Organic reaction mechanisms
IB Chemistry SL· 25 min read
1. Core Definitions and Reactive Species★★☆☆☆⏱ 6 min
A reaction mechanism describes the step-by-step sequence of bond breaking and bond forming that converts organic reactants into products. It helps explain why reactions produce specific products and allows us to predict outcomes for new reactions.
Reaction Mechanism
A detailed step-by-step description of the sequence of elementary reactions that converts reactants into products, showing electron movement and all intermediate species.
Example:
The two-step SN1 mechanism for tertiary haloalkane hydrolysis.
All organic mechanisms involve two key classes of reactive species, classified by their behavior: electrophiles and nucleophiles.
Classify each species as electrophile or nucleophile: (a) OH⁻, (b) CH₃⁺, (c) HBr (the H atom)
- 1
Recall the definitions: electrophiles accept electron pairs, nucleophiles donate electron pairs.
- 2
(a) OH⁻ has three lone pairs of electrons and a negative charge, so it donates an electron pair to form a bond. This makes it a nucleophile.
- 3
(b) CH₃⁺ has a positive charge and an empty p orbital, so it accepts an electron pair to form a bond. This makes it an electrophile.
- 4
(c) The H in HBr is δ⁺ due to polarity, so it accepts an electron pair from an electron-rich source. It acts as an electrophile.
Exam tip:
Use the mnemonic: Electrophile = Electron-loving (seeks negative charge), Nucleophile = Nucleus-loving (seeks positive charge) to avoid mixing them up.
2. Curly Arrow Notation Fundamentals★★☆☆☆⏱ 7 min
Curly arrows are the universal language of organic mechanisms, and correct placement is required for full marks on all IB exam mechanism questions. One small mistake in arrow placement will cost you marks even if your overall logic is correct.
Full Curly Arrow
A curved arrow that represents the movement of a pair of electrons from their original position (tail) to their new position (head).
Two core rules for correct placement:
- The tail of the arrow always starts at the source of the electron pair: either a lone pair on an atom, or the middle of a covalent bond that is breaking.
- The head of the arrow always points to where the new bond forms, or directly to the atom that receives the electron pair.
Describe the correct placement of a curly arrow for the attack of OH⁻ on a H⁺ to form water.
- 1
Identify the source of the electron pair: the lone pair on the oxygen atom of OH⁻.
- 2
Identify the destination of the electrons: H⁺, which accepts the electron pair to form a new O-H bond.
- 3
The correct arrow starts at the lone pair on the O atom of OH⁻, and points directly at the H⁺. Do not start the arrow at the negative charge sign.
3. Common Reaction Mechanism Types (IB SL)★★★☆☆⏱ 8 min
IB SL requires you to recognize, classify, and draw three core types of organic reaction mechanisms, summarized below.
Mechanism Type | Key Feature | Example Reaction |
|---|---|---|
Free radical substitution | Forms uncharged free radical intermediates via homolytic bond fission; has 3 stages: initiation, propagation, termination | Chlorination of methane |
Electrophilic addition | Electrophile adds across a C=C double bond; one pi bond breaks, two new sigma bonds form | Addition of HBr to ethene |
Nucleophilic substitution | Nucleophile replaces a leaving group on a saturated carbon; two variants: SN1 (two-step) and SN2 (one-step) | Hydrolysis of bromoethane with OH⁻ |
Identify the mechanism type for the reaction of propene with hydrogen bromide.
- 1
Propene is an alkene, so it contains an electron-rich carbon-carbon double bond.
- 2
HBr adds across the double bond, breaking the pi bond and forming two new single bonds, with no other reactant consumed.
- 3
The first step of the reaction is attack of the electron-rich double bond on the electrophilic Hδ+ of HBr.
- 4
This matches the definition of electrophilic addition, so the mechanism is classified as electrophilic addition.
