Classify characteristic organic reaction types
Use reactant and product changes to classify addition, substitution, elimination, hydrolysis and condensation.
Section 13 · An introduction to AS Level organic chemistry
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the published functional-group scope and nomenclature limits; draw formula and mechanism evidence with correct connectivity, electron movement and stereochemistry. Written practice is self-checked
Before you begin
- Use covalent structures, valency, molecular formulas, lone pairs, and single, double and triple bonds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens in a substitution reaction?
What you will learn
- Define five reaction-type terms
- Identify what changes in a reaction equation
- Distinguish addition from substitution and elimination
Compare atoms and groups before and after reaction
Addition joins atoms or groups across a multiple bond; no small molecule is necessarily removed. Substitution replaces one atom or group with another.
Elimination removes atoms or groups from a molecule, often forming a multiple bond. Hydrolysis uses water to break a bond; condensation joins molecules while eliminating a small molecule such as water.
Classify from evidence, not from reagent names alone
A reaction can be described by more than one useful feature, but choose the term that matches the stated change. For example, hydrolysis is a bond-breaking reaction with water as a reactant.
Addition and substitution are not interchangeable: addition changes the number of attached groups across a multiple bond, while substitution replaces one group and preserves the main skeleton.
Pause and explain
Which process is hydrolysis?
Worked example
Classify CH₂=CH₂ + HBr → CH₃CH₂Br and CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻.
Show the worked solution
- The first reaction adds H and Br across C=C.
- The second replaces Br with OH.
- They are addition and substitution, respectively.
Answer The alkene reaction is addition; replacement of Br by OH is nucleophilic substitution.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Classify hydrogenation of ethene to ethane by its reaction type.
Give me a hint
Compare the C=C reactant with the saturated product.
Compare my reasoning
- The C=C bond opens.
- Hydrogen atoms add to the two carbon atoms.
- This is addition.
Addition.
Look for these in your work
- Bond change identified
- New atoms included
- Reaction type named
Classify formation of an alkene and water from an alcohol as a common reaction type.
Give me a hint
Look for removal of groups and formation of C=C.
Compare my reasoning
- A hydrogen and an –OH-derived group are removed.
- A multiple bond forms between adjacent carbons.
- This is elimination.
Elimination.
Look for these in your work
- Groups removed
- C=C formation
- Type named
Two monomers join and release a water molecule. Classify the reaction and explain why addition is a poor label.
Give me a hint
Track the small molecule produced.
Compare my reasoning
- The molecules join to make a larger product.
- Water is eliminated as they join.
- This is condensation, not simple addition across a multiple bond.
Condensation; a small molecule, water, is released as the molecules join.
Look for these in your work
- Joining recognized
- Water release recognized
- Type distinguished
Common mistakes
- Calling every reaction involving two reactants addition
- Calling elimination hydrolysis
- Classifying only by whether a bond breaks
What can you explain now?
What happens in a substitution reaction?
Compare with the explanation
One atom or group is replaced by another
Substitution replaces a group in a molecule.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How can product evidence distinguish addition from substitution?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.