Curly Arrow Mechanisms: Electron Movement in Organic Reactions

What Are Curly Arrows?

Curly arrows are the language of organic chemistry mechanisms. They show exactly how electron pairs move during a reaction — from where they start (the tail) to where they end up (the head).

Every curly arrow represents the movement of two electrons (a pair). This is different from a single-headed "fishhook" arrow, which represents one electron (used in radical mechanisms).

Learning Goals: By the end of this guide, you should be able to:

Draw curly arrows correctly — tail at electron source, head at destination.

Apply curly arrows to nucleophilic substitution (SN1, SN2).

Apply curly arrows to electrophilic addition reactions.

Apply curly arrows to elimination (E1, E2) reactions.

Distinguish between mechanisms using arrow patterns.

The Rules

Rule 1: Arrows Start from Electrons

The tail of a curly arrow starts at:

A lone pair on an atom
A bonding pair (drawn from the bond itself)
A π bond (drawn from the double bond)

Rule 2: Arrows Point to Where Electrons Go

The head of the arrow points to:

An atom that accepts the electrons (forming a new bond)
An atom that receives the lone pair
Between two atoms (forming a new bond)

Rule 3: Charges Must Balance

After drawing curly arrows:

An atom that gains a pair of electrons becomes more negative (or less positive)
An atom that loses a pair of electrons becomes more positive (or less negative)

Key Mechanism Types

1. Nucleophilic Substitution (SN2)

A nucleophile (electron-rich species with a lone pair) attacks an electrophilic carbon, displacing the leaving group in one concerted step.

Example: $OH^-$ attacking bromoethane

HO^- + CH_3CH_2Br \rightarrow CH_3CH_2OH + Br^-

Curly arrows:

Arrow from lone pair on $O^-$ → to carbon bonded to Br (new C–O bond forms)
Arrow from C–Br bond → to Br (bond breaks, Br leaves with the pair)

Key features: one-step, backside attack, inversion of configuration.

2. Nucleophilic Substitution (SN1)

Two-step mechanism via a carbocation intermediate:

Step 1: Leaving group departs → forms carbocation

Arrow from C–Br bond → to Br

Step 2: Nucleophile attacks carbocation

Arrow from lone pair on nucleophile → to $C^+$

Key features: two steps, carbocation intermediate, racemisation.

3. Electrophilic Addition (to Alkenes)

An electrophile is attracted to the electron-rich $C=C$ π bond.

Example: $HBr$ adding to ethene

Step 1: Pi bond attacks H

Arrow from $C=C$ π bond → to H of HBr
Arrow from H–Br bond → to Br

Step 2: $Br^-$ attacks carbocation

Arrow from lone pair on $Br^-$ → to $C^+$

4. Elimination (E2)

A base removes a hydrogen from a carbon adjacent to the leaving group, forming a double bond:

Arrow from base lone pair → to H (removes proton)
Arrow from C–H bond → to between C–C (forms $\pi$ bond)
Arrow from C–LG bond → to leaving group (LG departs)

Three arrows in a concerted step — this is the classic E2 pattern.

Curly Arrow Mechanism Simulator

Watch arrow-pushing animations for SN1, SN2, E1, and E2 mechanisms. Draw your own curly arrows and see if they lead to the correct products.

Practice Curly Arrow Mechanisms

SN1 vs SN2 vs E1 vs E2: When Does Each Happen?

Factor	Favours SN2	Favours SN1	Favours E2	Favours E1
Substrate	Primary / methyl	Tertiary	Tertiary	Tertiary
Nucleophile/Base	Strong nucleophile	Weak nucleophile	Strong base	Weak base
Solvent	Polar aprotic	Polar protic	—	Polar protic
Temperature	Lower	—	Higher	Higher

Worked Examples

Example 1: SN2 on Chloromethane

Question: Draw the curly arrow mechanism for $OH^- + CH_3Cl \rightarrow CH_3OH + Cl^-$

Solution: One-step mechanism:

Arrow from lone pair on $OH^-$ → to the carbon of $CH_3Cl$ (new C–O bond)
Arrow from C–Cl bond → to Cl (Cl leaves as $Cl^-$ )

The nucleophile attacks from the back side, opposite to the leaving group.

Example 2: Electrophilic Addition of HBr to Propene

Question: Show the mechanism and predict the major product using Markovnikov's rule.

Solution:

Step 1: π bond of $CH_3CH=CH_2$ attacks H of HBr → H adds to the less substituted carbon ( $CH_2$ ), forming a secondary carbocation on the middle carbon.

Step 2: $Br^-$ attacks the carbocation.

Major product: 2-bromopropane ( $CH_3CHBrCH_3$ ), following Markovnikov's rule (H adds to the carbon with more H atoms).

Example 3: E2 Elimination of 2-Bromobutane

Question: Draw the E2 mechanism with ethoxide ( $C_2H_5O^-$ ) as base.

Solution: Concerted one-step:

Arrow from $C_2H_5O^-$ lone pair → to H on the β-carbon
Arrow from C–H bond → forms new $C=C$ double bond
Arrow from C–Br bond → to Br (leaves as $Br^-$ )

Products: but-2-ene (major, more substituted = more stable, Zaitsev's rule) + ethanol + $Br^-$ .

Common Mistakes

Starting arrows from atoms instead of electron pairs — The tail must start from a lone pair, bond, or π bond — never from a positive charge or "the atom."
Drawing arrows the wrong way — Electrons flow FROM nucleophile TO electrophile. Not the other way around.
Forgetting to show charges on intermediates — After each step, update the charges. If an atom gains electrons, it becomes more negative.
Confusing SN2 and E2 — Both are one-step mechanisms with a strong base/nucleophile, but SN2 attacks carbon while E2 attacks hydrogen on the adjacent carbon.
Not identifying the nucleophile/electrophile — Before drawing arrows, always ask: what is electron-rich (nucleophile)? What is electron-poor (electrophile)?

Exam Tips (A-Level / AP / IB)

Every question about mechanisms requires curly arrows. No arrows = no marks.
Draw each arrow clearly — examiners check that the tail starts from the correct source and the head points to the correct destination.
For multi-step mechanisms (SN1, E1), draw intermediates clearly with their charges.
When in doubt about SN vs E: strong nucleophile with primary substrate → SN2. Strong base with tertiary substrate → E2.

Frequently Asked Questions

What does a curly arrow represent?

A curly arrow represents the movement of an electron pair (two electrons). The tail shows where the electrons come from, and the head shows where they go.

What is the difference between SN1 and SN2?

SN2 is a one-step mechanism (nucleophile attacks while leaving group departs simultaneously). SN1 is two steps (leaving group departs first to form a carbocation, then nucleophile attacks). SN2 requires a strong nucleophile and primary substrate; SN1 favours weak nucleophiles and tertiary substrates.

Why do curly arrows always show pairs of electrons?

Full curly arrows show heterolytic bond breaking, where both electrons go to one atom (forming ions). For homolytic bond breaking (radical mechanisms), single-headed "fishhook" arrows are used instead.

How do I know if a reaction is substitution or elimination?

If the nucleophile/base attacks carbon → substitution (new bond to carbon). If it attacks hydrogen adjacent to the leaving group → elimination (forms a double bond). Strong bases with bulky structure and tertiary substrates favour elimination.

Hydrocarbons — The substrates for electrophilic addition reactions.
Hydrocarbon Derivatives — Products of substitution and elimination reactions.
Structural Isomerism — How different reaction pathways can give different structural isomers.