← Unit 4: Structure, synthesis and design

QLDChemistrySyllabus dot point

Topic 1: Properties and structure of organic materials

Describe and represent reaction pathways for the synthesis of organic compounds, including identifying reagents and conditions required for each step and predicting intermediates

A focused answer to the QCE Chemistry Unit 4 dot point on multi-step organic synthesis. Assembles the Unit 4 reaction toolkit (substitution, addition, oxidation, esterification, hydrolysis) into reaction pathway maps, with worked syntheses of ethyl ethanoate from ethene and a haloalkane from an alkane. Includes the QCAA pathway-diagram conventions for IA3 and EA.

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What this dot point is asking

QCAA wants you to combine the Unit 4 organic reaction toolkit (substitution, addition, oxidation, esterification, hydrolysis) into multi-step syntheses, drawing pathways that show feedstock, intermediates, products, and the reagents and conditions for each step. The dot point is the highest-yield organic item in the EA Paper 2 extended response (typically 6 to 8 marks) and a routine IA3 secondary-data interpretation step (proposing pathways to a target compound).

The answer

A reaction pathway (also called a synthetic route) is a sequence of reactions transforming a starting material into a target product. Each arrow is one reaction step with named reagents and conditions; each node is a named compound (feedstock, intermediate, or product).

The Unit 4 reaction toolkit

The following ten transformations are the building blocks for every Unit 4 pathway. Memorise them as both forward and reverse where possible.

From To Reagents Conditions
Alkane Haloalkane X2 (Br2 / Cl2) UV light, free-radical substitution
Alkene Haloalkane HX (HBr / HCl) room T, Markovnikov for unsymmetrical
Alkene Vicinal dihaloalkane X2 (Br2 / Cl2) room T, addition
Alkene Alkane H2 Ni or Pt catalyst, hydrogenation
Alkene Alcohol H2O dilute H2SO4 catalyst, heat, Markovnikov
Haloalkane Alcohol NaOH(aq) warm aqueous, nucleophilic substitution
Primary alcohol Aldehyde acidified K2Cr2O7 distillation, gentle heat
Primary alcohol Carboxylic acid acidified K2Cr2O7, excess reflux
Secondary alcohol Ketone acidified K2Cr2O7 reflux
Carboxylic acid + alcohol Ester concentrated H2SO4 catalyst reflux, equilibrium
Ester + water Acid + alcohol dilute H2SO4 catalyst reflux, acid hydrolysis

Tertiary alcohols cannot be oxidised. Alkanes are unreactive towards halogens without UV. These two negatives are common pathway "dead ends" that QCAA stimulus exploits.

How to draw a QCAA reaction pathway

QCAA accepts pathways drawn as a vertical or horizontal flow of named compounds connected by arrows. Each arrow is labelled with the reagent above and conditions below. Example template:

ethene  --[H2O, dilute H2SO4, heat]-->  ethanol  --[Cr2O7^2-/H+, reflux]-->  ethanoic acid

For a branching synthesis (where one intermediate is split into two streams), use a Y-shaped diagram with the split clearly labelled.

Worked example: ethyl ethanoate from ethene

The synthesis in the 2023 past question above is the canonical "make an ester from an alkene" exercise. The three steps are:

  1. Ethene -> ethanol (acid-catalysed hydration). Reagents: water. Conditions: dilute H2SO4, heat. Mechanism: addition.
  2. Ethanol -> ethanoic acid (full oxidation). Reagents: acidified K2Cr2O7. Conditions: reflux. Half the ethanol stream is held back for step 3.
  3. Ethanoic acid + ethanol -> ethyl ethanoate (Fischer esterification). Reagents: concentrated H2SO4 catalyst. Conditions: reflux. Equilibrium.

Pathway diagram:

                      [H2O, dilute H2SO4, heat]
ethene  -----------------------------------> ethanol
                                            /        \
                          [keep half]      /          \  [Cr2O7^2-/H+, reflux]
                                          v            v
                                       ethanol      ethanoic acid
                                          \            /
                                           \          /
                          [conc. H2SO4, reflux]
                                            v
                                       ethyl ethanoate

Worked example: propan-2-ol from propane

Propane is unreactive towards most reagents; the only Unit 4 reaction it undergoes is halogenation under UV. The pathway from propane to propan-2-ol uses substitution followed by hydrolysis.

  1. Propane -> 2-bromopropane (free-radical substitution; major product is secondary haloalkane). Reagents: Br2. Conditions: UV light. By-product: HBr.
  2. 2-bromopropane -> propan-2-ol (nucleophilic substitution). Reagents: NaOH(aq). Conditions: warm aqueous solution. By-product: NaBr.

An alternative route uses an alkene intermediate (propane -> propene by cracking, then propene -> propan-2-ol by Markovnikov hydration), but cracking is outside the Unit 4 syllabus, so the radical-substitution route is preferred.

Retrosynthesis: working backwards

When given a target product without a specified starting material, work backwards:

  1. Identify the functional groups in the target. What reactions produce these groups?
  2. List the immediate precursors. For an ester, the precursor pair is acid + alcohol. For a haloalkane, the precursor is either an alkane (radical substitution) or an alkene (HX addition).
  3. Repeat for each precursor until you reach a sensible feedstock (alkane, alkene, or simple alcohol).
  4. Reverse the chain. Write the pathway forward, with each step's reagents and conditions.

Example. Target: methyl propanoate. Precursors: propanoic acid + methanol. Propanoic acid from propan-1-ol (full oxidation). Methanol typically used as a feedstock; propan-1-ol from propene (anti-Markovnikov hydration is not in syllabus, so propan-1-ol is usually taken as feedstock or made via 1-bromopropane / NaOH). Working forward: propan-1-ol -> propanoic acid -> methyl propanoate.

