What is produced alongside the target?

Is the by-product valuable (then sell it), inert (then dump it), or hazardous (then treat it)? Atom economy is the percentage by mass of reactant atoms that end up in the product. 5.

What is generic statements about "high temperature is bad"?

Not always. Some reactions need it, and at industrial scale heat is often recovered through heat exchangers. Specifics matter.

§-Syllabus dot point

NSWChemistrySyllabus dot point

Inquiry Question 3: How, and why, are chemical reactions used to produce particular products?

Evaluate the factors that need to be considered when designing a chemical synthesis process, including availability of reagents, reaction conditions, yield and purity, industrial uses, and environmental, social and economic issues

A focused answer to the HSC Chemistry Module 8 dot point on chemical synthesis design. The factors a chemist must consider (reagent availability, reaction conditions, yield and purity, by-products, energy, environmental and economic issues), green chemistry principles, the case of aspirin synthesis as a worked example, and HSC past exam questions.

Generated by Claude Opus 4.811 min answerUpdated 2026-06-22

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

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What this dot point is asking
The answer
Examples in context
Try this

In plain English

Picture a chemist choosing how to cook a very large, very expensive meal for a stadium full of people, not just one plate for one guest. It is not enough for the recipe to taste good (that is just yield and purity). You also have to ask: can you actually buy the ingredients in bulk, reliably, without the price swinging wildly (reagent availability)? Does the recipe need a blowtorch and a pressure cooker, or will a normal oven do (reaction conditions)? What happens to the scraps and offcuts, do you compost them, sell them, or do they need a hazmat suit to dispose of (by-products and environmental impact)? And could you actually afford to run this kitchen every day at this scale (economic factors)? Designing an industrial chemical synthesis is exactly this kind of whole-kitchen decision: no single factor wins on its own, and the best route is the one whose overall package, cost, safety, conditions, yield and impact together, suits the scale and purpose of what you are making.

What this dot point is asking

NESA wants you to evaluate, not just list, the factors a chemist weighs when choosing how to make a target compound at industrial scale. The named factors are: availability of reagents, reaction conditions, yield and purity, by-products, industrial uses, and environmental, social and economic issues. A good evaluation compares at least two routes or makes explicit trade-offs.

The answer

The framework: six classes of factor

When choosing a synthesis, a chemist works through six considerations:

Reagent availability and cost. Are the starting materials abundant, cheap and consistent in quality? Petrochemical feedstocks are cheap but finite; biological feedstocks are renewable but variable.
Reaction conditions. Temperature, pressure, catalyst, solvent, time. Milder conditions mean cheaper, smaller, longer-lived equipment and lower energy cost.
Yield and purity. Yield is the percentage of theoretical product obtained. Purity is the fraction of that product that is the target compound. Pharmaceutical applications need very high purity; bulk industrial chemicals tolerate lower purity.
By-products and atom economy. What is produced alongside the target? Is the by-product valuable (then sell it), inert (then dump it), or hazardous (then treat it)? Atom economy is the percentage by mass of reactant atoms that end up in the product.
Environmental and social impact. Toxic intermediates, greenhouse gas emissions, water use, worker safety, community exposure, end-of-life disposal.
Economic factors. Capital cost of the plant, ongoing operating cost (energy, labour, maintenance), market price of the product, scale of demand.

A good evaluation links the factors to each other. A low yield with cheap reagents may beat a high yield with expensive reagents; a high yield with a toxic solvent may lose to a lower yield in water.

The two equations and the four green-chemistry principles

Yield: $\text{Yield} = \dfrac{\text{moles of product obtained}}{\text{moles of product theoretical}} \times 100\%$ (a practical recovery measure).
Atom economy: $\text{Atom economy} = \dfrac{\text{molar mass of desired product}}{\sum \text{molar masses of all products}} \times 100\%$ (a stoichiometric, in-principle measure, independent of yield).
Four HSC-relevant green chemistry principles (Anastas and Warner, 1998): atom economy; safer solvents and reagents; energy efficiency; renewable feedstocks and prevention of waste.
Purity benchmark: pharmaceutical-grade product typically needs 99.5%+ purity, confirmed by a sharp melting point and HPLC/TLC; bulk industrial chemicals tolerate far lower purity.

