Organic synthesis (A-level only) — AQA A-Level Chemistry
Test yourself on Organic synthesis (A-level only) with AQA A-Level practice questions.
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Organic synthesis (A-level only) explained
Most targets cannot be reached in one reaction, so you plan a route through intermediates, changing one functional group at a time.
Read the full explanation
One route that recurs is nitration of benzene using concentrated nitric acid with concentrated sulfuric acid, followed by reduction of the nitro group using tin and hydrochloric acid to give an aromatic amine. Another is halogenoalkane to nitrile with potassium cyanide in ethanol, which lengthens the carbon chain by one carbon, then reduction to an amine or hydrolysis to a carboxylic acid. Yields compound badly: three steps at seventy per cent each leave only thirty-four per cent overall, because the fractions multiply rather than average. Steps that give mixtures, such as an amine reacting further with a halogenoalkane, cost the most.
Students should be able to: explain why chemists aim to design processes that do not require a solvent and that use non- hazardous starting materials explain why chemists aim to design production methods with fewer steps that have a high percentage atom economy use reactions in this specification to devise a synthesis, with up to four steps, for an organic compound.
Percentage atom economy is the relative formula mass of the desired product divided by the total relative formula mass of all the reactants, multiplied by one hundred. For a balanced equation the products give the same total. An addition reaction reaches one hundred per cent, because every atom of the reactants ends up in the product; substitution and elimination cannot, because something always leaves. A high atom economy means less waste to treat and less raw material bought for the same output. Working without a solvent removes volatile organic compounds from the air, removes the energy cost of recovering the solvent, and removes an entire separation stage. Non-hazardous starting materials cut the risk to workers and to the environment. Fewer steps mean a higher overall yield, fewer separations, less energy and lower cost, which is why routes are designed backwards from the target.
Your focus
- Identify the reagents and conditions for each step of a given two-step or three-step route.
- Draw the intermediate formed at each stage of a stated synthesis.
- Calculate the overall percentage yield of a route from the yield of each separate step.
Show all 8 objectives
- Explain why a step that produces a mixture of products lowers the yield of the desired compound.
- Calculate the percentage atom economy of a reaction from its equation.
- Explain why addition reactions have a higher atom economy than substitution or elimination reactions.
- Justify a solvent-free process by naming two specific advantages over one that uses a solvent.
- Devise a route of up to four steps to a named target, giving reagents, conditions and intermediates.
Organic synthesis (A-level only) exam tips
Marking Points
- one mark for each reagent named in full, so concentrated nitric acid and concentrated sulfuric acid rather than nitric acid alone
- one mark for tin and hydrochloric acid, or iron and hydrochloric acid, as the system that reduces a nitro group
- one mark for the structure of each intermediate along the route
- one mark for multiplying the fractional yields of the separate steps to give the overall yield
- one mark for a correct atom economy expression, the relative formula mass of the desired product over the total relative formula mass of all the reactants, multiplied by one hundred
- one mark for stating that an addition reaction has an atom economy of one hundred per cent, and why
- one mark for each valid reagent and condition in a devised route, up to the number of steps allowed
- one mark for the structure of the intermediate at each stage of a devised route
- one mark for a reason such as less waste, lower energy demand or fewer separation stages
Examiner Tips
- 💡Write reagents in full, with concentration or state where it matters; an incomplete reagent is penalised even when the chemistry is right.
- 💡Draw every intermediate, because structure marks are awarded step by step even when a later step is wrong.
- 💡For a yield calculation work in decimals and multiply, converting back to a percentage only at the end.
- 💡Plan a synthesis backwards from the target: ask what functional group the final step must have started from.
- 💡Set a devised route out as a scheme of structures joined by arrows, with reagents and conditions written over each arrow.
- 💡Quote a number where you can; an atom economy calculation earns more than a sentence of general praise for green chemistry.
Common Mistakes
- omitting the word concentrated from nitric or sulfuric acid, which loses the reagent mark
- adding or averaging the percentage yields of the steps instead of multiplying them
- quoting a catalyst as though it were a reagent, or naming something that could not be taken from a bottle
- expecting a single product when an amine reacts with a halogenoalkane, where further substitution gives a mixture
- writing a route that changes two functional groups in one step with no reagent that could do it
- confusing atom economy with percentage yield, so working from the mass actually obtained
- leaving a reactant out of the denominator, which makes the atom economy come out too high
- answering that it is better for the environment without mentioning waste, energy or hazard
- devising a route that needs more steps than the question allows
- naming a reagent but no conditions, so the step could not actually be carried out