions are graded heavily on correct arrow placement, not just a correct final product — always double check that every arrow's tail sits on genuine electron density (a lone pair or bond) and that every arrow's head points to a sensible destination (a new bond or a stabilized lone pair), since a single backwards or misplaced arrow can invalidate an otherwise correct-looking mechanism.
⚠️ Most Common Curved Arrow Notation Mistakes
The most common mistake is drawing an arrow starting from a positively charged or electron-poor atom, which has no electron density available to donate in the first place — always trace an arrow's tail back to genuine electron density before drawing it. The other frequent trap is drawing an arrow that implies an atom physically moving, rather than correctly showing only electron movement between existing atomic positions.
✓ Quick Self-Test
1) What does a curved arrow represent — atom movement or electron movement? 2) Where does an arrow's tail need to be positioned? 3) Where does an arrow's head need to be positioned? 4) In the 'lone pair to new bond' pattern, what kind of mechanistic step does this typically represent? 5) Why is drawing an arrow from an electron-poor atom toward an electron-rich one always incorrect?
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Radical Stability
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