Curly Arrows and Mechanisms
A concise OCR A Level Chemistry A revision guide to the conventions used in reaction mechanisms: what a curly arrow means, how it shows heterolytic fission or the formation of a covalent bond, how dipoles show where an arrow points, and how to draw a mechanism with the relevant dipoles (4.1.1(h) and (i)).
GCSE Recap: Shared Pairs and Charged Ions
Three quick questions on the GCSE ideas that every mechanism on this page is built from.
What a Mechanism Shows
A reaction mechanism is a step-by-step description of how the bonds in the reactants break and the bonds in the products form. OCR 4.1.1(i) asks for reaction mechanisms drawn as diagrams that show clearly the movement of an electron pair with curly arrows and the relevant dipoles.
Every mechanism you draw at A Level uses one of two conventions, and the choice depends on how the bonds break.
| Type of mechanism | How bonds break (4.1.1(f)) | Symbol used | OCR examples |
|---|---|---|---|
| Radical mechanism | Homolytic fission: each bonding atom receives one electron, forming two radicals | A dot on each radical, for example Cl•, with equations for each step | Radical substitution of alkanes (4.1.2(f)) |
| Electron-pair mechanism | Heterolytic fission: one bonding atom receives both electrons | A curly arrow showing the movement of an electron pair | Electrophilic addition (4.1.3(h)), nucleophilic substitution (4.2.2(c)) |
Key idea: OCR 4.1.1(h): a curly arrow describes the movement of an electron pair, showing either heterolytic fission or the formation of a covalent bond. It never represents the movement of an atom, and it never starts on a positive charge.
Where a Curly Arrow Starts
There are only three places a curly arrow can start, because there are only three places an electron pair can be found: in a lone pair, in a bond that is being used to form a new bond, or in a bond that is breaking. Where the arrow finishes tells you what happens to the electrons.
| The arrow starts from | The arrow points to | What happens | Example |
|---|---|---|---|
| A lone pair on an atom | An atom that is short of electrons | The lone pair becomes a new covalent bond | The lone pair on the oxygen of a water molecule attacking a carbocation during hydration |
| A covalent bond | An atom outside that bond | The bond breaks and its electrons form a new bond to the atom pointed at | The C=C π bond attacking the H atom of HBr |
| A covalent bond | One of the two atoms in that bond | The bond breaks heterolytically; that atom keeps both electrons and becomes negative | The H-Br bond breaking to give Br– |
The three curly-arrow conventions: a bond forms from a lone pair, a bond forms from another bond, and a bond breaks by heterolytic fission.
Exam focus: Start the arrow exactly on the electron pair: on the lone pair, or on the middle of the bond. An arrow that starts on an atom or on a charge is marked wrong even if it points to the right place.
Quick Check: Where Must This Arrow Start?
Apply the rule to a reaction that is not drawn out above.
Dipoles: Where the Arrow Points
OCR specifically asks for the relevant dipoles to be shown on a mechanism. Electron pairs move towards atoms that are short of electron density. In a polar bond the more electronegative atom carries a partial negative charge, δ–, and the other atom a partial positive charge, δ+. Marking the dipole on the reactant tells you which atom the first arrow will point to.
- In H-Br, bromine is more electronegative, so the bond is Hδ+-Brδ–. The π bond of an alkene attacks the Hδ+ atom.
- In a C-Cl bond, chlorine is more electronegative, so the carbon is δ+. A nucleophile such as OH– attacks the Cδ+ atom.
- In Br2 there is no permanent dipole, but the electron-rich π bond of an alkene induces one as the molecule approaches, giving Brδ+-Brδ–.
Exam sentence: The curly arrow goes from the electron pair to the δ+ atom, and at the same time the polar bond breaks with its electrons moving to the δ- atom.
Quick Check: Describe Both Arrows
Drag the words into the gaps to describe a step that uses two arrows at once.
Drawing a Mechanism: A Worked Example
Take the hydrolysis of chloromethane by hydroxide ions, the nucleophilic substitution you will meet in 4.2.2. A complete OCR diagram shows every species with the correct charges and lone pairs, the relevant dipole, and each electron-pair movement as a curly arrow.