4. Exam Conventions for Drawing Mechanisms★★★☆☆⏱ 4 min
Many students lose easy marks for breaking simple drawing conventions, even when they understand the mechanism correctly. Follow these IB-approved rules:
Always draw all lone pairs on attacking nucleophiles or leaving groups
Never start a curly arrow on a negative charge sign (always start at the lone pair on the atom)
Always point the head of the arrow directly at the new bond location or accepting atom, do not stop between atoms
Show formal charges on all intermediate species
What is the mistake in this drawing: a curly arrow starts at the negative charge on CN⁻, attacking a haloalkane carbon.
- 1
The negative charge is just a label for charge, it is not the source of the electrons that form the new bond.
- 2
Wrong move: starting the arrow at the charge sign instead of the lone pair on the terminal carbon of CN⁻.
- 3
Correct drawing: the arrow tail starts at the lone pair on the carbon of CN⁻, with the negative charge drawn next to the carbon atom.
Exam tip:
Always double-check arrow direction before submitting your answer: reversed arrows or incorrect placement automatically lose marks, even if the overall mechanism is correct.
5. Common Pitfalls
Wrong move:
Starting a curly arrow on an atom's charge instead of a lone pair or bond
Why:
Curly arrows represent electron movement, not the location of the charge. Starting at the charge is ambiguous and marked wrong.
Correct move:
Start the arrow at the lone pair on the charged atom, and draw the charge sign next to the atom to indicate charge.
Wrong move:
Confusing electrophiles and nucleophiles
Why:
Similar names lead to mixing up their roles and definitions, which loses marks in classification questions.
Correct move:
Remember: Electrophile = Electron-loving, accepts electrons. Nucleophile = Nucleus-loving, donates electrons.
Wrong move:
Drawing the head of a curly arrow between two atoms
Why:
This is ambiguous, examiners cannot tell where you intend the electrons to go, so you lose the mark.
Correct move:
Point the head of the arrow directly at the atom that will receive the electrons or the location of the new bond.
Wrong move:
Omitting formal charges on intermediate species
Why:
Examiners require correct charges to confirm you understand the electron distribution in intermediates.
Correct move:
Always draw the formal charge on any charged intermediate after bond breaking.
6. Quick Reference Cheatsheet
Term | Core Definition | Exam Note |
|---|---|---|
Reaction mechanism | Step-by-step bond breaking/forming process | Draw full mechanisms for 3 core types |
Full curly arrow | Movement of one electron pair | Tail at electron source, head at destination |
Electrophile | Electron-deficient, accepts electron pair | Examples: H⁺, Brδ+, CH₃⁺ |
Nucleophile | Electron-rich, donates electron pair | Examples: OH⁻, CN⁻, H₂O |
Free radical substitution | Free radical intermediates | Alkane halogenation, 3 stages |
Electrophilic addition | Add across C=C double bond | Alkene reactions with electrophiles |
Nucleophilic substitution | Nucleophile replaces leaving group | Haloalkane hydrolysis, SN1/SN2 |
7. Frequently Asked
Do I need to memorize all mechanisms for SL?
For IB SL, you only need to draw full mechanisms for three core types: free radical substitution, electrophilic addition to alkenes, and nucleophilic substitution of haloalkanes. Other mechanisms only require classification, not full drawing.
What do curly arrows actually represent?
Curly arrows always represent the movement of electron pairs, not atoms or bonds. They show where electrons go when bonds break and new bonds form.
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.
- 2022 · 1
Classify electrophile/nucleophile
- 2023 · 2
Draw mechanism for addition reaction
- 2021 · 1
Identify correct curly arrow placement
What's Next
Mastering the fundamentals of organic reaction mechanisms is critical for success on the organic chemistry section of your IB SL Chemistry exam, which makes up approximately 25% of overall marks. The core conventions and rules you learned here apply to every mechanism you will encounter, so building a strong foundation now will make more specific mechanisms much easier to learn. Next, you can explore each of the core mechanism types in detail, learning how to draw full step-by-step mechanisms and predict products for different reaction conditions.