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 bond formation, how dipoles show where an arrow points, and how to draw a full mechanism.
- 4.1.1(f)(i)
- 4.1.1(f)(ii)
- 4.1.1(g)-i
- 4.1.1(g)-ii
- 4.1.1(h)-i
- 4.1.1(h)-ii
- 4.1.1(i)
What these spec points say
- 4.1.1(f)(i) the different types of covalent bond fission: homolytic fission (in terms of each bonding atom receiving one electron from the bonded pair, forming two radicals)
- 4.1.1(f)(ii) the different types of covalent bond fission: heterolytic fission (in terms of one bonding atom receiving both electrons from the bonded pair)
- 4.1.1(g)-i the term radical (a species with an unpaired electron)
- 4.1.1(g)-ii use of 'dots' to represent species that are radicals in mechanisms
- 4.1.1(h)-i a 'curly arrow' described as the movement of an electron pair, showing heterolytic fission
- 4.1.1(h)-ii a 'curly arrow' described as the movement of an electron pair, showing formation of a covalent bond
- 4.1.1(i) reaction mechanisms, using diagrams, to show clearly the movement of an electron pair with 'curly arrows' and relevant dipoles
Breaking a Covalent Bond: Homolytic and Heterolytic Fission
Choose a step to watch where the two electrons of a bond go when it breaks: one each (free radicals), or both to one atom (ions).
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
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 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 | 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 |
| Electron-pair mechanism | Heterolytic fission: one bonding atom receives both electrons | A curly arrow showing the movement of an electron pair | Electrophilic addition, nucleophilic substitution |
Key idea: In OCR wording, 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.
Homolytic fission gives radicals shown with a dot, while heterolytic fission gives ions and each electron-pair movement is shown with a curly arrow.
Quick Check: Pairs, Single Electrons and Dots
Four rapid questions on which bonds break into radicals, which into ions, and what each symbol shows.
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.
It can be 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.
Marking the dipole first shows which atom is δ+, and the attacking electron pair always moves towards that 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 with the haloalkanes. 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.
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 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, haloalkanes | Alkanes |
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 Description Is Right?
In each round, pick the one description of the arrow or symbol that would earn the mark.
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.
Curly Arrows and Mechanisms FAQs
These questions check the arrow and dot conventions that examiners mark hardest.
What does a curly arrow show?
The movement of a pair of electrons, either in heterolytic fission or in forming a covalent bond. It never shows an atom moving.
Where can a curly arrow start?
Only on an electron pair: a lone pair on an atom, or the middle of a covalent bond. An arrow that starts on an atom or a charge loses the mark.
Do I have to show the dipoles?
Yes. OCR awards a mark for the relevant dipole. The attacking pair moves towards the δ+ atom, and the breaking bond’s electrons move to the δ– atom.
Do I draw curly arrows in a radical mechanism?
No. Write an equation for each step of initiation, propagation and termination, with a dot on every radical, as in Br• + C2H6 → HBr + •C2H5.
What is the difference between homolytic and heterolytic fission?
In homolytic fission each bonding atom receives one electron, forming two radicals. In heterolytic fission one atom receives both electrons, forming ions.
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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