• Chemical Equation. At the equation level, we simply have one or more balanced chemical equations. We have identified the reactants, the desired product, and any undesired coproducts that will accompany that desired product. The principal green measure at this level is atom economy.

• Chemical Reaction. At the reaction level, we identify the amounts of reactants we plan to use in the reaction. Before we carry out the reaction, we have access to three additional measures: theoretical yield, relative excess, experimental atom economy. After the reaction is over, we add three more: actual yield, theoretical yield, and reaction mass efficiency.

• Chemical Process. The ultimate goal of the Green Chemistry Assistant is to get us thinking at the overall process level. At this point, the actual chemical equations are not as important as the actual mass amounts of all materials used in the overall process, including catalysts, solvents, and all the other reagents necessary to isolate a final product. The measures we highlight in this regard are process mass efficiency, which takes into account all these masses, and the environmental impact E-factor, which takes into account the possibility that we are able to recycle at least some of our overall process mass.

The discussion below defines these terms more precisely. The figures here represent the distribution of mass specifically at the end of the process. The idea is that ultimately these Green Chemistry measures focus on output -- what is being recovered and what is being lost -- from a chemical process. As you progress through your own analysis, click on the process analysis icon to see how your process stacks up.

Where can you make changes to make your process greener?

Atom economy is a calculation based simply on the overall balanced chemical equation. It is simply the mass of desired product divided by the total mass of products. Or, since the mass of products equals the mass of reactants in a balanced chemical equation, atom economy is the mass of desired product divided by the total mass of reactants.

(Atom Economy) = D/(D+C)

The actual process is carried out on a scale different from the molar scale represented by the balanced chemical equation. But (at least theoretically) the ratios are the same. Atom economy can be calculated much the same way, this time using the theoretical yield and the expected mass of undesired products.

(Atom Economy) = D/(D+C) = T/(T+U)

A preliminary Green Process Analysis identifies the quantities of reactants involved in the actual process, as planned. If there is a limiting reactant, we can now calculate the relative excess and the experimental atom economy.

• The relative excess represents the extent to which reactant mass is being wasted because it is present in a greater amount than is actually necessary. It is the ratio of excess reactant mass to the total product mass. A relative excess of 100% means we plan to have as much reactant left over in the end as used to form products.

(Relative Excess) = E/(T+U)

• The experimental atom economy represents the atom economy taking into account the fact that some of the atoms of reactant will not be utilized in any products -- desired or otherwise -- because they are in excess. The experimental atom economy is less than or equal to the atom economy, because the excess mass (E) is always greater than or equal to zero.

(Experimental Atom Economy) = T/(E+T+U)

(Experimental Atom Economy) = (Atom Economy)/[(Relative Excess) + 1]

So when there is no excess, we have that the experimental atom economy equals the atom economy, and when we have a 100% excess of reactants, the experimental atom economy is just half of the (theoretical) atom economy.

At the reaction level, where we are considering a simple process consisting of a single chemical reaction or process step, two measures include percent yield and Reaction Mass Efficiency.

• Percent yield is the standard means of accounting for product loss. It is simply the ratio of actual yield to theoretical yield.

  Percent Yield = (A/T)*100%

• Reaction Mass Efficiency (RME) measures the efficiency with which reactant mass ends up in the desired product, usually expressed as a percentage. It does not (as originally defined) include catalysts, solvents, or other reagents not specifically in the balanced chemical equations for the process. Shown here is the original definition of RME as discussed by Constable, Curzons, and Cunningham (Green Chemistry, 2002, 4, 521-527). Over the years, however, other interpretations of this measure have been introduced into the literature.

  RME = [(actual yield)/(mass of all reactants)]*100%

RME = A/(E+T+U)*100% = (Percent Yield)*(Experimental Atom Economy)

In the end, there are going to be losses that go beyond just the product losses considered in the individual reaction steps. These losses might include byproducts (mass lost to side reactions), product lost in handling and purification, loss of catalysts and other reagents, and loss of solvents. A major goal of Green Chemistry is to think at the process level and to maximize the recovery of useful materials and to minimize these losses. At the process level, where several steps may be involved, we (currently) focus on two particular measures for describing "greenness."

• Process Mass Efficiency (PME) (we propose) measures the efficiency with which all process inputs end up in the desired product. PME can be thought of as a "global RME measure."

  PME = (actual yield)/(total mass) = A/M

• E-factor, representing environmental impact, takes into account that some of the excess reactants, catalyst, solvent, and other reagents might be recycled. E-factor measures the amount of waste generated by a chemical process relative to the amount of product obtained.

  E-factor = (total loss)/(actual yield) = L/A

The Green Chemistry Assistant is under active development. Currently it can do all of the preliminary calculations for reasonably complicated processes (involving up to six process steps). One final tab is planned -- Actual Amounts Recovered -- which will complete the picture and allow more than just a preliminary set of calculations. We hope to have this stage completed early in the fall of 2005. Your feedback is appreciated! Bob Hanson