Selectivity and product separation

Real syntheses rarely give a single product. Pathway-design questions are marked on:

  • Selecting the correct major product for each step (Markovnikov for HX and H2O, secondary alcohol for radical halogenation of propane, etc.).
  • Stating realistic conditions. "Reflux" is required for slow reactions (oxidation, esterification). "Distillation" is required to isolate volatile aldehydes before they over-oxidise.
  • Acknowledging by-products. HX from radical substitution, water from esterification, NaX from nucleophilic substitution. The QCAA pathway-diagram convention is to show by-products on the arrow as "+ by-product" rather than as a separate stream.

Pathway pitfalls

Trying to oxidise a tertiary alcohol. Dead end. If your retrosynthesis lands on (CH3)3C-OH as an alcohol precursor for a ketone, the route is wrong.

Hydrogenating across a C=O bond with H2/Ni. H2 with Ni reduces C=C only, not C=O. Use NaBH4 (outside the Unit 4 syllabus) or design around the limitation.

Markovnikov in the wrong direction. "Add HBr to propene" gives 2-bromopropane (major), not 1-bromopropane. If you want 1-bromopropane, you need a different route (radical addition with peroxides, outside syllabus, so use a haloalkane substitution from a primary alcohol).

Drawing a pathway without conditions. QCAA marks for the conditions explicitly; "step 2: oxidation" earns half the marks of "step 2: acidified K2Cr2O7, reflux, full oxidation to carboxylic acid."

Forgetting equilibrium for esterification. Ester syntheses must use a reversible arrow and an excess of one reactant or removal of water to push the equilibrium right.

In one sentence

A reaction pathway assembles the Unit 4 reaction toolkit (radical substitution, electrophilic addition, nucleophilic substitution, oxidation, esterification, hydrolysis) into a sequence transforming a starting material into a target product, with each step's reagents and conditions explicitly labelled, intermediates named, and any byproducts or equilibrium considerations stated.

Past exam questions, worked

Real questions from past QCAA papers on this dot point, with our answer explainer.

2023 QCAA-style6 marksPropose a three-step synthesis of ethyl ethanoate starting from ethene as the only organic feedstock. For each step, write a balanced equation, identify the reagents and conditions, and name any intermediate.
Show worked answer →

A 6-mark answer needs three balanced equations, the conditions for each, and the named intermediates.

Step 1: ethene to ethanol (acid-catalysed hydration).

CH2=CH2+H2O→H2SO4 catalyst, heatCH3−CH2−OHCH_2=CH_2 + H_2O \xrightarrow{H_2SO_4 \text{ catalyst, heat}} CH_3-CH_2-OH

Intermediate / product: ethanol. Conditions: dilute H2SO4 catalyst, heat, water as reagent.

Step 2a: oxidation of ethanol to ethanoic acid.

CH3−CH2−OH+2[O]→Cr2O72−/H+, refluxCH3−COOH+H2OCH_3-CH_2-OH + 2[O] \xrightarrow{Cr_2O_7^{2-} / H^+, \text{ reflux}} CH_3-COOH + H_2O

Intermediate: ethanoic acid (the carboxylic acid). Conditions: acidified potassium dichromate, reflux (full oxidation through ethanal to the carboxylic acid).

Step 2b (parallel branch): keep some ethanol unoxidised. Half the original ethanol stream stays as ethanol for Step 3. Markers accept this as part of Step 2 or noted separately.

Step 3: Fischer esterification.

CH3−COOH+CH3−CH2−OH⇌H2SO4CH3−COO−CH2−CH3+H2OCH_3-COOH + CH_3-CH_2-OH \xrightleftharpoons[]{H_2SO_4} CH_3-COO-CH_2-CH_3 + H_2O

Product: ethyl ethanoate. Conditions: concentrated H2SO4 catalyst, reflux.

A clean pathway diagram (ethene -> ethanol -> ethanoic acid + ethanol -> ethyl ethanoate) earns the structural-representation marks. Markers reward correctly named intermediates, correct conditions (especially dichromate reflux for full oxidation, sulfuric acid for esterification), and a balanced ester equation with a reversible arrow.

2022 QCAA-style4 marksStarting from propane, propose a synthesis of propan-2-ol. Write a pathway with at least two steps, naming reagents, conditions and any intermediate.
Show worked answer →

A 4-mark answer needs the two-step pathway with reagents and the named intermediate.

Step 1: propane to 2-bromopropane (free-radical substitution).

CH3−CH2−CH3+Br2→UVCH3−CHBr−CH3+HBrCH_3-CH_2-CH_3 + Br_2 \xrightarrow{UV} CH_3-CHBr-CH_3 + HBr

Intermediate: 2-bromopropane (the secondary haloalkane is the major product because secondary C-H is more readily abstracted than primary). Conditions: bromine gas or solution, ultraviolet light. By-product: HBr.

Note. Markers also accept the alternative route via dehydration of propan-1-ol to propene, followed by acid-catalysed hydration (Markovnikov) of propene to propan-2-ol, but this is longer and starts from a different feedstock; from propane the radical-substitution route is shorter.

Step 2: 2-bromopropane to propan-2-ol (nucleophilic substitution).

CH3−CHBr−CH3+NaOH(aq)→CH3−CH(OH)−CH3+NaBrCH_3-CHBr-CH_3 + NaOH_{(aq)} \rightarrow CH_3-CH(OH)-CH_3 + NaBr

Product: propan-2-ol. Conditions: warm aqueous sodium hydroxide. By-product: sodium bromide.

Markers reward the two distinct steps, correct major-product identification for the substitution, valid conditions for each step, and a balanced equation per step. Drawing the pathway as a flow diagram with arrows is encouraged.

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