Reagent availability

Feedstock	Source	Comment
Ethene	Cracking of naphtha (petroleum)	Cheap but tied to oil prices
Methanol	$CO + 2H_2$ syngas	Made from natural gas
Glucose	Cane sugar, corn starch	Renewable but agricultural
Salt $NaCl$	Solar evaporation, mining	Effectively unlimited
Iron ore	Mining	Major mineral

For a multi-step synthesis, the supply of the rarest intermediate dominates. A natural product synthesis using a rare plant extract may be prohibitively expensive at scale.

Reaction conditions

Conditions set the scale of the engineering. The Haber process (450 degrees C, 200 atm) needs steel reactors with thick walls and energy-intensive compressors. Aspirin synthesis (70 degrees C, atmospheric pressure) runs in ordinary glass-lined reactors.

Catalysts reduce activation energy and let the reaction run at lower temperature. The biological route (fermentation of glucose to ethanol) runs at 30 degrees C; the petrochemical route (hydration of ethene with $H_3PO_4$ catalyst) runs at 300 degrees C and 70 atm. Both produce ethanol, but the energy and capital differences are enormous.

Yield and purity

Yield is the actual mass divided by the theoretical mass, expressed as a percentage:

\text{Yield} = \frac{\text{moles of product obtained}}{\text{moles of product theoretical}} \times 100\%

Industrial reactions rarely give 100% because of side reactions, incomplete conversion, and losses during work-up.

Purity is measured by melting point, chromatography (TLC, HPLC, GC), and spectroscopy (NMR, MS, IR). Pharmaceuticals need 99.5%+ purity, achieved by recrystallisation, distillation or column chromatography. Bulk plastics tolerate less.

Atom economy and by-products

Atom economy is the percentage of the total mass of reactants that ends up in the desired product:

\text{Atom economy} = \frac{\text{molar mass of desired product}}{\sum \text{molar masses of all products}} \times 100\%

An addition reaction (alkene plus $H_2$ ) has 100% atom economy. A substitution reaction (aspirin synthesis releasing ethanoic acid) has a finite atom economy. Atom economy and yield are different measures: a reaction can have 100% atom economy but 50% yield, or vice versa.

By-products can be classified:

Recoverable (ethanoic acid from aspirin synthesis is sold separately or recycled).
Treatable (acidic waste neutralised with lime).
Hazardous (chlorinated organic waste, heavy metal residues; needs incineration or specialist disposal).

Environmental and social factors

The twelve principles of green chemistry (Anastas and Warner, 1998) provide the framework. The four most relevant at HSC are:

Prevention of waste rather than treatment.
Atom economy to maximise mass into the product.
Use of less hazardous chemicals (water over benzene, biocatalysts over heavy metals).
Energy efficiency through milder conditions and catalysis.

Social factors include worker exposure, community air and water quality, transport risks, and end-of-life disposal of the product itself (e.g. polymer pollution).

Economic factors

Plants are designed for a specific scale. A penicillin plant making 100 tonnes per year of an active pharmaceutical ingredient looks very different from a polyethylene plant making 500,000 tonnes per year. Capital cost typically scales as the 0.6 power of capacity ("six-tenths rule"), and operating cost is dominated by feedstock for bulk chemicals or by labour and purification for pharmaceuticals.

Market dynamics also matter. A new drug under patent commands a price set by its therapeutic value, not by its production cost. A generic version after patent expiry is priced by competition. Synthesis design follows the economics.

A worked comparison: ethanol production

Ethanol can be made two ways. Both give the same product, but the trade-offs differ.

Factor	Hydration of ethene	Fermentation of glucose
Feedstock	Ethene (from oil)	Glucose (from cane, corn)
Renewable	No	Yes
Conditions	300 degrees C, 70 atm, $H_3PO_4$	30 degrees C, 1 atm, yeast
Yield	95% (single pass)	Saturates at 15% ethanol; yeast dies
Purity	High after distillation	Needs distillation, dilute feed
By-products	Polyethene, ethers (minor)	$CO_2$ , biomass
Energy intensity	High	Low
Capital cost	High	Low to moderate
Economic logic	At low oil price, beats fermentation per tonne	At policy support or high oil price, competitive

Either is preferred depending on local feedstock costs and government policy. Brazil produces nearly all its ethanol by fermentation of sugarcane (subsidised, plentiful cane); the United States by fermentation of corn (mandated by biofuel law); much of the world's industrial-solvent ethanol is still made by ethene hydration.

