A multi-stage synthesis converts a starting material into a target molecule through two or more steps. OCR exam questions ask for up to about four steps, with the reagents and conditions for each step and the structure of every intermediate.A systematic approach:1. Compare the starting material and the target: which functional groups change, and are they aliphatic or aromatic?
2. Count the carbon atoms. If the chain gets one carbon longer, a nitrile step (KCN with a haloalkane, or NaCN/H+ with a carbonyl) is needed. On a benzene ring, a Friedel-Crafts reaction adds carbon.
3. Work backwards from the target: what could the target be made from in one step? Repeat until you reach the starting material.
4. Check each step: correct reagents and conditions, no step that would also attack another group, and, for benzene rings, the right order for directing effects.Worked example: make propanoic acid from ethanol.
Carbons: 2 → 3, so a nitrile step is needed.
Working backwards: propanoic acid ← propanenitrile ← bromoethane ← ethanol.
Step 1: ethanol → bromoethane: NaBr and conc. H2SO4.
Step 2: bromoethane → propanenitrile: KCN in ethanol.
Step 3: propanenitrile → propanoic acid: dilute HCl(aq), heat.The overall yield falls with every step: three steps each with 70% yield give 0.70 × 0.70 × 0.70 × 100 = 34% overall, so the shortest workable route is preferred.
2. Count the carbon atoms. If the chain gets one carbon longer, a nitrile step (KCN with a haloalkane, or NaCN/H+ with a carbonyl) is needed. On a benzene ring, a Friedel-Crafts reaction adds carbon.
3. Work backwards from the target: what could the target be made from in one step? Repeat until you reach the starting material.
4. Check each step: correct reagents and conditions, no step that would also attack another group, and, for benzene rings, the right order for directing effects.Worked example: make propanoic acid from ethanol.
Carbons: 2 → 3, so a nitrile step is needed.
Working backwards: propanoic acid ← propanenitrile ← bromoethane ← ethanol.
Step 1: ethanol → bromoethane: NaBr and conc. H2SO4.
Step 2: bromoethane → propanenitrile: KCN in ethanol.
Step 3: propanenitrile → propanoic acid: dilute HCl(aq), heat.The overall yield falls with every step: three steps each with 70% yield give 0.70 × 0.70 × 0.70 × 100 = 34% overall, so the shortest workable route is preferred.