A great yield.
A terrible business?
The difference between yield, atom economy and cost per kilogram, with the ibuprofen comparison and a worked cost example.
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A high yield is not a cost model.
A route can convert most of its limiting reactant and still consume expensive inputs or create a difficult separation. Three different questions need three different measures:
- Yield
- How much desired product did you isolate compared with the theoretical amount from the limiting reactant?
- Atom economy
- What fraction of the mass of the stoichiometric reactants could end up in the desired product? It is a theoretical reaction metric, not a measure of conversion. Solvents and catalysts are not counted unless they are consumed stoichiometrically.
- Cost per kilogram
- What did the inputs and the rest of the process cost for each kilogram of saleable product?
Definitions: ACS, atom economy and green chemistry metrics.
Ibuprofen: fewer steps, more atoms used.
The EPA’s account of the BHC ibuprofen process compares six stoichiometric steps with three catalytic steps. Its reported atom-utilisation numbers make the difference concrete:
| Measure | Older route | BHC route |
|---|---|---|
| Steps | 6 | 3 |
| Atom utilisation | <40% | About 80% |
| Including recovered acetic acid | Not stated | Virtually 99% |
The near-99% figure includes a recovered byproduct. It is not a 99% isolated yield of ibuprofen. On a simplified 100 kg stoichiometric-reactant basis, 40% and 80% correspond to 40 kg and 80 kg allocated to desired product theoretically. They are not measured plant yields or complete material balances.
The EPA also reports a large reduction in aqueous salt waste. Fewer steps and better material use can improve a route, but this source does not provide a comparative dollars-per-kilogram cost model.
Source: US EPA, 1997 Greener Synthetic Pathways Award: BHC ibuprofen process.
The line a yield percentage cannot give you.
Raw-material cost / kg product
= Σ (kg of purchased input / kg product × price / kg input)
Here is an explicitly hypothetical one-input comparison. Assume each route needs 1 kg of its priced feed per theoretical kilogram of product, with no feed recovery. Different stoichiometry would change that basis.
| Quantity | Route A | Route B |
|---|---|---|
| Isolated yield | 95% | 80% |
| Feed price | $2,000 / kg | $20 / kg |
| Feed per kg product | 1 / 0.95 = 1.053 kg | 1 / 0.80 = 1.250 kg |
| Feed cost per kg product | $2,105.26 | $25.00 |
The higher-yield route loses this particular raw-material comparison. That does not mean high yield is bad. It means yield cannot answer a price question on its own.
For a real route, sum every purchased input and account consistently for recovery or recycle. Then add solvent losses, separation duties, utilities, waste treatment, labour, quality requirements and capital. Keep atom economy separate from process mass intensity, which includes the broader material inputs.
The chemistry is only part of the process.
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