Yield

The Problem of Yield

A high yield is desirable, but is not the whole story. The reaction once used to manufacture phenol from benzene generates 1 mole of sodium sulphite (126g), for every mole of phenol (88g) produced. This may be acceptable if there is sufficient demand for the sodium sulphite, but if not, it presents a serious waste management problem, and adds significantly to costs.
It is much better to reduce the amount of waste generated in the first place, rather than treat it at a later date, and this is an important aim of green chemistry. A yield calculation does not indicate how efficiently the reactants have been used in generating the desired product, and so does not indicate how effectively waste is being reduced.

Chemists need to measure the efficiency of chemical reactions in order to compare alternative routes to products and their associated economic and environmental costs. Percentage yield has long been used for this purpose, as it compares the expected product quantity with the actual obtained. Click here for an example.

Atom Economy

The concept of Atom Economy was developed by Barry Trost of Stanford University (US), for which he received the Presidential Green Chemistry Challenge Award in 1998. It is a method of expressing how efficiently a particular reaction makes use of the reactant atoms.

equation: calculating atom economy

This approach does not take yield into account, and does not allow for the fact that many real-world processes use deliberate excess of reactants. It does, however, help in comparing different pathways to a desired product.

Examples

Benzene can be oxidised to make maleic anhydride, an important intermediate chemical.

chemical equation

The Atom Economy of this reaction is 43%, calculated using the relative formula masses. This means that 43% of the mass of the reactants ends up in the desired product.

equation: calculating an atom economy

Phenol has been produced by a variety of methods, and each alternative has shown an improved atom economy. See the phenol section for more details.

Ibuprofen became obtainable without prescription in the 1980s. At that stage the method used for production used six steps with an overall atom economy of just 40.1%. In the 1990s the Hoechst Celanese Corporation developed a new three-stage process with an atom economy of 77.4%, now a classic example of improving the route to a commercial product.

 


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