Understanding Reaction Mechanisms Revision Notes | A Level AQA Chemistry

1. Why Mechanisms Matter in Organic Chemistry

Every organic reaction you encounter at A Level — whether it is the chlorination of methane or the addition of HBr to an alkene — follows a precise sequence of electron movements. A reaction mechanism is the map of those movements: it shows which bonds break, which bonds form, and what happens to the electrons at each stage.

Without mechanisms, organic chemistry is a collection of disconnected facts to memorise. With mechanisms, you start to see the same patterns repeating across completely different reactions. That pattern recognition is exactly what AQA examiners reward in higher-mark questions.

The foundation of every mechanism is understanding how bonds break. There are two fundamentally different ways a covalent bond can break, and the type of fission determines everything that follows — the reactive intermediate formed, the type of mechanism, and the products obtained.

2. Bond Fission: The Two Ways a Bond Can Break

Picture a covalent bond as a shared pair of electrons sitting between two atoms. When that bond breaks, the central question is: where do those two electrons go? The answer splits all of organic chemistry into two broad families of mechanism.

The dot notation is not decorative — it is the AQA-specified way of showing that a species is a radical. In an exam, writing Cl without the dot when you mean a chlorine radical will cost you the mark. The dot tells the examiner you understand the species is electrically neutral but highly reactive because of that unpaired electron.

Heterolytic fission is far more common in A Level organic chemistry than homolytic fission. It underlies nucleophilic substitution, electrophilic addition, and nucleophilic addition — the three mechanism families you will encounter most frequently across the course.

3. Free Radicals and the Dot Notation

A free radical is a species with one unpaired electron. That single unpaired electron makes radicals extraordinarily reactive — they will react with almost anything to pair up that electron and reach a more stable configuration. This reactivity is what drives radical chain reactions such as the free-radical substitution of alkanes.

The AQA specification is explicit: the unpaired electron in a radical is represented by a dot placed next to the species symbol. So a chlorine radical is written as Cl·, a methyl radical as CH₃·, and a hydrogen radical as H·. The dot is not optional — it is the defining feature of radical notation at A Level.

Radicals are electrically neutral. This distinguishes them from ions: Cl· has 17 protons and 17 electrons (neutral), whereas Cl⁻ has 17 protons and 18 electrons (negative). Confusing radicals with ions is one of the most common errors in mechanism questions.

Notice that in homolytic fission, half-headed (fish-hook) curly arrows are used — one fish-hook arrow per electron, showing one electron moving to each atom. This is in contrast to heterolytic mechanisms, where full-headed (double-barbed) curly arrows show an entire electron pair moving together. Using the wrong arrow type in an exam mechanism will cost marks.

4. Reactive Intermediates: Radicals, Carbocations, and Carbanions

Bond fission produces reactive intermediates — short-lived species that form during a reaction but do not appear in the overall equation. The type of fission determines which intermediate forms, and the intermediate determines the mechanism type. Knowing these three intermediates gives you the framework to classify any A Level organic mechanism.

At A Level, carbocations appear most frequently in electrophilic addition to alkenes and in S_N1 substitution of halogenoalkanes. Carbanions are less prominent at A Level; species such as CN⁻ and H⁻ (from NaBH₄) act as carbanion-like nucleophiles in nucleophilic addition to carbonyl compounds, but they are the attacking nucleophiles rather than intermediates formed by fission of the substrate. Free radicals dominate the radical substitution of alkanes.

5. Curly Arrow Notation: The Language of Mechanisms

Curly arrows are the notation system used to show electron movement in reaction mechanisms. Every curly arrow tells a precise story: it starts at an electron-rich site (a lone pair or a bond) and points towards an electron-poor site. The arrow shows where the electrons are going, not where the atoms are moving.

There are two types of curly arrow, and using the correct type is non-negotiable in AQA mark schemes. A full-headed (double-barbed) curly arrow represents the movement of an electron pair — two electrons moving together. This is used in all heterolytic mechanisms. A half-headed (fish-hook) curly arrow represents the movement of a single electron and is used exclusively in radical (homolytic) mechanisms.

• Full curly arrow: starts from a lone pair or a bond; shows movement of 2 electrons; used in heterolytic mechanisms (nucleophilic substitution, electrophilic addition, nucleophilic addition)
• Half-headed (fish-hook) arrow: shows movement of 1 electron; used in homolytic/radical mechanisms
• Arrow must start from the electron-rich site — never from an atom label or a positive charge
• Arrow must point towards the electron-poor site — the atom or bond that will receive the electrons

Worked example: Drawing a Heterolytic Mechanism: Worked Example

Describe the mechanism for the reaction of bromomethane (CH₃Br) with hydroxide ions (OH⁻), showing curly arrow notation and identifying the type of bond fission involved.

1. The C–Br bond breaks. Bromine is more electronegative than carbon, so the bond is polar: C⁺–Br⁻. — Identifying bond polarity tells you which atom is the electrophile (C) and which will leave as an anion (Br⁻).
2. OH⁻ is the nucleophile — it has lone pairs on oxygen and carries a negative charge, making it electron-rich. — Nucleophiles are electron-rich species that attack electron-poor (electrophilic) centres. The carbon in C–Br is the electrophilic centre here.
3. Draw a full curly arrow starting from a lone pair on the oxygen of OH⁻, pointing towards the carbon atom of CH₃Br. — The arrow starts from the electron-rich site (lone pair on O) and points to the electron-poor site (C). Never start the arrow from the negative charge symbol — start from the lone pair itself.
4. Draw a full curly arrow starting from the C–Br bond, pointing towards the Br atom. — This shows the C–Br bond breaking heterolytically — both electrons go to Br, forming Br⁻. The carbon simultaneously forms a new bond with oxygen.
5. Bond fission type: heterolytic. No stable intermediate in S_N2 — the reaction is concerted (both arrows operate simultaneously). Products: CH₃OH and Br⁻. — Stating 'heterolytic fission' explicitly is worth a mark in AQA questions that ask you to 'describe' or 'explain' the mechanism.

Answer: The C–Br bond undergoes heterolytic fission. OH⁻ acts as a nucleophile, with a full curly arrow from its lone pair to the electrophilic carbon, and a second full curly arrow from the C–Br bond to bromine. Products are CH₃OH and Br⁻.

6. Putting It Together: Mechanism Types at A Level

Now that you have the building blocks — bond fission types, reactive intermediates, and curly arrow notation — you can classify any A Level organic mechanism. The table below maps the key mechanism types you will encounter across the course to the fission type and intermediate involved.

Each of these mechanism types will be covered in depth in later lessons. What matters here is that you can already predict the mechanism family from the type of bond fission — homolytic always leads to radical mechanisms, and heterolytic always leads to ionic mechanisms involving nucleophiles or electrophiles.

7. Exam Technique: Writing Mechanisms for Full Marks

Mechanism questions are among the most mark-dense questions in AQA organic chemistry papers. A single mechanism question can carry 3–5 marks, and each mark typically corresponds to one specific feature: a correct curly arrow, a correctly drawn intermediate, or a correctly labelled product. The following guidance is drawn directly from AQA mark scheme patterns.

Every mechanism you write in an AQA exam is an application of the two principles covered in this lesson: bond fission type determines the intermediate, and the intermediate determines the mechanism. Radical dot notation and curly arrow direction are the two technical details that separate a full-mark mechanism from a partial one — both are now within your grasp.