Design multistep synthesis pathways and evaluate them using yield and green chemistry principles

Designing multistep synthesis routes, reaction pathways between functional groups, percentage yield and atom economy, and green chemistry principles, with worked TASC-style examples.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking

Chemical synthesis is the deliberate making of a desired product, the target molecule, from available starting materials. Designing a synthesis means choosing a sequence of reactions that converts one functional group into another until the target is reached. This draws together the reactions of the organic families, because you must know which reagent and conditions transform one group into the next.

A synthesis pathway is a map of these conversions. For example, an alkene can be converted to an alcohol by hydration, the alcohol can be oxidised to a carboxylic acid, and the carboxylic acid can react with another alcohol to form an ester. Each arrow in such a map represents a specific reaction with particular reagents and conditions. Being able to read and construct these pathways is the core skill of this dot point.

Some key organic transformations to remember include addition of water to an alkene to give an alcohol, oxidation of a primary alcohol to a carboxylic acid (via an aldehyde), oxidation of a secondary alcohol to a ketone, dehydration of an alcohol to an alkene, and esterification of a carboxylic acid with an alcohol. Knowing the conditions, such as an acid catalyst for esterification or an oxidising agent for alcohol oxidation, is essential when proposing a route.

The efficiency of a synthesis is measured in several ways. Percentage yield compares the actual amount of product obtained with the theoretical maximum predicted by the balanced equation:

\text{percentage yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100

Yields are rarely 100 percent because of side reactions, incomplete reactions, and losses during purification. In a multistep synthesis the overall yield is the product of the yields of each step, so even moderate per-step yields multiply to a low overall figure. This is why short routes with high-yielding steps are preferred.

Atom economy measures how much of the starting material ends up in the desired product rather than in waste by-products. A reaction can have a high yield but poor atom economy if it produces large amounts of unwanted by-product. Addition reactions tend to have high atom economy because all atoms are incorporated, whereas substitution and elimination reactions produce by-products and so have lower atom economy.

Green chemistry provides principles for designing synthesis that is safer and more sustainable. These include preventing waste rather than treating it afterwards, maximising atom economy, using safer solvents and reagents, designing for energy efficiency, using renewable feedstocks where possible, and reducing the use and generation of hazardous substances. Evaluating a route against these principles is increasingly expected in exam answers.

In exams, present synthesis routes as clear step-by-step pathways with reagents and conditions for each step, show yield and atom economy calculations fully, and justify a chosen route using green chemistry where asked.

Exam-style practice questions

Practice questions written in the style of TASC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

TCE 20225 marksAmmonia is produced by the Haber-Bosch process:

3\text{H}_2(g) + \text{N}_2(g) \rightleftharpoons 2\text{NH}_3(g)

\Delta H = -92\ \text{kJ mol}^{-1}

. Explain the effect of increased temperature on (i) the rate of reaction and (ii) the yield of ammonia, and (iii) describe how industry reconciles these two effects.

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(i) Increasing the temperature gives molecules more kinetic energy, so collisions are more frequent and a greater fraction exceed the activation energy; the rate increases. (2 marks)

(ii) The forward reaction is exothermic, so by Le Chatelier's principle raising the temperature shifts the equilibrium towards the reactants (the endothermic direction), and the equilibrium yield of ammonia decreases. (2 marks)

(iii) The two effects oppose each other, so industry uses a compromise temperature of about $400\ ^\circ\text{C}$ to $450\ ^\circ\text{C}$ , high enough for an acceptable rate but not so high that yield collapses; an iron catalyst raises the rate without affecting yield, and high pressure favours the product side. (1 mark)

TCE 20213 marksMethanol is produced by

\text{CO}(g) + 2\text{H}_2(g) \rightleftharpoons \text{CH}_3\text{OH}(g)

\Delta H = -91\ \text{kJ mol}^{-1}

. By considering both yield and rate, explain why a compromise temperature must be used.

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A higher temperature increases the rate (more frequent, more energetic collisions), which is desirable for producing methanol quickly. (1 mark)

However, the forward reaction is exothermic, so a higher temperature shifts the equilibrium back towards the reactants (Le Chatelier), reducing the equilibrium yield of methanol. (1 mark)

These effects pull in opposite directions, so a compromise (moderate) temperature is chosen: high enough for an economically acceptable rate but low enough to keep the yield reasonably high. (1 mark)