Curly Arrows: Bond Formation and Breaking Revision Notes | A Level AQA Chemistry

1. Why Curly Arrows Matter

Every organic reaction you will ever study at A Level — nucleophilic substitution, electrophilic addition, elimination — is described using the same tool: the curly arrow. Get this right once, and every mechanism you meet becomes a pattern you can follow rather than a list to memorise.

The core idea is this: chemical reactions involve electrons moving from one place to another. A curly arrow is a map of that movement. The tail of the arrow marks where the electron pair starts; the head marks where it ends up.

Think of it like a delivery route. The arrow tells you the parcel (the electron pair) is leaving address A and arriving at address B. If you draw the arrow backwards, you are describing the wrong reaction entirely — which is why precision here is non-negotiable.

2. The Two Valid Starting Points for a Curly Arrow

AQA is explicit on this: a curly arrow must start from either a lone electron pair or from an existing covalent bond. Those are the only two permitted origins. Electrons do not materialise from thin air, so an arrow starting from an atom's nucleus, from a partial charge symbol, or from empty space is always wrong.

The second valid starting point is an existing covalent bond. This applies both when a bond is breaking and when a pi bond is acting as an electron source in electrophilic attack. The arrow starts from the bond line and moves towards the atom that will gain the electrons.

3. Where the Arrow Head Points: Bond Formation vs Bond Breaking

The destination of the arrow head is just as important as its origin. When a new bond is forming, the arrow head points to the atom that will share the incoming electron pair — typically an electrophilic carbon or a hydrogen being transferred. When a bond is breaking, the arrow head points to the atom that will retain both electrons as a lone pair.

A useful mental check: after you draw an arrow, ask yourself what has changed. If an arrow leaves a lone pair and points to an atom, a new bond has formed between the donor and that atom, and the donor atom has lost a lone pair (and gained a positive charge if it was neutral). If an arrow leaves a bond and points to an atom, that bond has broken and the receiving atom has gained a lone pair (and a negative charge if it was neutral).

Notice that in the SN2 mechanism above, both arrows are drawn at the same time — bond formation and bond breaking happen in a single, concerted step. The two arrows together tell the complete story of electron movement in that step.

4. Drawing Complete Mechanisms: Putting It All Together

A complete mechanism requires you to draw the structures of every species involved — reactants, intermediates, and products — and to use curly arrows to show every electron pair movement between those structures. AQA mark schemes award marks for each correct arrow and for correctly drawn intermediates, so missing either loses you marks.

The discipline of drawing full structures matters because it forces you to track charges. Every time an arrow leaves a neutral atom, that atom becomes positive. Every time an arrow arrives at a neutral atom, that atom becomes negative. If your intermediate has the wrong charges, your arrows are wrong — the two must always be consistent.

Worked example: Drawing the Mechanism for Electrophilic Addition of HBr to Ethene

Draw the full mechanism for the reaction of ethene with hydrogen bromide, showing all curly arrows and the structure of the intermediate.

1. Ethene has a C=C pi bond (electron-rich). HBr is polarised: H carries a partial positive charge (δ+) because Br is more electronegative. — The pi bond will act as the nucleophile; the H of HBr is the electrophile. This tells us where the first arrow starts and ends.
2. Arrow 1: starts from the C=C pi bond, points to H of H–Br. — This forms a new C–H bond. The pi bond electrons are used up, so the other carbon is left with only three bonds and a positive charge — this is the carbocation intermediate.
3. Arrow 2: starts from the H–Br bond, points to Br. — Both electrons from H–Br move to Br, generating Br⁻. This happens simultaneously with arrow 1 in the first step. Draw the carbocation intermediate: CH₃–CH₂⁺ (or the secondary carbocation if a longer chain is used).
4. Arrow 3: starts from a lone pair on Br⁻, points to the positively charged carbon. — The lone pair on bromide forms a new C–Br bond. The carbocation is neutralised. This gives the final product: bromoethane (CH₃CH₂Br).
5. Reactants: CH₂=CH₂ (neutral) + HBr (neutral). Product: CH₃CH₂Br (neutral). Intermediate: CH₃CH₂⁺ and Br⁻ (charges balance to zero overall). — Charge must be conserved at every stage. If your intermediate charges do not balance, recheck which atom each arrow points to.

Answer: Step 1 uses two arrows: C=C pi bond → H (forming C–H) and H–Br bond → Br (giving Br⁻ and a carbocation intermediate). Step 2 uses one arrow: lone pair on Br⁻ → carbocation carbon (forming C–Br and giving bromoethane).

5. Homolytic vs Heterolytic Fission: Knowing Which Arrow to Use

AQA distinguishes between two types of bond breaking. Heterolytic fission — the type used in ionic and polar mechanisms — is shown with a full curly arrow (a full-headed or double-barbed arrow representing two electrons). Homolytic fission — where one electron goes to each atom, producing radicals — is shown with a half-headed curly arrow (sometimes called a fish-hook arrow), representing a single electron.

For the majority of A Level mechanisms (nucleophilic substitution, electrophilic addition, elimination, nucleophilic addition), you will use full curly arrows throughout. Homolytic fission and fish-hook arrows appear specifically in radical mechanisms, such as the free-radical substitution of alkanes with halogens under UV light.

6. Exam Technique: Winning and Losing Marks on Mechanisms

Mechanism questions are among the most mark-efficient questions on the paper — a well-practised student can score full marks in under two minutes. The marking points are predictable: one mark per correct curly arrow, one mark for the correct intermediate structure (including charges), and sometimes one mark for the correct product. Knowing this, you can plan your answer before you draw.

Before drawing any arrow, write out the full structural formula of the reactant. Skeletal formulae are fine for organic products, but when drawing mechanisms, showing all bonds explicitly helps you identify exactly where each arrow starts and ends. Examiners cannot award a mark for an arrow whose origin is ambiguous.

When an exam question says 'outline the mechanism', that is AQA's instruction to draw structures and curly arrows — not to write a paragraph. Use the command word as your checklist: structures of all species, correct curly arrows with correct starting points, and charges on intermediates.

7. Consolidation: Applying Curly Arrows Across Mechanism Types

Earlier in this lesson, you saw curly arrows applied to nucleophilic substitution and electrophilic addition. The same two rules — start from a lone pair or a bond, point to where the electrons go — apply without exception across every mechanism at A Level. Recognising the pattern is what allows you to work out an unfamiliar mechanism rather than recalling a memorised sequence.

When you encounter a new mechanism in an exam, work through these questions in order: What is the electron-rich species (nucleophile or pi bond)? What is the electron-poor species (electrophile or leaving group)? Which bond forms first, and which breaks? Each answer tells you where to place the next curly arrow.

Curly arrow notation is the universal language of organic mechanisms at A Level: master the two starting points and the charge-tracking rule, and every mechanism you meet becomes a logical sequence rather than a memory task.