white polo hoodie Green Chemistry
In this case the NaBr (reagent 2) is used in excess (.0129 mole) as compared to the 1 butanol (.0108 mole), which is the limiting reagent. Thus in Table 4 in the fourth column where the mass of the reagents that are actually used is calculated, it can be seen that even though the bromine (from the NaBr) is utilized in the desired product there is an excess of this element and therefore the excess must be added into the unutilized weight of reagents. A term we have coined called the Atom Economy now calculated. The percentage experimental atom economy is simply the expected mass of the reagents that are utilized in the desired product (which works out to be the same as the theoretical yield), divided by the actual total mass of all the reagents used in the experiment. In Table 4 we see in column 4, row 4, the utilized mass is 1.48 g and the total mass of the reagents is 4.13 g (row 4, column 2) thus the % experimental atom economy is (1.48/4.13 X 100) 36%. Although the same conclusion was obtained before a discussion of the concept of atom economy, it is helpful to table such as Table 4 so that one can see which reagent(s) are leading to the poor atom economy.
Rather than consider the percentage yield and (experimental) atom economy separately,
an even better indication of the efficiency of a reaction is to consider a combination of the two. We have coined the term Yield X Experimental Atom Economy (%PE illustrate this. This is calculated as follows:Notice, that in this calculation, the theoretical yield cancels leaving the ratio of the actual yield to the total mass of all the reactants X 100. If again one assumes that the actual yield is 1.20 g and this is divided by 4.13 g (total mass of all reactants from Table 4) then the % Yield X Experimental Atom Economy is only 29% (previously we arrived at this same conclusion in a much less formal way; see the first paragraph of this section). This means that only 29% of the total mass of all the reactants is actually isolated in the desired product while 71% is wasted! Measuring the efficiency of a reaction in this manner puts it into an entirely new perspective. While many chemists would consider a yield of 81% to be a very acceptable, not many would consider the isolation of only 29% of the mass of the reactant atoms in the desired product to be satisfactory.
Although consideration of both the atom economy and the % yield, gives a much better measure of the efficiency of a reaction and its environmental acceptability, other factors need to be considered. To prompt a discussion of the environmental acceptability of a reaction it is prudent to study some of the Twelve Principles of Green Chemistry (see below). Principle numbers 1 and 2 are directly addressed by atom economy and yield.