Chemistry • Year 12 • Module 7 • Lesson 19
Organic Reaction Pathways: Synthesis & Multi-Step Problems
Recall functional group names, reaction conditions and pathway vocabulary so every arrow on the Module 7 reaction map has a meaning you can write from memory.
1. Term–definition match
Match each term (column A) to its correct definition (column B) by writing the definition letter next to each term. 8 marks
| Column A — Term | Your answer | Column B — Definition |
|---|---|---|
| 1.1 Reaction pathway |
A — The reversible reaction between a carboxylic acid and an alcohol to form an ester and water, requiring a concentrated acid catalyst. B — Working backwards from a target molecule to identify suitable precursors and reagents at each step. C — A sequence of organic reactions connecting a starting material to a target product via named intermediates. D — The conditions that stop oxidation at the aldehyde stage by removing it from the reaction vessel as it forms. E — Systematic conversion of one functional group class into another using known reagents and conditions. F — The overall fractional yield of a multi-step synthesis, equal to the product of each individual step yield. G — A secondary alcohol oxidation product that cannot be oxidised further to a carboxylic acid under Module 7 conditions. H — Choosing reagents or apparatus that convert one functional group without disturbing another present in the same molecule. | |
| 1.2 Retrosynthetic analysis | ||
| 1.3 Functional group interconversion | ||
| 1.4 Esterification | ||
| 1.5 Distillation (in oxidation) | ||
| 1.6 Ketone dead end | ||
| 1.7 Selectivity | ||
| 1.8 Overall yield in multi-step synthesis |
2. True or false — with correction
Circle T or F. If the statement is false, write the corrected version on the line below. 10 marks (1 T/F + 1 correction each)
2.1 A primary alcohol can be converted directly to an aldehyde by heating with excess K₂Cr₂O₅ and H₂SO₄ under reflux. T / F
2.2 Alkane cannot be converted to alcohol in a single Module 7 step; it must pass through a haloalkane intermediate. T / F
2.3 Esterification is an irreversible reaction; the arrow in the equation should be written as →. T / F
2.4 Saponification of an ester with concentrated NaOH(aq) and reflux is irreversible. T / F
2.5 A secondary alcohol oxidised with K₂Cr₂O₅/H₂SO₄ under reflux produces a carboxylic acid. T / F
3. Fill in the blanks
Complete the paragraph by writing one term from the word bank in each blank. Each term is used once only. 9 marks
Word bank: distillation — reflux — aldehyde — carboxylic acid — haloalkane — NaOH(aq) — UV light — H₂PO₄ — esterification
To convert an alkene to an alcohol, steam is added in the presence of __________ (1) as catalyst at approximately 300°C and high pressure. Alternatively, an alkane can be converted to a __________ (2) by reacting it with a halogen under __________ (3). This intermediate is then converted to a primary alcohol by refluxing with __________ (4). Mild oxidation of the primary alcohol using K₂Cr₂O₅/H₂SO₄ and __________ (5) produces an __________ (6), because the volatile product is removed from the flask before further oxidation occurs. If oxidation is carried out with excess K₂Cr₂O₅/H₂SO₄ under __________ (7) instead, the product is a __________ (8). This compound can then be combined with an alcohol in a process called __________ (9) using concentrated H₂SO₄ catalyst to produce an ester.
4. Function recall — conditions & their purpose
Answer each prompt in 1–2 sentences using precise chemical terms. 8 marks (2 each)