An owned illustrative energy profile shows why the fermentation route needs so much less energy input to reach product than the hydration route, even though both start from the same energy reference point:

The gap between the two Ea barriers is the whole reason fermentation runs at 30 degrees C in a simple vat while hydration needs a steel pressure reactor at 300 degrees C: the enzyme catalyst in fermentation lowers the activation energy so far that ordinary ambient conditions supply enough successful collisions, while the uncatalysed or acid-catalysed hydration pathway needs high temperature and pressure to reach the same rate.

Worked example: aspirin

Aspirin synthesis: equation, conditions and a yield calculation

The standard HSC synthesis: salicylic acid plus ethanoic anhydride, catalysed by a few drops of concentrated $H_2SO_4$ :

C_6H_4(OH)COOH + (CH_3CO)_2O \rightarrow C_6H_4(OCOCH_3)COOH + CH_3COOH

Conditions are mild (50 to 70 degrees C, 15 to 30 minutes). Crude yield is 60 to 80%. The product is recrystallised from hot water and tested for purity by melting point (135 degrees C) and by an iron(III) chloride test (pure aspirin gives no purple colour; residual salicylic acid does). The by-product ethanoic acid is recovered by distillation and recycled.

The alternative acetylating agent, ethanoyl chloride, gives a higher reaction rate but co-produces HCl gas, which is a worse environmental and safety problem than ethanoic acid. The anhydride route wins on green-chemistry grounds.

Worked calculation. A school laboratory reacts 2.00 g of salicylic acid ( $M = 138.12\ \text{g mol}^{-1}$ ) with excess ethanoic anhydride and recovers 1.65 g of pure aspirin ( $M = 180.16\ \text{g mol}^{-1}$ ). Find the percentage yield.

Step 1: moles of salicylic acid (limiting reagent).

n(\text{salicylic acid}) = \frac{2.00\ \text{g}}{138.12\ \text{g mol}^{-1}} = 0.014479\ \text{mol}

Step 2: theoretical moles and mass of aspirin (1:1 ratio from the equation).

n(\text{aspirin, theoretical}) = 0.014479\ \text{mol}

m(\text{aspirin, theoretical}) = 0.014479\ \text{mol} \times 180.16\ \text{g mol}^{-1} = 2.609\ \text{g}

Step 3: percentage yield.

\text{Yield} = \frac{1.65\ \text{g}}{2.609\ \text{g}} \times 100\% = 63.2\%

This sits inside the expected 60 to 80% crude yield range for this synthesis, confirming the calculation is realistic, and it is reported to 3 significant figures to match the 3 s.f. precision of the given masses.

Marker's note: full marks require the 1:1 mole ratio to be stated explicitly (not assumed silently), molar masses shown with units, and the final answer rounded to match the significant figures of the given data.

Common traps

Listing factors without evaluating them: "Yield is important. Conditions are important. Cost is important." This is not evaluation. Evaluate means weigh up: route A wins on X, route B wins on Y, and on balance the choice is...
Confusing yield with atom economy: Yield measures how much of the theoretical product you actually got. Atom economy measures how much of the input mass is built into the product even before yield is considered. Both can be improved independently.
Treating environmental issues as a footnote: A modern HSC answer is expected to integrate green chemistry, not bolt it on.
Forgetting the by-product: Every reaction has at least one. State what it is, where it goes, and whether it is a problem.
Generic statements about "high temperature is bad": Not always. Some reactions need it, and at industrial scale heat is often recovered through heat exchangers. Specifics matter.

