Part 1: Planning routes and improving yield

Plan AQA organic syntheses of up to four steps, track carbon atoms and compare yield, atom economy and practical choices.

Start with the target, then work backwards

Identify the target functional group and the carbon skeleton separately. Ask which reaction makes the final bond or functional group, then which starting material that reaction needs. Repeat until you reach the supplied reactant. Finally read the route forwards and check every intermediate, reagent and condition.

Changing an alcohol into an aldehyde does not lengthen its carbon chain. Replacing a halogen by CN adds one carbon; reducing that nitrile keeps this new carbon. A reagent name alone does not demonstrate a route: draw or write each intermediate unambiguously.

A compact reaction toolkit

These are route-planning reminders. The individual topic notes explain mechanisms and limitations. Separate steps may need isolation or a change of conditions; listing mutually incompatible reagents together is not a valid one-pot method.

Worked four-step route: ethanol to propylamine

The target contains three carbons but ethanol contains two. Plan a nitrile intermediate to supply the extra carbon. This is one chemically valid four-step route; it is not a claim that industry would choose it.

Step 1: dehydrate ethanol with concentrated phosphoric acid and heat to form ethene. Step 2: add HBr to ethene to form bromoethane. Step 3: heat bromoethane with KCN in aqueous ethanol under reflux to form propanenitrile. Step 4: reduce propanenitrile with H2/Ni under suitable conditions to give propylamine (propan-1-amine).

Check compatibility and competing products

A molecule with two functional groups can react at both. An oxidant intended to oxidise a primary alcohol can also oxidise an aldehyde elsewhere. Strong acid or base can hydrolyse an ester. Check the whole structure before choosing a reagent.

A racemic intermediate may remain racemic unless a later step separates or selectively transforms its enantiomers. A constitutional isomer mixture may need separation. Do not give a single pure product merely because it is the one you want.

For a synthesis ending in an amide, distinguish the carbonyl-containing acyl fragment from the amine fragment. N-ethylpropanamide needs a propanoyl group and an ethylamine group; ethanoyl chloride plus propylamine makes a different amide.

Yield multiplies; atom economy answers a different question

For consecutive steps with one-to-one stoichiometry, multiply fractional yields. If stoichiometric ratios change, calculate moles at each stage instead of blindly multiplying product masses.

A route with yields of 80.0%, 75.0% and 90.0% gives 0.800 × 0.750 × 0.900 = 0.540, or 54.0% overall. Starting with 0.200 mol of the limiting reactant can therefore produce 0.108 mol of the final product for this one-to-one route.

Atom economy = Mr of the desired product, including its stoichiometric coefficient, ÷ total Mr of all reactants with their coefficients × 100. It is calculated from a balanced equation, not the measured mass collected. High atom economy does not guarantee a high experimental yield.

When comparing routes, consider number of steps, yield, selectivity, atom economy, energy, separation, solvent use and reagent hazards. Avoid unnecessary solvents and hazardous substances where a workable alternative exists. A shorter route is not automatically better if it needs difficult purification or a much more hazardous reagent.

Explain why your route works

In AQA June 2022 Paper 2 Q10, the route involves amine chemistry and structural deduction. The report discusses difficulties with reagents and further reactions; use that context to practise connecting a named transformation to the actual structure.

For each arrow, check the functional-group change, carbon count, charge and conditions. State a necessary work-up, such as adding alkali after reducing nitrobenzene in acid, rather than presenting an ammonium salt as the free amine.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources