# Reaction mechanisms

> CIE A-Level Chemistry · 9701
> Source: https://www.owlsprep.com/study/cie-9701-u12-reaction-mechanisms/

A reaction mechanism describes the step-by-step movement of electrons and breaking/forming of bonds during an organic reaction. This guide covers core notation, key terms, and rules for interpreting mechanisms for CIE A-Level exams.

**Prerequisites:** [Organic bonding and functional group identification](https://www.owlsprep.com/study/cie-9701-u12-organic-bonding/)

## Learning objectives

- Define what a reaction mechanism is in organic chemistry
- Interpret and use curly arrow notation for electron movement correctly
- Distinguish between homolytic and heterolytic bond fission
- Identify common types of reaction intermediates and their properties

## Core Definitions and Curly Arrow Notation

**Reaction Mechanism** — A detailed step-by-step description of how bonds are broken and formed as reactants convert to products, showing the position of all electrons involved in bond changes.

*Example:* The substitution of methane with chlorine is described by a three-step free radical mechanism.

**Curly Arrow Notation** — Curly arrows show the movement of electrons: double-headed arrows represent movement of an electron pair, while single-headed arrows represent movement of one unpaired electron.

*Notation:* Double-headed / single-headed (fishhook)

All curly arrows follow a core rule: the tail starts at the source of electrons (a lone pair or a covalent bond), and the head ends at the electron sink (an electron-deficient atom).

> **tip**
>
> CIE examiners require the tail of the arrow to be clearly positioned at the electron source, not just near it, to gain full marks.

**Worked example:** What does a double-headed arrow starting from the lone pair on a hydroxyl oxygen and ending at an adjacent carbocation represent?

1. Identify the arrow type: a double-headed arrow always represents movement of a full electron pair.
2. Identify the electron source: a lone pair on the electronegative oxygen atom.
3. Identify the electron sink: the electron-deficient positively charged carbocation.
4. This arrow shows the oxygen lone pair forming a new covalent bond between oxygen and the carbocation carbon.

## Types of Bond Fission

**Heterolytic Fission** — A covalent bond breaks unevenly, with both electrons from the bonding pair moving to one bonded atom. This produces two charged species: a cation (electron-deficient) and an anion (electron-rich).

*Example:* Breaking the C-Br bond in a tertiary halogenoalkane gives a tertiary carbocation ($R_3C^+$) and a bromide anion ($Br^-$).

**Homolytic Fission** — A covalent bond breaks evenly, with each bonded atom taking one electron from the bonding pair. This produces two neutral free radicals, species with one unpaired electron.

*Example:* Breaking the Cl-Cl bond in chlorine gas under UV light gives two chlorine free radicals ($Cl \cdot$).

**Worked example:** A bromine molecule ($Br_2$) absorbs UV light and splits into two products. What type of fission is this, and what products form?

1. UV light provides the energy for homolytic bond cleavage, the standard initiation step for free radical reactions.
2. The Br-Br bond breaks evenly, so each bromine atom takes one electron from the bonding pair.
3. Each product is a neutral bromine atom with an unpaired electron, called a bromine free radical.

*Conclusion:* This is homolytic fission, producing two $Br \cdot$ free radicals.

## Common Reaction Intermediates

Multi-step reactions proceed via short-lived intermediates: species formed in one step and consumed in a later step, so they do not appear as overall reactants or products. The three most common intermediates in CIE A-Level organic chemistry are summarized below:

| Intermediate | Charge | Key Property | Common Use |
| --- | --- | --- | --- |
| Carbocation | +1 Positive | Electron-deficient electrophile | Electrophilic addition, $S_N1$ substitution |
| Carbanion | -1 Negative | Electron-rich nucleophile | Condensation reactions |
| Free Radical | Uncharged | Unpaired electron, highly reactive | Free radical substitution, addition polymerization |

**Worked example:** A reaction step produces the species $CH_3CH_2^+$. What type of intermediate is this, and is it an electrophile or nucleophile?

1. The species is a carbon atom with a single positive charge.
2. Positively charged carbon intermediates are defined as carbocations.
3. Carbocations are electron-deficient, so they accept electrons from electron-rich donors = they are electrophiles.

*Conclusion:* This is a carbocation intermediate, classified as an electrophile.

## Interpreting Multi-Step Mechanisms

Most organic reactions have 2-3 distinct steps, with each step showing 1-2 curly arrows for electron movement. The overall reaction is the sum of all individual steps, with intermediates canceling out in the final equation.

**Exam command terms**

Common command terms for mechanism questions in CIE exams, with expected responses:

- **Add curly arrows** — Draw all arrows showing electron movement for the given step *(1-2 arrows per step, start at electron source)*

- **Draw the missing intermediate** — Show the full structure and any charge of the intermediate *(Always add the correct charge for full marks)*

- **Outline the mechanism** — Draw all steps, with curly arrows and intermediates *(Show every bond change for full marks)*

**Worked example:** Identify the intermediate in this 3-step free radical mechanism: 1. Initiation: $Cl_2 \rightarrow 2 Cl \cdot$; 2. Propagation 1: $Cl \cdot + CH_4 \rightarrow HCl + CH_3 \cdot$; 3. Propagation 2: $CH_3 \cdot + Cl_2 \rightarrow CH_3Cl + Cl \cdot$

1. Intermediates are defined as species formed in an early step and fully consumed in a later step, they do not appear in the net reaction.
2. $Cl \cdot$ is regenerated at the end of the mechanism, so it is a catalyst, not an intermediate.
3. $CH_3 \cdot$ is formed in propagation 1 and fully consumed in propagation 2, so it fits the definition of an intermediate.

*Conclusion:* The intermediate in this mechanism is the methyl free radical $CH_3 \cdot$.

## Common pitfalls

- **Wrong:** Starting the tail of a curly arrow on the positive charge of a carbocation
  - Why it fails: A positive charge indicates the carbocation is an electron sink, not an electron source
  - Correct: Start the arrow from the electron source (e.g. a nucleophile's lone pair) and end the arrow head at the positively charged carbon atom
- **Wrong:** Confusing homolytic and heterolytic fission products, writing charged products for homolytic fission
  - Why it fails: Even bond splitting does not create a net charge on either product
  - Correct: Remember: Homolytic = Even split = uncharged free radicals; Heterolytic = Uneven split = charged cations + anions
- **Wrong:** Ending the head of a curly arrow between two atoms when forming a new bond
  - Why it fails: CIE examiners require the arrow to clearly indicate which atom accepts the electrons
  - Correct: End the arrow head directly at the electron-deficient atom that will accept the electron pair
- **Wrong:** Forgetting to add the charge to reaction intermediates in drawing
  - Why it fails: Mark schemes require the correct charge to award full marks for intermediate structures
  - Correct: Always explicitly add the positive or negative charge to every charged intermediate you draw

## Cheatsheet

| Concept | Key Rule for CIE Exams |
| --- | --- |
| Curly arrow start | Always at electron source: lone pair or covalent bond |
| Curly arrow end | Always at electron sink: the accepting atom, not between atoms |
| Double-headed arrow | Movement of a full electron pair |
| Single-headed arrow | Movement of one unpaired electron (free radicals) |
| Homolytic fission | Even split → 2 neutral free radicals |
| Heterolytic fission | Uneven split → 1 cation + 1 anion |
| Carbocation | Positive charge, acts as an electrophile |
| Free radical | Uncharged, has one unpaired electron |

## What's next

Reaction mechanisms are the foundation of all organic chemistry in CIE A-Level, and you will apply the core notation and rules you learned here to every organic reaction you study in later units. Mastering curly arrow notation and key definitions early will save you significant time when learning more complex mechanisms like nucleophilic substitution and electrophilic addition in upcoming topics. Understanding how bond breaking and forming works step-by-step also helps you predict products of unfamiliar reactions, a common skill tested in both multiple choice and extended response questions. Next, you will apply these core rules to specific mechanism types common in A-Level organic chemistry.

- [Organic molecule shapes](https://www.owlsprep.com/study/cie-9701-u12-organic-molecule-shapes/)
- [Hydrocarbons](https://www.owlsprep.com/study/cie-9701-u13-overview/)
- [Alkanes](https://www.owlsprep.com/study/cie-9701-u13-alkanes/)

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