Examples in context

Example 1. Choosing the sulfuric acid route at Port Kembla. Incitec Pivot's sulfuric acid plant near Port Kembla manufactures around 200 kt year $^{-1}$ via the contact process. Engineers evaluated alternatives: feedstock options (elemental sulfur shipped from Canadian refineries versus on-site pyrite roasting), catalysts (vanadium pentoxide versus platinum, which is more active but 1000 times more expensive), and operating temperature (a thermodynamic-kinetic compromise at 450 degrees C). The chosen design balances atom economy at 99 percent (almost no sulfur waste), catalyst cost, and a captive market in Australian fertiliser manufacture. The HSC framework of yield, purity, environmental cost and economic constraint is the exact framework Incitec used in commissioning the plant.

Example 2. Aspirin yield trade-offs in a NSW HSC depth study. Stage 6 students synthesise aspirin by reacting salicylic acid with ethanoic anhydride in the presence of phosphoric acid catalyst. Acetic anhydride is the preferred reagent over acetyl chloride (which would give HCl as toxic by-product) and gives 90 percent theoretical yield. Students recrystallise from ethanol-water to lift purity to above 99 percent, verified by melting point. The depth study explicitly asks them to evaluate why a pharmaceutical synthesis cannot tolerate the contamination tolerable in a fertiliser synthesis. NESA marking emphasises this trade-off between yield, purity, cost and end-use sensitivity.

Try this

Q1. List four factors that a chemist must evaluate when designing an industrial synthesis. [4 marks]

Cue. Reagent availability and cost; reaction conditions (temperature, pressure, catalyst); yield and purity; environmental and social impact.

Q2. A new synthesis offers 95 percent atom economy versus 60 percent for the existing route. Calculate the additional kilograms of useful product obtained from 100 kg of reagents and explain why atom economy matters for green chemistry. [3 marks]

Cue. New: 95 kg product; existing: 60 kg product; difference 35 kg; high atom economy reduces waste and feedstock cost.

Q3. Compare two routes to ethanol: fermentation of sugar versus hydration of ethene. (a) State one advantage of each. (b) Identify which is preferred for pharmaceutical-grade alcohol. (c) Explain the environmental trade-off between the two. [2+1+2 marks]

Cue. (a) Fermentation: renewable feedstock; hydration: high purity, high throughput. (b) Hydration (consistent purity profile, no fermentation residues). (c) Fermentation lower carbon footprint but uses arable land; hydration uses fossil feedstock but lower land use.

Exam-style practice questions

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

2022 HSC6 marksEvaluate the factors that must be considered in the industrial synthesis of aspirin from salicylic acid and ethanoic anhydride. Refer to availability of reagents, reaction conditions, yield and purity, environmental considerations, and economic factors.

Show worked answer →

A 6 mark answer needs the reaction, a claim on each named factor, and an overall judgement.

The reaction. Salicylic acid is acetylated by ethanoic anhydride with a few drops of concentrated $H_2SO_4$ catalyst:

C_6H_4(OH)COOH + (CH_3CO)_2O \rightarrow C_6H_4(OCOCH_3)COOH + CH_3COOH

Ethanoic acid is the by-product.

Reagent availability: Salicylic acid (from phenol via the Kolbe-Schmitt reaction with $CO_2$ ) and ethanoic anhydride (from acetic acid) are both produced at the megatonne scale and are inexpensive globally.
Reaction conditions: Mild: 50 to 70 degrees C, atmospheric pressure, acid catalyst, 15 to 30 minutes. Low energy input and cheap glass-lined reactors.
Yield and purity: Crude yield 60 to 80%. Recrystallisation from hot water removes unreacted salicylic acid (tested by $FeCl_3$ , which gives a purple complex with the phenol but not with aspirin). Purity is confirmed by melting point (135 degrees C) and HPLC for pharmaceutical grade.
Environmental factors: The by-product ethanoic acid is recovered by distillation and recycled. Using ethanoyl chloride instead would give a faster reaction but co-produce HCl gas, a worse environmental and safety problem; the anhydride route is the greener choice.
Economic factors: Salicylic acid is the costlier reagent per kilogram, so its conversion efficiency dominates the economics. Mild conditions mean cheap equipment and easy scaling in batch reactors.
Judgement: The salicylic acid plus ethanoic anhydride route is well suited to large-scale pharmaceutical production: cheap reagents, mild conditions, acceptable yield, recyclable by-product. Markers reward (1) the equation, (2) a claim on each factor, (3) an evaluative judgement, (4) a comparison or green-chemistry point.

