← Properties of selected organic compounds

Multi-step synthesis and incomplete reaction schemes

Every year gives an incomplete scheme (missing structures, reagents, or reaction-type names) to complete — treat it as a chain of single steps, not one large puzzle.

The method for an incomplete scheme

  • Work through the scheme ONE ARROW AT A TIME — identify the starting functional group and the ending functional group for that single step.
  • Match that single-step transformation to the reaction web: which reagent and condition convert THAT starting group into THAT ending group.
  • Name the reaction TYPE (substitution, elimination, addition, oxidation) for each arrow — this is graded separately from getting the structure or reagent right.
  • Where a step could give more than one product, decide and justify the major/minor split before moving to the next step.
Worked example Proposing a synthesis route between two named alcohols Propose a series of reactions converting a primary alcohol (pent-1-ol) into a secondary alcohol with the OH at a different position (pentan-2-ol equivalent structure), stating reagents, conditions and reaction types. STEP 1 — dehydrate the primary alcohol to an alkene: concentrated H₂SO₄, heat (elimination). STEP 2 — rehydrate the alkene with the OH added at the new position: H₂O/H⁺, heat (addition, Markovnikov orientation determines where the OH lands). Justify orientation: Markovnikov addition places the OH on the more substituted carbon, which is exactly the position needed for the target secondary alcohol. State the reaction type at each arrow explicitly (elimination, then addition) — this is checked separately from whether the correct structures are drawn. ✓ A genuine multi-step route needs each arrow's reagent, condition AND reaction-type name stated — a scheme with only structures and no reaction-type labels is an incomplete answer even if every structure is correct. ✕

Attempting a scheme in one giant leap instead of step by step

Trying to see the whole multi-step transformation at once often causes a wrong reagent to be chosen for an intermediate step. Work through ONE arrow at a time, identifying exactly which functional group interconversion that single arrow represents, before considering the next step — this reduces the multi-step problem to a sequence of single-step problems you already know how to solve. ℹ

Working backwards from the target can also help

When a scheme feels difficult to plan forward, try working BACKWARDS from the target compound: ask what reagent could have produced it from a compound one step earlier, and repeat until you reach the given starting material. Meeting in the middle (forward from the start, backward from the target) often reveals the route faster than forward planning alone.

Practise just this