VCE Chemistry organic synthesis pathways: the 2026 guide

What this guide is for

VCE Chemistry Unit 3-4 includes organic synthesis pathways: combining multiple reactions to convert one organic compound into another. The Section B exam often asks for a 5-step or 8-step synthesis with reagents and conditions. This guide covers the reaction toolkit, retrosynthesis, and three worked syntheses.

The reaction toolkit

Memorise each as both forward and reverse direction.

From	To	Reagents	Conditions
Alkane	Haloalkane	X2 (Br2, Cl2)	UV light
Alkene	Vicinal dihaloalkane	X2	room T
Alkene	Haloalkane	HX	room T; Markovnikov
Alkene	Alcohol	H2O	dilute H2SO4 catalyst, heat; Markovnikov
Alkene	Alkane	H2	Ni or Pt catalyst
Haloalkane	Alcohol	NaOH (aq)	warm aqueous, nucleophilic substitution
Primary alcohol	Aldehyde	acidified K2Cr2O7	distillation
Primary alcohol	Carboxylic acid	acidified K2Cr2O7, excess	reflux
Secondary alcohol	Ketone	acidified K2Cr2O7	reflux
Carboxylic acid + alcohol	Ester	concentrated H2SO4 catalyst	reflux
Ester + water	Carboxylic acid + alcohol	dilute H+ catalyst	reflux (acid hydrolysis)
Ester + NaOH	Carboxylate salt + alcohol	reflux	base hydrolysis (saponification)

Retrosynthesis

Working backwards from the target:

1. What is the target compound? Identify its functional groups.
2. What is the immediate precursor? What reaction could have produced this functional group?
3. What is the precursor's precursor? Continue.
4. Reach a feasible starting material.
5. Write the synthesis forwards with reagents and conditions.

Example. Synthesise propyl ethanoate from propene.

Target: propyl ethanoate (CH3-CO-O-CH2-CH2-CH3). Ester.

Precursors: propan-1-ol + ethanoic acid (Fischer esterification).

Propan-1-ol from propene: addition of water (Markovnikov gives propan-2-ol, the wrong isomer). Need an alternative: propene + HBr (Markovnikov gives 2-bromopropane). Then nucleophilic substitution with NaOH to propan-2-ol. But we want propan-1-ol.

Anti-Markovnikov pathways are outside VCE scope. The cleanest route from propene to propan-1-ol involves intermediate steps that produce 1-bromopropane (radical addition with peroxides), outside standard VCE.

Practical route: assume propan-1-ol is available as starting material (or use a different starting point).

Ethanoic acid from ethanol (oxidation under reflux with acidified dichromate). Ethanol from ethene (addition of water, Markovnikov gives ethanol directly).

Pathway:

1. Ethene + H2O (dilute H2SO4, heat) → ethanol.
2. Half the ethanol: ethanol + acidified Cr2O7^2- (reflux) → ethanoic acid.
3. The remaining ethanol stays as ethanol... wait, we want propan-1-ol, not ethanol, for the propyl ester.

Re-read the target. Propyl ethanoate is the ester of propan-1-ol + ethanoic acid. So we need both propan-1-ol and ethanoic acid.

A cleaner alternative starting material: start with propan-1-ol and ethanol both available; oxidise ethanol to ethanoic acid; combine.

Worked example 1: Ethyl ethanoate from ethene

Target: ethyl ethanoate (CH3-CO-O-CH2-CH3).

Pathway:

1. Ethene (CH2=CH2) + H2O (dilute H2SO4, heat) → ethanol (CH3-CH2-OH).
2. Some ethanol kept; the rest oxidised: ethanol + acidified K2Cr2O7 (reflux) → ethanoic acid.
3. Ethanoic acid + ethanol (concentrated H2SO4 catalyst, reflux) → ethyl ethanoate + water (equilibrium).

This is the canonical "synthesis ethyl ethanoate from ethene" question.

Worked example 2: Propan-2-ol from propane

Target: propan-2-ol (CH3-CHOH-CH3).

Pathway:

1. Propane + Br2 (UV) → 2-bromopropane (Markovnikov-equivalent for substitution: secondary radical is more stable) + HBr.
2. 2-bromopropane + NaOH (aq) → propan-2-ol + NaBr.

Worked example 3: Pentan-2-one from pentan-2-ol

Target: pentan-2-one (CH3-CO-CH2-CH2-CH3). Ketone.

Pentan-2-ol is a secondary alcohol. Oxidation:

Pentan-2-ol + acidified K2Cr2O7 (reflux) → pentan-2-one + H2O.

(The dichromate orange-to-green colour change is the visible indicator.)

Pathway diagrams

VCAA expects clear pathway diagrams. Standard format:

ethene
  | + H2O (dilute H2SO4, heat)
  v
ethanol
  | + acidified Cr2O7^2- (reflux)
  v
ethanoic acid + ethanol (kept separately)
  | + conc. H2SO4 catalyst, reflux
  v
ethyl ethanoate + water

Each arrow labelled with reagent above and conditions below.

Common errors

Trying to oxidise a tertiary alcohol. Dead end.

Forgetting Markovnikov direction. HX and H2O addition to unsymmetrical alkenes follows Markovnikov in standard VCE.

Wrong conditions for primary alcohol oxidation. Distillation gives aldehyde; reflux gives carboxylic acid.

Missing the equilibrium for esterification. Use a reversible arrow.

Mixing up esterification and hydrolysis. Esterification: acid + alcohol → ester + water (catalysed by H+, equilibrium). Hydrolysis: ester + water → acid + alcohol (reverse).

In one sentence

VCE Chemistry organic synthesis pathways combine the reaction toolkit (alkane halogenation under UV, alkene addition reactions following Markovnikov, alcohol oxidation with acidified dichromate distillation/reflux, Fischer esterification with H2SO4 catalyst) using retrosynthesis (working backwards from the target) to design multi-step syntheses; pathway diagrams must specify reagents, conditions and named intermediates at each step.