2019 HSC4 marksDiscuss how green chemistry principles influence the choice of synthetic route for a target compound. Refer to specific principles in your answer.

Show worked answer →

Green chemistry, introduced by Anastas and Warner in 1998, sets out twelve principles for the design of chemical processes that minimise harm. The four most relevant to a HSC synthesis choice are:

Atom economy: The percentage of reactant mass that ends up in the desired product. High atom economy means little waste. An addition reaction across a double bond (100% atom economy) is greener than a substitution that releases a leaving group.
Use of safer solvents and reagents: Water and ethanol are greener than benzene or dichloromethane. Catalysts allow lower temperatures, smaller reactor volumes, and less waste.
Energy efficiency: Reactions running at or near room temperature consume far less energy and require less expensive equipment than high-temperature, high-pressure processes. The Haber process (450 degrees C, 200 atm) is famously energy-intensive and is a target of ongoing green-chemistry research.
Renewable feedstocks and prevention of waste: Bioethanol from fermentation uses a renewable feedstock (sugarcane); ethene from petroleum cracking does not. Designing the reaction to avoid waste at source is cheaper and cleaner than treating waste downstream.

A green synthesis trade-off: a less efficient (lower yield) green route may still be preferred to a higher-yield toxic route, because solvent recovery and waste disposal costs dominate the overall economics.

Markers reward (1) at least two named principles, (2) a specific example per principle, (3) the trade-off between yield and environmental cost.

Practice questions

Original practice questions graded from foundation to exam level, each with a full worked solution. Try them before revealing the solution.

foundation3 marksList three factors, other than yield and purity, that a chemist must evaluate when designing an industrial synthesis, and give a one-sentence reason for each.

Show worked solution →

A 3-mark identify needs a named factor AND a reason for each.

Reagent availability and cost: Cheap, consistently supplied feedstocks lower operating cost and reduce the risk of supply disruption.
Reaction conditions: Milder temperature and pressure mean cheaper, smaller and longer-lived equipment, and lower energy cost.
Environmental and social impact: Toxic by-products, emissions and worker exposure carry regulatory, health and reputational costs that can outweigh a small gain in yield.

Marking criteria: 1 mark per correctly named factor with a matching, specific reason (a factor with no reason, or a reason that is really "yield" or "purity" restated, earns no mark).

foundation3 marksExplain why atom economy and percentage yield are different measures, using aspirin synthesis as your example.

Show worked solution →

Atom economy is fixed by the reaction's stoichiometry alone: it is the percentage of the mass of all reactants that ends up in the desired product, calculated as if the reaction went to completion with no losses. For aspirin synthesis, ethanoic acid is unavoidably co-produced, so the atom economy is below 100% even in principle.

Percentage yield measures how much of that theoretically possible product a chemist actually recovers in the lab or plant, after side reactions, incomplete conversion and losses during recrystallisation and filtration.

The key distinction. A reaction can have high atom economy (little mass wasted in the equation itself) but low yield (poor practical recovery), or the reverse. Aspirin synthesis has a fixed, moderate atom economy set by the equation, while its yield (60 to 80% crude) depends on how carefully the reflux, cooling and recrystallisation steps are carried out.

Marking criteria: 1 mark for a correct definition of atom economy, 1 mark for a correct definition of yield, 1 mark for explicitly stating they are independent (one fixed by stoichiometry, one by practical recovery) with reference to aspirin.

core5 marksA pilot-scale reactor converts 250.0 g of salicylic acid (

M = 138.12\ \text{g mol}^{-1}

) with excess ethanoic anhydride to aspirin (

M = 180.16\ \text{g mol}^{-1}

). The plant recovers 245.0 g of crude aspirin. Calculate the percentage yield to 3 significant figures, and state one reason the actual yield is below 100%.

Show worked solution →

Step 1: write the equation (1:1 stoichiometry).

C_6H_4(OH)COOH + (CH_3CO)_2O \rightarrow C_6H_4(OCOCH_3)COOH + CH_3COOH

One mole of salicylic acid produces one mole of aspirin.

