Reaction mechanisms

CIE A-Level Chemistry· Unit 12: Introduction to organic chemistry· 25 min read

1. Core Definitions and Curly Arrow Notation★☆☆☆☆⏱ 8 min

📘 Definition

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.

📘 Definition

Curly Arrow Notation

Double-headed / single-headed (fishhook)

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.

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).

📐 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.

2. Types of Bond Fission★★☆☆☆⏱ 7 min

📘 Definition

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 ( $R_{3} C^{+}$ ) and a bromide anion ( $B r^{-}$ ).

📘 Definition

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 ( $C l \cdot$ ).

📐 Worked Example

A bromine molecule ( $B r_{2}$ ) 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.

3. Common Reaction Intermediates★★☆☆☆⏱ 6 min

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, $S_{N} 1$ substitution
Carbanion	-1 Negative	Electron-rich nucleophile	Condensation reactions	organometallic reactions
Free Radical	Uncharged	Unpaired electron, highly reactive	Free radical substitution, addition polymerization

📐 Worked Example

A reaction step produces the species $C H_{3} C H_{2}^{+}$ . 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.

4. Interpreting Multi-Step Mechanisms★★★☆☆⏱ 4 min

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.

📐 Worked Example

Identify the intermediate in this 3-step free radical mechanism: 1. Initiation: $C l_{2} \to 2 C l \cdot$ ; 2. Propagation 1: $C l \cdot + C H_{4} \to H C l + C H_{3} \cdot$ ; 3. Propagation 2: $C H_{3} \cdot + C l_{2} \to C H_{3} C l + C l \cdot$

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
$C l \cdot$ is regenerated at the end of the mechanism, so it is a catalyst, not an intermediate.
3
$C H_{3} \cdot$ is formed in propagation 1 and fully consumed in propagation 2, so it fits the definition of an intermediate.

5. Common Pitfalls

Wrong move:

Starting the tail of a curly arrow on the positive charge of a carbocation

Why:

A positive charge indicates the carbocation is an electron sink, not an electron source

Correct move:

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 move:

Confusing homolytic and heterolytic fission products, writing charged products for homolytic fission

Why:

Even bond splitting does not create a net charge on either product

Correct move:

Remember: Homolytic = Even split = uncharged free radicals; Heterolytic = Uneven split = charged cations + anions

Wrong move:

Ending the head of a curly arrow between two atoms when forming a new bond

Why:

CIE examiners require the arrow to clearly indicate which atom accepts the electrons

Correct move:

End the arrow head directly at the electron-deficient atom that will accept the electron pair

Wrong move:

Forgetting to add the charge to reaction intermediates in drawing

Why:

Mark schemes require the correct charge to award full marks for intermediate structures

Correct move:

Always explicitly add the positive or negative charge to every charged intermediate you draw

6. Quick Reference 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

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · 12
Identify correct curly arrow placement
2022 · 21
Classify type of bond fission
2021 · 32
Name reaction intermediate type

Going deeper

syllabusCIE 9701 Latest SyllabusVerify current learning outcomes

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.