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9476 / 2027
Organic Chemistry

Lesson 34 of 34

Build a route by changing one functional group at a time

Check carbon count, conditions, competing groups and the final chemical form.

A-Level 9476 (2026-2027)

  • I can describe alkene chemistry using ethene and analogous structures.Syllabus

    Syllabus 9476, 11.4(b). Describe alkene chemistry using ethene and analogous structures.

  • I can describe the specified halogenoalkane reactions.Syllabus

    Syllabus 9476, 11.5(a). Describe the specified halogenoalkane reactions.

  • I can describe alcohol reactions using ethanol.Syllabus

    Syllabus 9476, 11.6(a). Describe alcohol reactions using ethanol.

  • I can interconvert alcohols and carbonyl compounds.Syllabus

    Syllabus 9476, 11.7(a). Interconvert alcohols and carbonyl compounds.

  • I can prepare carboxylic acids by oxidation and hydrolysis.Syllabus

    Syllabus 9476, 11.8(a). Prepare carboxylic acids by oxidation and hydrolysis.

  • I can describe four carboxylic-acid transformations.Syllabus

    Syllabus 9476, 11.8(b). Describe four carboxylic-acid transformations.

  • I can describe acid and base ester hydrolysis.Syllabus

    Syllabus 9476, 11.8(h). Describe acid and base ester hydrolysis.

  • I can prepare the specified amines.Syllabus

    Syllabus 9476, 11.9(a). Prepare the specified amines.

  • I can describe amide reactions using ethanamide.Syllabus

    Syllabus 9476, 11.9(h). Describe amide reactions using ethanamide.

Build a route by changing one functional group at a time

Check carbon count, conditions, competing groups and the final chemical form.

Worked exampleLengthen ethanol by one carbon to make propanoic acid

Suggest a route from ethanol to propanoic acid using the specified reactions.

Try it first. Then show the worked stepsHide the worked steps
  1. Replace OH with Br using HBr and suitable heating, giving bromoethane.
  2. Heat with KCN in ethanol to substitute Br by CN, giving propanenitrile. This is the carbon-chain extension step.
  3. Hydrolyse the nitrile with dilute aqueous acid and heat, or with aqueous alkali and heat followed by acidification.
  4. Check the final product contains three carbons and is the acid rather than its carboxylate salt.
Answer

CH3CH2OH → CH3CH2Br → CH3CH2CN → CH3CH2COOH. Direct oxidation of ethanol would give only the two-carbon ethanoic acid.

Worked exampleUse tests to narrow a structure, then check all evidence

A compound with formula C3H6O gives 2,4-DNPH, does not give Tollens, and gives yellow CHI3 with alkaline iodine. What structure fits the ordinary aldehyde/ketone candidates?

Try it first. Then show the worked stepsHide the worked steps
  1. 2,4-DNPH supports an aldehyde or ketone.
  2. The negative Tollens result supports a ketone rather than propanal.
  3. The iodoform result requires a CH3CO unit.
  4. The three-carbon formula permits propanone, CH3COCH3.
Answer

Propanone fits all the evidence. A test identifies a feature within an assumed comparison set; avoid claiming that one isolated observation proves a complete structure.

In preparation questions, name each reagent, the medium, the conditions and the main product. For separating the product, ask what dissolves in each layer. Washing with acid or base can change whether a substance is ionised, and so how soluble it is. Extraction separates layers. Drying removes water. Distillation separates liquids with different boiling points. Choose the steps that suit the mixture given, rather than learning one fixed sequence.

Check your understanding

Which reagents and conditions turn CH3CH2Br into CH3CH2CN, and CH3CHBrCH3 into CH3CH=CH2?

  1. KCN in ethanol, heat under reflux; then NaOH in ethanol, heat under reflux
  2. HCN with a little KCN at 10-20 degrees C; then NaOH(aq), heat under reflux
  3. KCN(aq) at room temperature; then concentrated H3PO4, heat
Show the answer

KCN in ethanol, heat under reflux; then NaOH in ethanol, heat under reflux

Ethanolic KCN substitutes Br with CN. Ethanolic NaOH favours elimination of HBr to give the alkene. Mark schemes need the medium (aqueous or ethanolic) and the condition as well as the reagent.

Exam-style question

Suggest a three-step synthesis of 2-hydroxy-2-methylpropanenitrile, (CH3)2C(OH)CN, from propene. For each step give the reagents and conditions and the structure of the intermediate. [3]

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Exam-style question

Compound A, C4H8O, is chiral. Use these observations to deduce the structure of A, giving the deduction from each one.
1. A decolourises orange Br2(aq) at room temperature.
2. A gives effervescence with sodium metal.
3. A gives no precipitate with 2,4-DNPH.
4. A gives a yellow precipitate when warmed with alkaline aqueous iodine.
5. A reacts with H2 and a Ni catalyst to give B, C4H10O, which also gives a yellow precipitate with alkaline aqueous iodine. [5]

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