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

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:

1. 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.
2. Reaction conditions. Temperature, pressure, catalyst, solvent, time. Milder conditions mean cheaper, smaller, longer-lived equipment and lower energy cost.
3. 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.
4. 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.
5. Environmental and social impact. Toxic intermediates, greenhouse gas emissions, water use, worker safety, community exposure, end-of-life disposal.
6. 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.

Reagent availability

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:

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:

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.

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.

Worked example: aspirin

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

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.

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.

In one sentence

Designing a chemical synthesis means evaluating reagent cost and supply, reaction conditions, yield, purity, atom economy and by-products, and the environmental and economic context together, choosing the route whose overall package is best for the scale, the product, and the regulatory and social setting in which it will operate.