4.1 What is the purpose of using distillation (rather than reflux) when oxidising a primary alcohol with K₂Cr₂O₅?
4.2 Why must aqueous NaOH (not ethanolic NaOH) be used when converting a haloalkane to an alcohol?
4.3 What is the function of concentrated H₂SO₄ as a catalyst in esterification?
4.4 Why is the secondary alcohol → ketone conversion described as a ‘dead end’ in Module 7 synthesis?
5. Reaction map — concept connections
Six functional group names are provided in the chips below. Draw labelled arrows between them to show valid Module 7 transformations. Each arrow must carry: the reagent/condition AND the direction of conversion. Aim for at least 7 labelled arrows. 7 marks
Q1 — Term–definition match
1.1 C • 1.2 B • 1.3 E • 1.4 A • 1.5 D • 1.6 G • 1.7 H • 1.8 F
Q2 — True / false with correction
2.1 False. Correction: using excess K₂Cr₂O₅ and reflux produces a carboxylic acid, not an aldehyde. To stop at the aldehyde stage, distillation must be used to remove the more volatile product before further oxidation occurs.
2.2 True.
2.3 False. Correction: esterification is a reversible reaction; the equation should show a reversible arrow (⇌). Saponification (with NaOH) is the irreversible process.
2.4 True.
2.5 False. Correction: oxidation of a secondary alcohol with K₂Cr₂O₅/H₂SO₄ under reflux produces a ketone, not a carboxylic acid. Ketones cannot be oxidised further under Module 7 conditions — this is the dead end.
Q3 — Fill in the blanks
(1) H₂PO₄ • (2) haloalkane • (3) UV light • (4) NaOH(aq) • (5) distillation • (6) aldehyde • (7) reflux • (8) carboxylic acid • (9) esterification
Q4.1 — Purpose of distillation in oxidation
The aldehyde product (e.g., ethanal, BP 20°C) has a significantly lower boiling point than the corresponding alcohol (e.g., ethanol, BP 78°C). By using distillation, the aldehyde vapour is collected as it forms and removed from contact with the excess dichromate oxidant, preventing it from being further oxidised to a carboxylic acid.
Q4.2 — Aqueous vs ethanolic NaOH
Aqueous NaOH provides hydroxide ions (OH¹¯) dissolved in water. This promotes nucleophilic substitution at the carbon–halogen bond, replacing the halogen with –OH to give an alcohol. Ethanolic (alcoholic) NaOH promotes elimination (E2 mechanism) instead, producing an alkene rather than an alcohol.
Q4.3 — Function of H₂SO₄ in esterification
Concentrated H₂SO₄ donates H¹♠ to protonate the carbonyl group of the carboxylic acid, activating it toward nucleophilic attack by the alcohol. It is also a dehydrating agent that absorbs the water produced, shifting the equilibrium toward more ester by Le Chatelier’s principle. H₂SO₄ is not consumed overall — it is a true catalyst.
Q4.4 — Secondary alcohol / ketone dead end
Ketones do not have a hydrogen directly bonded to the carbonyl carbon (R–CO–R′), so there is no C–H bond adjacent to the C=O that can be removed in the oxidation mechanism to form a carboxylic acid. Under the reagents and conditions used in Module 7 (K₂Cr₂O₅/H₂SO₄), no further oxidation of a ketone is possible, making it a pathway dead end.
Q5 — Sample reaction map arrows
Acceptable arrows include (minimum 7):
- Alkene → Alcohol: H₂O (steam), H₂PO₄ cat., 300°C, high pressure (hydration)
- Alcohol → Alkene: conc. H₂SO₄, heat (dehydration, reverse)
- Alkene → Haloalkane: + HX, r.t. (hydrohalogenation, Markovnikov)
- Haloalkane → Alcohol: NaOH(aq), reflux (substitution)
- Alcohol → Haloalkane: + HX, reflux
- Alcohol → Aldehyde: K₂Cr₂O₅/H₂SO₄, distillation (1° alcohol only)
- Aldehyde → Carboxylic Acid: K₂Cr₂O₅/H₂SO₄ excess, reflux
- Alcohol → Carboxylic Acid: K₂Cr₂O₅/H₂SO₄ excess, reflux (direct, 1° alcohol)
- Carboxylic Acid + Alcohol ⇌ Ester: conc. H₂SO₄ cat., reflux