Multi-step synthesis questions ask you to get from compound A to compound B. Learn the map. Then any route is just joining arrows. Alkene and haloalkane are the two central “hubs”.
Work backwards from the target. Need to add a carbon? The only chain-lengthening step at L3 is haloalkane → nitrile (KCN). Need an amine? Go via a nitrile or a haloalkane + NH₃. Need a carboxylic acid? Oxidise a 1° alcohol/aldehyde, or hydrolyse a nitrile. Worked example Ethene → ethanoic acid (2 steps + why) Convert CH₂=CH₂ into CH₃COOH. Step 1, hydrate the alkene: CH₂=CH₂ + H₂O (H⁺ catalyst) → CH₃CH₂OH (ethanol). Step 2, fully oxidise the 1° alcohol under reflux: CH₃CH₂OH + K₂Cr₂O₇/H⁺, reflux → CH₃COOH. (Reflux, not distillation: distilling would stop at the aldehyde ethanal.) ✓ Ethene → ethanol (hydration) → ethanoic acid (oxidation, reflux).