A product can usually be made by more than one route

1. Different reactions can lead to the same product from different starting materials.
2. Choosing between them means comparing the routes on several measures at once.
3. The measures used are atom economy, percentage yield, rate, position of equilibrium and the use made of by-products.
4. No single measure decides the choice, because a route can score well on one and badly on another.
5. The cost of running the process is what all of these feed into.

Atom economy and percentage yield measure different things

1. The equation alone fixes atom economy, which says how much of the mass formed is the wanted product.
2. Measuring the product gives the percentage yield, which says how much of the possible product was recovered.
3. A route can have a high atom economy and a low percentage yield, or the reverse.
4. A low percentage yield can sometimes be improved by changing the conditions, while the atom economy cannot.
5. Both matter, because raw material is wasted either by becoming a by-product or by never being converted.

Rate and equilibrium position decide how fast and how far

1. A faster reaction makes more product per hour, so it needs smaller plant for the same output.
2. A very slow route may be rejected however good its atom economy.
3. A reversible reaction in a closed system settles at an equilibrium that limits how much product forms.
4. A position lying well to the right means a large proportion of the reactants is converted.
5. A route whose equilibrium lies to the left needs its product removing continually to be worth running.

Useful by-products change the economics

1. A by-product that can be sold offsets the cost of the raw materials.
2. A by-product that must be treated as waste adds a disposal cost instead.
3. A by-product that is fed back into the process reduces what has to be bought in.
4. Steam produced by an exothermic route can be used to heat another part of the plant.
5. A modest atom economy can still be acceptable when the other product has a market.

Weighing the factors against one another

1. A judgement begins by stating which route wins on each measure separately.
2. The measures are then weighed, and the weighting depends on what the product is worth.
3. Where the raw material is expensive, atom economy and yield carry the most weight.
4. Where demand is high and the material cheap, rate matters more than either.
5. A conclusion is only sound when the reason for the weighting is given alongside it.