Green Chemistry and Atom Economy: Why Percentage Yield Is Not Enough

Two different questions about efficiency

Percentage yield compares what you got with what the equation says you should have got. It measures how well the reaction was carried out. A reaction that goes to completion with no losses gives 100 per cent yield.

Atom economy compares the mass of the desired product with the total mass of everything you put in, assuming the reaction works perfectly. It measures how well the reaction was chosen. A reaction whose balanced equation produces by-products can never reach 100 per cent atom economy however carefully it is run.

A worked comparison

Consider two ways of making ethanol.

Route 1: hydration of ethene

Reactant masses: 28 + 18 = 46. Product mass: 46. Atom economy = 46/46 × 100 = 100 per cent. Every atom put in appears in the product, because this is an addition reaction with a single product.

Route 2: substitution from bromoethane

Reactant masses: 109 + 40 = 149. Desired product: 46. Atom economy = 46/149 × 100 = 30.9 per cent. Roughly 69 per cent of the mass becomes sodium bromide. Even at a perfect yield, two thirds of the input mass is waste.

This is why industry hydrates ethene rather than hydrolysing a halogenoalkane, and it is the standard calculation asked in this topic.

Reaction type sets the ceiling

The Wittig example is worth carrying into higher study. It is a superb reaction for controlling alkene position, and its atom economy is poor because a large phosphine oxide is thrown away every time. Green chemistry does not say never use it; it says count the cost honestly and look for an alternative when one exists.

The twelve principles, grouped so they are usable

Anastas and Warner set out twelve principles of green chemistry. Learning them as a list of twelve is unhelpful; they fall into four groups.

• Do not make the waste. Prevention is better than treatment, maximise atom economy, and use catalytic rather than stoichiometric reagents. A catalyst is recovered and reused; a stoichiometric reagent is consumed and discarded.
• Make what you make safer. Design less hazardous syntheses, design safer chemicals and safer solvents, and minimise the chance of accidents.
• Use less energy and renewable input. Run reactions at ambient temperature and pressure where possible, and prefer renewable feedstocks.
• Think about the end of life. Design products that degrade to harmless substances rather than persisting, avoid unnecessary protecting-group steps, and monitor reactions in real time to prevent by-product formation.

Real examples that appear in questions

• Ibuprofen. The original six-step synthesis had an atom economy of about 40 per cent. A later three-step catalytic route reached roughly 77 per cent, and higher still when the recovered acetic acid by-product is counted. Fewer steps, catalytic reagents, less waste.
• Hydrogen peroxide as an oxidant. Its only by-product is water, which makes it close to ideal where it can be used in place of a metal oxidant.
• Supercritical carbon dioxide as a solvent for dry cleaning and decaffeination, replacing chlorinated organic solvents. It is non-toxic, non-flammable and removed simply by releasing the pressure.
• Enzymes as catalysts work in water, at body temperature and ordinary pressure, and are highly selective, which removes the need for protecting groups.

How to answer a calculation question

1. Balance the equation first. An unbalanced equation gives the wrong reactant total and therefore the wrong answer, and this is the most common error.
2. Add the molar masses of every reactant, each multiplied by its coefficient. Do not leave out reagents such as sodium hydroxide simply because they look like helpers.
3. Take the molar mass of the desired product only. If two useful products form, the question will say which one to count.
4. Divide, multiply by 100, and comment. A sentence identifying the by-product responsible for the loss usually carries a mark of its own.

Frequently asked questions

Can a reaction have 100 per cent yield but poor atom economy?

Yes, and this is the central idea of the topic. Yield measures how completely the reaction you chose was carried out; atom economy measures how much of the input mass that reaction was ever capable of converting into product. A substitution reaction discards atoms by design regardless of how well it runs.

Which reaction types can reach 100 per cent atom economy?

Addition and rearrangement reactions, because every atom of the reactants appears in the single product. Substitution and elimination reactions always generate a by-product, so they are capped below 100 per cent.

Do I include the catalyst in the atom economy calculation?

No. A catalyst is not consumed and is recovered, so it does not appear in the balanced equation and is not counted. This is one of the reasons catalytic routes score better than stoichiometric ones.

What if a reaction produces two useful products?

Then the waste is smaller than the simple calculation suggests. Examination questions normally specify a single desired product, but in industry the atom economy is often quoted for the combination when both products are sold, as with the acetic acid recovered in the modern ibuprofen route.

Why is preventing waste ranked above treating it?

Treatment costs energy, reagents and money, and often produces waste of its own. Waste that is never formed needs no disposal at all, which is why prevention is the first of the twelve principles rather than a later one.