Step 2: moles of salicylic acid (limiting reagent, since anhydride is in excess).

n(\text{salicylic acid}) = \frac{m}{M} = \frac{250.0\ \text{g}}{138.12\ \text{g mol}^{-1}} = 1.8100\ \text{mol}

Step 3: theoretical moles and mass of aspirin (1:1 ratio).

n(\text{aspirin, theoretical}) = 1.8100\ \text{mol}

m(\text{aspirin, theoretical}) = n \times M = 1.8100\ \text{mol} \times 180.16\ \text{g mol}^{-1} = 326.09\ \text{g}

Step 4: percentage yield.

\text{Yield} = \frac{m(\text{actual})}{m(\text{theoretical})} \times 100\% = \frac{245.0\ \text{g}}{326.09\ \text{g}} \times 100\% = 75.14\%

Step 5: round to 3 significant figures.

\text{Yield} = 75.1\%

Reason for yield below 100%. Some product mass is lost during recrystallisation (aspirin remains dissolved in the cold filtrate and mother liquor) and during transfer between vessels; side reactions and incomplete conversion also reduce yield below the theoretical maximum.

Marking criteria: 1 mark for correct moles of salicylic acid, 1 mark for correctly identifying the 1:1 mole ratio, 1 mark for the theoretical mass of aspirin, 1 mark for the correct percentage yield to 3 significant figures with a percent sign, 1 mark for a valid, specific reason for yield loss (a vague "some was lost" without a mechanism earns no mark).

core5 marksThe graph below is an owned illustrative rate curve showing the concentration of ester product formed over time for two synthesis routes to the same ester, Route A (uncatalysed, 25 degrees C) and Route B (concentrated

H_2SO_4

catalyst, reflux). (a) Describe ONE difference in the curves' shape. (b) Explain the difference using collision theory. (c) State which route a chemist would choose for large-scale synthesis and justify your choice using the factors framework (reaction conditions and economics).

Show worked solution →

(a) Description: Route B rises far more steeply at first and reaches a higher plateau concentration in a much shorter time (equilibrium reached by around 20 minutes) than Route A, which rises slowly and has not yet plateaued by 60 minutes on the graph.
(b) Collision theory explanation: The concentrated $H_2SO_4$ catalyst in Route B provides an alternative reaction pathway with a lower activation energy. At a given temperature, a much larger fraction of collisions between acid and alcohol molecules then have enough energy to react successfully, so the initial rate is far higher and equilibrium (or completion) is reached sooner. Route A, with no catalyst, has a smaller fraction of sufficiently energetic collisions, so the same extent of reaction takes far longer to reach.
(c) Choice and justification: A chemist would choose Route B for large-scale synthesis. Reaction conditions: the catalysed route reaches a useful yield in a fraction of the time, allowing far higher reactor throughput (more batches per day from the same capital equipment). Economics: although the catalyst adds a marginal reagent cost, the dramatic reduction in reaction time lowers energy and labour cost per unit of product by far more, so Route B has the better overall economic and conditions profile despite Route A needing no catalyst at all.

Marking criteria: (a) 1 mark for a clearly stated shape difference (steepness or time-to-plateau). (b) 1 mark for citing activation energy/lower Ea, 1 mark for linking this to a greater fraction of successful collisions at a given temperature. (c) 1 mark for the correct choice (Route B), 1 mark for a justification that uses BOTH named factors (conditions and economics), not just "it's faster".

core6 marksTwo proposed industrial routes both make the target compound Z from a common feedstock. Route 1 uses a stoichiometric reagent, has 92% atom economy, 88% yield, and produces a hazardous chlorinated by-product requiring incineration. Route 2 uses a catalyst, has 99% atom economy, 70% yield, and produces only water as a by-product. Evaluate which route better satisfies green chemistry principles, referring to at least THREE named principles.

Show worked solution →

A 6-mark evaluate needs a claim on each of at least three named principles plus an overall judgement, not just a recitation of the numbers.

