Organic Structure & Electron Movement · Section 12 of 64

Curved arrows

Practice this — interactive lesson

A curved arrow is the notation organic chemists use to say where electrons went. Every mechanism in this course — every substitution, elimination, addition, and carbonyl reaction — is written in this one symbol. It has exactly two rules, and almost every mechanism error you will make for the next several months is a violation of one of them.

The two rules

A curved arrow represents two electrons moving together. The tail starts wherever those electrons currently are. The head points at wherever they end up.

A tail may start on exactly two things: a lone pair, or an existing bond. Nothing else. A tail can never start on a bare atom, on a positive charge, or on empty space, because there are no electrons there to move.

A head may point at two things: at an atom, meaning a new bond forms between the source and that atom, or at the middle of a bond, meaning a new pi bond forms there. A head pointing at an atom that already has a full octet is a signal that something else must break simultaneously — which usually means you need a second arrow.

A tail may start on exactly two thingsOa lone pairCBran existing bondA head may point at exactly two thingsC+an atom — a new bond formsCCthe middle of a bond — a new π formsAnd the one that is always wrongC+a tail starting on a positive chargethere are no electrons there to moveA + atom is a DESTINATION, never a source.Arrows track electrons,not attack direction.
The entire notation. Two legal places for a tail, two legal places for a head, and one error that accounts for more wrong mechanisms than everything else combined: starting an arrow at a positive charge. A positively charged atom is short of electrons — it has nothing to give. If your arrow starts there, you have drawn the atoms moving rather than the electrons.
Worked example — ammonia protonated by H⁺

Ammonia's nitrogen has a lone pair; H⁺ has an empty 1s orbital and no electrons at all.

One arrow: tail on nitrogen's lone pair, head on the hydrogen. The lone pair becomes the new N–H bond.

Check the result: nitrogen now has 4 bonds and no lone pairs, so FC = 5 − 0 − 4 = +1. The proton, now sharing a pair, is neutral. Charge in: +1. Charge out: +1. ✓

Drawing this arrow the other way round — from the H⁺ to the nitrogen — is wrong even though it "shows what connects to what," because H⁺ has no electrons to donate. Arrows track electrons, not atoms.

Worked example — two arrows at once

Hydroxide attacking bromomethane, the canonical SN2. If you drew only the arrow from hydroxide's lone pair to the carbon, you would produce a carbon with five bonds, which does not exist.

So a second arrow is required, and it must fire at the same instant: tail on the C–Br bond, head onto the bromine, which leaves with both electrons as bromide.

Every arrow that would overfill a second-row atom demands a partner arrow that empties it again. That pairing is the shape of most mechanism steps.

One arrow: ammonia takes a protonNHHHH+N+HHHHthe lone pair BECAME the new N–H bond4 bonds, no lone pair → FC = +1charge in +1, charge out +1 ✓HO−CBrHHHarrow 1 — a bond formsarrow 2 — a bond breaks
Top: one arrow is enough, because the proton had an empty orbital waiting and nothing needed to break. Note the direction — from the nitrogen's pair to the hydrogen, never the other way, because H⁺ has no electrons to offer. Bottom: the carbon already has four bonds, so forming a fifth is impossible; the second arrow is not optional decoration, it is what makes the step legal. Check the charge on both sides of each step and these errors surface immediately.Two arrows, fired at the same instant: hydroxide displaces bromide Without arrow 2 you have just drawn a carbon with five bonds, which does not exist.

Spotting invalid arrows

Four checks catch nearly everything.

Does the tail start on electrons? An arrow starting from a positive charge is always wrong. A positively charged atom is electron-deficient: it can only be a destination, never a source. This is the single most common beginner's error, and it comes from thinking of arrows as showing attack direction rather than electron flow.

Does any atom end up with too many bonds? Carbon never exceeds four, nitrogen never exceeds four, oxygen never exceeds three. If your product violates one of these, you formed a bond without breaking one.

Does charge balance? Total charge on the left must equal total charge on the right, every single step. Recompute formal charges after each arrow until it becomes automatic.

Does the arrow go from rich to poor? Electrons flow from an electron-rich site — a lone pair, a pi bond, a negative charge — toward an electron-poor one. An arrow running from a δ+ carbon toward a δ− oxygen is going uphill and is almost certainly backwards.

Arrows are not "what attacks what." They are "where the electrons go." In practice the two often point the same way, since the nucleophile supplies the electrons — but not always, and the exceptions are exactly where people trip. When a leaving group departs, the arrow points away from the carbon and onto the leaving group, even though nothing is attacking anything.

Arrows in resonance versus arrows in mechanism

The same symbol does two related jobs, and it helps to keep them straight. In a resonance context, arrows show how to get from one drawn structure to another drawn structure of the same molecule; nothing physically moves, and the arrows are a bookkeeping device for generating valid contributors. In a mechanism context, arrows describe electrons genuinely moving as one species becomes another.

The rules for drawing them are identical, which is why learning them here pays twice. The difference is only in what you write between the structures: ↔ for resonance, → for a mechanism step.

Fishhook arrows

A full curved arrow with a double-barbed head moves two electrons, and that covers essentially everything in this course. A fishhook arrow, drawn with a single barb, moves just one electron, and is used for radical mechanisms — homolytic bond cleavage, radical halogenation, radical addition of HBr. You should recognize the visual difference; you will not need to draw one for SN1, SN2, E1 or E2.

Full arrow — two electronsheterolytic: the pair goes to one sideeverything in Modules 1–13Fishhook — one electronhomolytic: the pair splits, one eachradical chemistry only
Count the barbs. Two barbs move a pair — one atom ends up with both electrons, which is what happens in every mechanism in this course. One barb moves a single electron, which is what radical chemistry needs. You should be able to tell them apart on sight; you will not be asked to draw the single-barbed one for SN1, SN2, E1 or E2.
Every mechanism in the remaining twelve modules is built from these two rules: tail on the electrons, head on the destination. There is no second notation to learn and no special cases to memorize. Practising arrows until they are automatic is the highest-return investment available this early in the course — and the arrow-pushing tool is built for exactly this drill.

What carries forward

Immediately: generating resonance structures reliably, which is the previous section's payoff. Then every mechanism from Module 6 onward. When you meet a reaction you have never seen, the way in is always the same — find the electron-rich site, find the electron-poor site, and draw the arrow between them. Much of what looks like memorization in organic chemistry is really this one skill applied to new molecules.