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Curved-Arrow
Formalism

Master the language of electron movement in organic reactions: how to read, draw, and verify curved arrows for mechanistic bookkeeping.

Organic Chemistry 1

What Are Curved Arrows?

Curved arrows are the universal notation organic chemists use to show how electrons move during a chemical reaction. They do not show atoms moving: they show electron pairs flowing from one location to another.

Full Arrow
A double-barbed arrow represents the movement of two electrons (a pair).
Fishhook Arrow
A single-barbed (half) arrow represents the movement of one electron (radical).

Why Mechanisms Matter

A balanced equation tells you what forms. A mechanism tells you how it forms. Curved arrows are the grammar of that story: without them you are memorising reactions instead of understanding them.

1
Predict products of unfamiliar reactions by following electron flow.
2
Explain selectivity: why one product forms over another.
3
Verify bookkeeping: confirm that charge, atoms, and electrons are conserved at every step.

Sources and Sinks

Every curved arrow starts at an electron-rich site (source) and ends at an electron-poor site (sink). Identifying these correctly is the first step in drawing any mechanism.

Electron Sources
Lone pairs, \(\pi\) bonds, \(\sigma\) bonds, negative charges
Electron Sinks
Positive charges, atoms with empty or low-energy \(\sigma^*\) / \(\pi^*\) orbitals, electronegative atoms

The Core Rule of Curved Arrows

Electron Source Electron Sink 2 electrons move Tail = source Head = sink

A double-barbed arrowhead means two electrons move. A single-barbed (fishhook) arrowhead means one electron moves (radical mechanism).

Full Arrow vs Fishhook

Full (Double-Barbed) 2 electrons (ionic / polar) Fishhook (Single-Barbed) 1 electron (radical)
Used in: \(\text{S}_\text{N}2\), \(\text{E}2\), \(\text{additions}\)
Used in: \(\text{halogenation, polymerization}\)

Curved Arrows vs Reaction Arrows

Curved Arrow Reaction Arrow
What it shows Electron movement Transformation of reactants to products
Shape Curved, originates at bonds/lone pairs Straight (\(\rightarrow\)) or double (\(\rightleftharpoons\))
Shows direction of Electrons (negative charge) Overall reaction progress
Appears in Mechanism steps Balanced equations

Mechanistic Bookkeeping

After drawing every curved arrow, you must audit your mechanism. Bookkeeping means verifying three conservation laws at every step of a mechanism:

Charge
Total formal charge on the left must equal total formal charge on the right.
Atoms
Every atom present before the step must appear after the step: nothing created or destroyed.
Electrons
Total electron count (bonds + lone pairs) is conserved across each mechanistic step.

Formal Charge Formula

Every time a curved arrow creates or breaks a bond, formal charges change. Use this formula to verify each atom:

1
An atom that uses a lone pair to form a new bond gets a \(+1\) shift in formal charge.
2
An atom that loses a bond pair and gains a lone pair gets a \(-1\) shift in formal charge.
3
Sum all formal charges before and after: they must be equal.

How to Draw a Mechanism

1
Identify the nucleophile (electron source) and the electrophile (electron sink).
2
Draw curved arrow(s) from the source to the sink. Tail on electrons, head on target atom/bond.
3
Draw the product of that step, showing all new bonds, broken bonds, and updated lone pairs.
4
Assign formal charges to every atom that changed its bonding. Verify total charge is conserved.
5
Repeat for the next elementary step until you reach the final product.

Example 1: Proton Transfer

\(\text{HO}^-\)
\(\text{H}\)
\(\text{Br}\)
\(\text{H}_2\text{O}\)
 +
\(\text{Br}^-\)
 nucleophile electrophile Bookkeeping Check
Charge: \((-1) + (0) = (0) + (-1)\) ✓
Atoms: \(2\text{H}, 1\text{O}, 1\text{Br}\) on both sides ✓

Example 2: \(\text{S}_\text{N}2\) Mechanism

\(\text{HO}^-\)
\(\text{CH}_3\)
\(\text{Br}\)
\(\text{HOCH}_3\)
 +
\(\text{Br}^-\)
 bond forms bond breaks simultaneous (concerted) Bookkeeping
Charge: \((-1) + (0) = (0) + (-1)\) ✓ | Atoms: balanced ✓

Example 3: Multi-Step Mechanism

Some reactions proceed in two or more elementary steps. Each step has its own set of curved arrows and its own bookkeeping check.

Step 1: Ionisation
\(\text{R-Br} \rightarrow \text{R}^+ + \text{Br}^-\)
Arrow: \(\text{C-Br}\) bond electrons \(\rightarrow \text{Br}\)
 Step 2: Capture
\(\text{R}^+ + \text{H}_2\text{O} \rightarrow \text{R-OH}_2^+\)
Arrow: \(\text{O}\) lone pair \(\rightarrow \text{C}^+\)
 Overall Bookkeeping Each step: charge conserved ✓ | Atoms conserved ✓ | Electrons conserved ✓

Common Arrow Patterns

Lone pair \(\rightarrow\) Bond
A nucleophile donates a lone pair to form a new bond to an electrophile.
Bond \(\rightarrow\) Lone pair
A bond breaks heterolytically: both electrons become a lone pair on the more electronegative atom.
Bond \(\rightarrow\) Bond
A \(\pi\) bond breaks while a new \(\sigma\) bond forms (electrophilic addition).

Common Mistakes

✕ Reversed Arrow
Drawing the arrow from the electrophile to the nucleophile. Remember: electrons flow from source to sink.
✕ Exceeding Octet
Forming a new bond to an atom already at \(8\) electrons without breaking another bond first (except for third-row expansion).
✕ Ignoring Charges
Forgetting to update formal charges after drawing arrows. Always recalculate and check conservation.

Summary Table

Concept Description Key Rule
Full curved arrow Moves \(2\) electrons Tail on source, head on sink
Fishhook arrow Moves \(1\) electron Radical reactions only
Charge conservation Total charge is constant Sum charges before = after
Atom conservation No atoms created/destroyed Count every element each step
Octet rule Max \(8\) electrons on 2nd-row atoms Break a bond before making one

Question 1 of 5

A curved arrow with a double-barbed head represents the movement of how many electrons?

Question 2 of 5

When drawing a curved arrow, the tail of the arrow should be placed on:

Question 3 of 5

In the reaction \(\text{HO}^{-} + \text{CH}_3\text{Br} \rightarrow \text{CH}_3\text{OH} + \text{Br}^{-}\), what is the total formal charge on the reactant side?

Question 4 of 5

A student draws a curved arrow from the carbon of \(\text{CH}_3\text{Br}\) to the hydroxide ion. What is wrong?

Question 5 of 5

Which of the following is not an electron source for a curved arrow?