1. Percentage yield and atom economy

2. The percentage yield has been used as the deciding factor when working out if a particular reaction is economically viable. This is the equation you need to calculate percentage yield.

3. The conversion of a ketone to an alkene is a reaction commonly used in the manufacture of vitamins and pharmaceuticals.

4. For example, cyclohexanone forms a useful product called methylene cyclohexane.

5. The rather complex looking phosphine oxide made in the reaction is a waste product.

6. How much methylene cyclohexane could be produced from 1000 tonnes of cyclohexanone?

7. From the relative formula masses, you can see that 98 tonnes of cyclohexanone can produce 96 tonnes of methylene cyclohexane.

8. So 979.6 tonnes of methylene cyclohexane could be produced from 1000 tonnes of cyclohexanone.

9. What is the percentage yield if 842 tonnes are actually produced?

10. The percentage yield is 86%.

11. The atom economy of a reaction takes into account how efficiently the atoms in the reactants are used.

12. The relative formula mass of the useful product, methylene cyclohexane, is 96.

13. The total relative formula mass of the reactants is 374.

14. The atom economy of the reaction is just 26% – not very good.

15. The different types of reaction you meet at AS have different atom economies.

16. Rearrangement and addition reactions have atom economies of 100%.

17. Other types of reaction have lower atom economies.

18. You also study elimination reactions at A2. These usually have even lower atom economies.