How can the efficiency of a chemical reaction be expressed?
Answers
In addition to percentage yield you can use atom economy.
It is very efficient to do an addition reaction where you combine 100% of your reactants together with (theoretically) no byproducts. Or where you combine reagents and kick out H2 as the only byproduct. Incidentally these reactions cover most of my chemical research to date.
It is not very efficient to say, use a very high molecular mass compound just to add a methyl group onto another compound.
Two methods of making the same thing can have different yields as well as different atom economies. You may consider a reaction with a 5% lower yield to be more efficient if the atom economy is much higher.
Atom economy is generally not important if you can easily convert the unwanted byproduct back into a reagent!
It is still expressed as a percentage. In this case you are adding up the molecular masses of your starting materials and comparing them to the molecular mass of the product that you want.