- Draw the reactants with their dipoles and lone pairs. Chloromethane is drawn with the C-Cl bond marked Cδ+-Clδ–. The hydroxide ion is drawn as –:OH with its lone pair and negative charge shown.
- Draw the arrow that forms the new bond. A curly arrow starts on the lone pair of the hydroxide oxygen and points to the δ+ carbon atom.
- Draw the arrow that breaks the old bond. A second curly arrow starts on the middle of the C-Cl bond and points to the chlorine atom, which takes both electrons: heterolytic fission.
- Draw the products. Methanol, CH3OH, and a chloride ion, Cl–. Check that the charges balance: one negative charge on each side.
A complete mechanism diagram: dipole marked, the arrow from the lone pair forms the C-O bond while the arrow from the C-Cl bond breaks it heterolytically.
Exam focus: Two arrows in the same step must be consistent: if a new bond forms at a carbon atom, an old bond at that carbon must break in the same step or the carbon would have five bonds.
Quick Check: Build a Mechanism in Order
Put the steps of a different mechanism into the order in which you would draw them.
Quick Check: Which Description Is Right?
In each round, pick the one description of the arrow that would earn the mark.
Radical Mechanisms Are Different
In a radical mechanism the bonds break homolytically, so only single electrons move. OCR shows radicals with a dot for the unpaired electron (4.1.1(g)) and describes the mechanism as equations for initiation, propagation and termination rather than with curly arrows.
For example, in the propagation steps of the bromination of ethane you write Br• + C2H6 → HBr + •C2H5, then •C2H5 + Br2 → C2H5Br + Br•. Every radical must carry its dot.
| Feature | Electron-pair mechanism | Radical mechanism |
|---|---|---|
| Bond fission | Heterolytic | Homolytic |
| Species formed | Ions (carbocations, halide ions) and molecules | Radicals with an unpaired electron |
| What OCR asks you to draw | Structures with curly arrows, dipoles, lone pairs and charges | Equations for initiation, propagation and termination with dots on the radicals |
| Where you meet it | Alkenes (4.1.3), haloalkanes (4.2.2) | Alkanes (4.1.2(f)) |
Key idea: Dot for a radical, curly arrow for a pair. Mixing the two conventions on one diagram is a common way to lose marks.
Quick Check: Which Convention Goes Where?
Rapid questions on pairs of electrons, single electrons and dots.
Common Exam Mistakes
- Starting a curly arrow on an atom, a δ+ symbol or a positive charge instead of on the electron pair.
- Pointing the arrow the wrong way, from the electron-poor atom towards the electron pair.
- Forgetting the lone pair on a nucleophile such as OH–, H2O or NH3, so the first arrow has nowhere to start.
- Leaving out the dipole, or drawing it the wrong way round, so the arrow attacks the δ– atom. OCR awards a mark for the relevant dipole.
- Omitting the charge on an intermediate, for example a carbocation drawn without its positive sign.
- Using curly arrows in a radical mechanism, or leaving the dots off the radicals.
Exam sentence: A curly arrow shows the movement of a pair of electrons: it starts on a lone pair or a bond and points to where the electrons go.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Free Radical Substitution FAQs
These questions summarise the core exam points on alkane substitution and free radicals.
Why are alkanes usually unreactive?
Alkanes are usually unreactive because they contain strong C-C and C-H bonds. These bonds require a large amount of energy to break.
What condition is needed for alkanes to react with chlorine?
Ultraviolet light is needed. UV light provides enough energy to break the Cl-Cl bond by homolytic fission, forming chlorine radicals.
What is a free radical?
A free radical is a species with an unpaired electron. The unpaired electron is usually represented using a dot, such as Cl•.
Why is the reaction called substitution?
It is called substitution because a hydrogen atom in the alkane is replaced by a halogen atom.
Why can a mixture of products form?
After the first substitution, the haloalkane product can undergo further substitution. This can replace more hydrogen atoms and produce a mixture of chlorinated products.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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