QCE Chemistry - Unit 4 - Properties and structure of organic materials

Organic Reactions and Reaction Pathways | QCE Chemistry

Learn the major organic reaction types and how to map reaction pathways in QCE Chemistry.

Updated 2026-05-18 - 5 min read

Organic reaction pathways are maps from one functional group to another. The goal is not to memorise random equations. The goal is to recognise what changes in the molecule and choose conditions that make that change happen.

Major reaction types

Addition

Addition reactions often involve alkenes. The carbon-carbon double bond opens and atoms are added across it.

Examples:

• alkene + hydrogen -> alkane, using a metal catalyst
• alkene + halogen -> dihaloalkane
• alkene + hydrogen halide -> haloalkane
• alkene + water -> alcohol, using acid-catalysed hydration conditions

Addition reduces unsaturation because the $\mathrm{C=C}$ bond becomes a $\mathrm{C-C}$ bond.

Substitution

In substitution, one atom or group is replaced by another. Haloalkanes are common examples because the carbon-halogen bond can be attacked by nucleophiles.

For example, a haloalkane can react with hydroxide ions to form an alcohol under suitable conditions.

Elimination

Elimination removes atoms from a molecule and forms a multiple bond. An alcohol can be dehydrated to form an alkene. A haloalkane can also undergo elimination to form an alkene under appropriate conditions.

Elimination is often the reverse idea of addition: instead of adding across a double bond, you create a double bond by removing a small molecule.

Oxidation and reduction

Oxidation in organic chemistry often means increasing bonding to oxygen or decreasing bonding to hydrogen.

For alcohols:

• primary alcohol -> aldehyde -> carboxylic acid
• secondary alcohol -> ketone
• tertiary alcohols are not easily oxidised under typical school-level conditions because there is no suitable hydrogen on the carbon bearing the $\mathrm{-OH}$ group

Reduction reverses some of these changes, such as reducing an aldehyde or ketone to an alcohol.

Esterification and hydrolysis

Esterification joins a carboxylic acid and an alcohol to form an ester and water. It is usually carried out with an acid catalyst and heat.

General pattern:

$ \text{carboxylic acid} + \text{alcohol} \rightleftharpoons \text{ester} + \text{water} $

Hydrolysis breaks an ester back into smaller molecules. Acid hydrolysis can form a carboxylic acid and an alcohol. Base hydrolysis forms a carboxylate salt and an alcohol.

Polymerisation

Addition polymerisation joins alkene monomers by opening the $\mathrm{C=C}$ double bond. The repeating unit keeps the same carbon skeleton but no longer has the double bond.

Condensation polymerisation joins monomers with two functional groups and releases a small molecule, often water. Polyesters and polyamides are common examples.

Pathway strategy

Use this process:

1. Identify the starting functional group.
2. Identify the target functional group.
3. Decide whether the carbon skeleton changes.
4. Choose known one-step conversions.
5. Add reagents and conditions above each arrow.
6. Check for side products, selectivity and practical separation.

If you are stuck, work backwards from the target. For example, if the target is an ester, ask: "What alcohol and carboxylic acid would form this ester?"

Reflux vs distillation

Some pathway questions mention practical conditions. Two common heating methods are:

| Method | What it does | Why it matters | | --- | --- | --- | | Reflux | heats a reaction mixture while condensing vapours back into the flask | allows prolonged heating without losing volatile reactants/products | | Distillation | heats and collects a volatile product as it evaporates and condenses elsewhere | can separate a product as it forms |

For alcohol oxidation, conditions can affect the product. Gentle oxidation with distillation can help collect an aldehyde before further oxidation. Stronger oxidation under reflux can push a primary alcohol toward a carboxylic acid.

Writing pathway answers

A complete pathway answer usually needs:

• the structure or name of each organic compound
• reagents and conditions for each step
• major products, including inorganic products where relevant
• practical details if the question asks about preparation or separation

Do not just write "oxidation" above an arrow. Say what oxidising agent or conditions are being used if the course expects it.

Quick check

Exam traps worth knowing

• Writing a product without showing conditions.
• Forgetting that primary and secondary alcohols oxidise differently.
• Treating reaction pathways as linear memorisation instead of functional group conversion.
• Ignoring the position of the functional group when naming products.
• Assuming every reaction goes to completion. Esterification is reversible, so equilibrium matters.

Sources

• QCAA Chemistry subject page
• QCAA Chemistry 2025 syllabus
• OpenStax Chemistry 2e: Hydrocarbons
• OpenStax Chemistry 2e: Alcohols and Ethers
• OpenStax Chemistry 2e: Aldehydes, Ketones, Carboxylic Acids, and Esters
• OpenStax Chemistry 2e: Amines and Amides