Curved Arrows and Electron Movement

Introduction

Curved arrows are a grammar for mechanisms. Their tails identify where electrons start and their heads show where electron density is placed after a proposed step. A correct arrow must produce a chemically sensible structure with conserved atoms and charge. Decorative arrows without electron sources do not explain a reaction.

Core explanation

A full-headed curved arrow represents movement of an electron pair. Its tail begins at a lone pair or a bond line. Its head may point to an atom to make a bond, to a bond position to create a pi connection, or to an atom receiving the electrons from a broken bond. In protonation of an amine, the arrow begins at N's lone pair and ends at an acidic H. If the acid is H–Cl, a second arrow begins at the H–Cl bond and ends at Cl, yielding an ammonium species and chloride. The arrows explain both bond formation and cleavage in one charge-balanced step.

In nucleophilic addition to a carbonyl, the nucleophile's pair goes toward carbonyl carbon and the C=O pi pair goes onto oxygen. Without the second arrow, carbon would acquire an extra bond without losing pi bond order and could exceed ordinary valence. The resulting alkoxide may then be protonated in a later step. Do not combine separate proton-transfer and addition events into one arbitrary drawing unless the mechanism actually proposes a concerted process.

In a substitution such as HO⁻ attacking CH₃Br, draw an arrow from oxygen's lone pair to methyl carbon and an arrow from the C–Br bond to Br. The second arrow gives bromide its bond pair as the bond breaks. For a concerted S N2 example, both changes occur in the same step; for another substrate and solvent, a stepwise ionisation pathway may be more appropriate. Curved arrows describe a chosen mechanism, not a universal certainty from a net equation.

Radical reactions use different notation. Homolytic cleavage splits a bonding pair so each fragment takes one electron, shown by two single-headed fishhook arrows. Using one full-headed pair arrow would depict heterolytic cleavage instead. Radical addition or abstraction also requires tracking individual unpaired electrons. Mixing full and half arrowheads without an electron count can generate impossible charges or spins.

Resonance arrows and reaction arrows must be distinguished. Curved pair arrows can be drawn above resonance contributors to show alternative electron placements while nuclei and sigma connectivity remain fixed. A double-headed resonance symbol separates contributor drawings. A single reaction arrow separates species before and after a real chemical step, which may change bonds and positions. Moving a proton is a chemical transformation, not resonance.

After each step, count formal charges. If a neutral amine's lone pair forms a fourth bond, N ordinarily gains +1 formal charge. If a carbonyl pi bond pair moves to O, that oxygen often gains −1 until protonated. The total charge on all species must match before and after an electron-conserving step. Also track explicit H atoms; an H cannot vanish merely because an arrow was drawn near it.

Step-by-step reasoning

1. Draw all relevant lone pairs, bonds and charges in reactants. 2. Mark the electron donor and electron acceptor for each intended change. 3. Draw full-headed arrows for pairs or fishhooks for single electrons. 4. Redraw all products, including leaving groups and proton-transfer partners. 5. Check atoms, net charge, carbon valence and formal charges.

Visual explanation

Draw NH₃ + H–Cl with an arrow from N lone pair to H and another from H–Cl bond to Cl. Under the product NH₄⁺ + Cl⁻, write total charge zero before and after to display conservation.

Real-world analogy

An accounting transfer records both the source and destination of funds. A curved arrow likewise records where an electron pair originates and where it ends; a missing source is not a valid transaction.

Real-world example

Mechanism diagrams in synthesis reports help explain why one product forms and where side products might arise. They also reveal when a proposed pathway requires an unstable ion or an impossible carbon valence.

Why?

Why is a second arrow needed when a nucleophile attacks a carbonyl? Moving the C=O pi pair to oxygen prevents overbonding carbon and accounts for the new charge distribution.

Common misconception

“The arrow points from the positive atom to the negative atom because attraction goes that way.” Electron-flow arrows start at an electron source, often negative or lone-pair-rich, and point toward the acceptor.

Worked example

Show protonation of methylamine by HCl. Start with CH₃NH₂ and H–Cl. Draw N lone pair → H and H–Cl bond pair → Cl. Products are CH₃NH₃⁺ and Cl⁻. Nitrogen now has four bonds and +1 formal charge; Cl has the former bond pair and −1. Net charge remains zero, and every H and Cl atom is accounted for.

Quick check

1. What kind of curved arrow is used to show one electron moving during homolytic cleavage? Answer: A single-headed fishhook arrow.

Exam focus

Draw electron sources explicitly. Use full heads for pairs and fishhooks for single electrons, then verify product valence and charge. Distinguish resonance notation from reaction steps.

Advanced insight

Curved arrows are a compact model for changes in electron occupancy, not literal trajectories of individual electrons through space. A transition state describes the continuous quantum process more precisely than a sequence of static line drawings.

Summary

Mechanism arrows map electron sources to destinations while atoms and charge are conserved. Paired and radical electron movements use different arrowheads. Every proposed step needs a chemically valid product and complete bond bookkeeping.

Practice questions

1. Where does a full-headed arrow begin in amine protonation? Answer: At nitrogen's lone pair. 2. What charge does N acquire when a neutral amine lone pair forms a fourth bond? Answer: +1 formal charge in the usual Lewis accounting. 3. Why move the C=O pi pair during nucleophilic addition? Answer: To avoid overbonding carbon and place electron density on oxygen. 4. What distinguishes fishhook from full-headed arrows? Answer: Fishhooks move one electron; full-headed arrows move an electron pair.