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Organic Chemistry

Topic 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)

Build a route by changing one functional group at a time

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

Worked example

Lengthen ethanol by one carbon to make propanoic acid

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

  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 example

Use 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?

  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.

For preparation questions, name each reagent, essential medium and condition, plus the main product. For work-up, ask what remains dissolved in each phase: an acid/base wash can change ionisation and solubility; extraction separates phases; drying removes water; distillation separates sufficiently different volatilities. Choose these steps from the stated mixture instead of memorising one universal purification sequence.