Atom economy: Route 2's 99% atom economy is far closer to ideal than Route 1's 92%: almost all reactant mass ends up in the product rather than in a by-product stream, satisfying the atom economy principle more strongly.
Use of safer chemicals and prevention of waste: Route 1's chlorinated by-product is hazardous and requires incineration, an energy-intensive, emissions-producing treatment step; Route 2's only by-product is water, which needs no special treatment. This strongly favours Route 2 on both the "less hazardous substances" and "prevention of waste" principles.
Catalysis: Route 2's use of a catalyst is itself a named green chemistry strategy: catalysts are reusable, often allow milder conditions, and reduce the stoichiometric reagent input compared with Route 1's stoichiometric approach.
The trade-off (yield): Route 1's yield (88%) is higher than Route 2's (70%), so a purely yield-focused comparison would favour Route 1. However, green chemistry evaluation weighs waste and hazard more heavily than yield alone, especially when the hazardous by-product carries ongoing disposal cost and environmental risk that a lower yield does not.
Judgement: On balance, Route 2 better satisfies green chemistry principles: its advantages in atom economy, by-product safety and catalysis outweigh its lower yield, provided the lower yield does not make the process commercially unviable at the required production scale.

Marking criteria: 1 mark per correctly named and applied principle (max 3), 1 mark for correctly identifying the yield trade-off as a competing consideration, 1 mark for comparing the two routes directly rather than describing them separately, 1 mark for a clear, reasoned final judgement.

exam8 marksAssess the claim that yield is the single most important factor in designing an industrial chemical synthesis, using the aspirin synthesis case (salicylic acid + ethanoic anhydride) as a worked example, and comparing it against the alternative ethanoyl chloride route.

Show worked solution →

This is an 8-mark ASSESS: markers reward a judgement backed by a worked comparison, not just a description of aspirin synthesis.

Band 6 PLAN.

Thesis: yield is an important but not the single most important factor; environmental, safety and economic factors can outweigh a higher yield, as shown by the aspirin case where the higher-yielding, faster ethanoyl chloride route is rejected industrially in favour of the lower-yielding, slower ethanoic anhydride route.
Set out both routes: (1) salicylic acid + ethanoic anhydride, $H_2SO_4$ catalyst, mild conditions (50 to 70 degrees C), crude yield 60 to 80%, by-product ethanoic acid (recoverable); (2) salicylic acid + ethanoyl chloride, faster reaction, typically higher local yield, by-product HCl gas (hazardous, corrosive, needs scrubbing).
Apply the six-factor framework directly: reagent availability (both routes' reagents are available at scale); reaction conditions (similar, both mild); yield and purity (ethanoyl chloride route can be faster/higher-yielding in isolation); by-products (decisive difference: recoverable acetic acid vs hazardous HCl gas); environmental/social (HCl gas poses worker exposure and corrosion risk, tipping the balance against the higher-yield route); economic (HCl scrubbing and containment equipment adds capital and operating cost that can exceed the value of the extra yield).
Judgement: industry chooses the anhydride route despite its lower/slower yield, which directly falsifies the claim that yield alone determines the best synthesis; the overall balance of factors, not any single one, drives real design decisions.
Address the counter-case briefly: for a bulk commodity chemical with a benign by-product profile, yield genuinely can dominate the choice between otherwise similar routes, so the claim has partial validity in narrower contexts.

Model paragraph (excerpt). The claim that yield is the single most important factor in synthesis design does not hold for aspirin manufacture. Although the ethanoyl chloride route can outperform the ethanoic anhydride route on reaction rate and local yield, it co-produces corrosive HCl gas, a hazard that raises worker-safety, containment and environmental-compliance costs well beyond what the extra product yield would recover. Industrial aspirin production therefore favours the anhydride route, evidence that reaction-condition mildness, by-product safety and economic containment cost can together outweigh a purely yield-based comparison. This does not make yield unimportant; it makes it one input among several that must be weighed jointly, exactly as the six-factor NESA framework requires.

Marker's note: top-band answers (1) name and apply at least four of the six factors, not just yield and one other, (2) use the aspirin-versus-ethanoyl-chloride contrast as concrete evidence rather than assertion, (3) acknowledge a counter-case where yield could dominate, and (4) close with an explicit judgement rather than a neutral summary. Answers that only describe the aspirin synthesis without a comparative judgement cap out in the mid-range bands.