Yield and Atom Economy

9 ATOM ECONOMY AND THE E FACTOR IN GREEN CHEMISTRY 14.9.1 Yield and Atom Economy 

Although atom economy is a useful concept, a more accurate measurement of the environmental acceptability of a chemical manufacturing process is the E factor, defined as follows  

Total mass of waste from process Total mass of product 

The E factor takes into account waste by-products, leftover reactants, solvent losses, spent catalysts and catalyst supports, and anything else that can be regarded as a waste. Its calculation depends on what is defined as waste. For example, water is a significant by-product of many chemical processes and is generally harmless, so its mass is usually omitted from the total mass of waste in the calculation. However, it may be included in those processes in which it is severely contaminated and difficult to reclaim in a form pure enough to use or discharge to a publicly owned wastewater treatment facility. A leftover reactant that can be easily reclaimed and recycled to the process is not included as waste, whereas a reactant that cannot be salvaged is counted in the waste. 

By-products generated from reaction making desired product 

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Because RME accounts for the mass of all reactants, that is, the actual stoichiometric quantities used, and therefore includes yield and atom economy, this combined metric is probably one of the most helpful metrics for chemists to focus attention on how far from green their current processes actually is. However, like many green chemistry metrics, it does take a little bit of thought to calculate in practice, as one has to work to strict definitions of what to include and what to exclude [46] ... 

Let s use the Wittig reaction (see Figure 1.2.2.1) to demonstrate how percent yield and atom economy can differ. 

Finding the eoneentration of an aeid solution Manufaeturing halogens and their eompounds Hydrochloric acid -an industrial success Concentration of solutions (titrations) Percentage of yield and atom economy (atom economy)... 

Nueleophilic substitution reaetion mechanism How do halogenoalkanes differ in reactivity Making of halogenalkane Treasures of the sea Halogenalkanes Percentage yield and atom economy (percentage yield)... 

As an example, let us consider the stoichiometric oxidation of diphenylmethanol to benzophenone, one of the most commonly used photosensitizers in photochemistry (Figure 1.3). We will evaluate this reaction using the measures of product yield, product selectivity, E-factor, and atom economy. In this reaction, three equivalents of diphenylmethanol react with two equivalents of chromium trioxide and three equivalents of sulfuric acid, giving three equivalents of benzophenone. First, let us see how the reaction measures with respect to product yield and selectivity. Assume that this is an ideal chemical reaction which goes to completion, so one obtains 100% yield of the product, benzophenone. If no other (organic) by-product is obtained, the product selectivity is also 100%. This is all well and good, and indeed for many years this has been the way that chemical processes were evaluated, both in academia and in the (fine-) chemical industry. 

Because reaction mass efficiency includes all the mass used for a given reaction (whether or not it includes or excludes water), and includes yield, stoichiometry, and atom economy, we believe that this metric is the most helpful metric for chemists to focus their attention on how far from green a given reaction or reaction scheme may be. 

Design reactions with higher yields and better atom economy. See Chemical Connection 1.2.2.1 Yield versus Atom Economy. ... 

A striking synergistic application of the TST in combination with RCM involves the catalytic enantiospecific desymmetriza-tion of meso silyl ether 34. Subjecting 34 to enantiopure Mo catalyst 35 generates siloxane 36 with near-perfect yield, enantiomeric excess, and atom economy (eq 10). ... 

Using benzene typical selectivities of around 65% are obtained commercially whilst for butene it is approximately 55%. If we multiply the theoretical atom economies by these figures we obtain practical atom economies of 28.7% for the benzene route and 35.6% for butene. This is a useful illustration of how the atom economy concept is a valuable additional tool in measuring overall reaction efficiency, and how good atom economy can compensate for poorer yields or selectivities. 

The atom economy for this process is 86.5% (100 X 116/134), which is reasonable. To calculate the E-factor and EMY further information is needed. From published literature (Vogel s Practical Organic Chemistry ), a standard procedure is to mix butanol (37 g) with glacial acetic acid (60 g), and a small amount of sulfuric acid catalyst (ignored in all calculations). Following completion of the reaction the mixture is added to water (250 g). The crude ester is washed further with water (100 g), then saturated sodium bicarbonate solution (25 g) and finally water (25 g). After drying over 5 g of anhydrous sodium sulfate the crude ester is distilled to give product (40 g) in a yield of 69%. 

Review a recent synthetic reaction you have carried out in the laboratory. Write a balanced equation for the reaction(s) and calculate the atom economy. From your experimental results calculate the Yield, E-factor and Effective Mass Yield (ignoring any water used). Identify ways in which this reaction could be made greener. 

Other than energy considerations, on which there is little comparative data, the most important green role for photochemistry is in improving atom economy. Although only a preliminary research result, an excellent example of this is the avoidance of the need for stoichiometric amounts of Lewis acid catalysts in the synthesis of some acylated aromatic compounds. Benzoquinone can be reacted with an aldehyde under a sunlamp to yield an acylhydroquinone in up to 88% yield. The alternative procedure would involve reaction of an acyl chloride with hydroquinone and a... 

Figure 4.6 (a) Graph showing full domains of atom economy, reaction yield and region where reaction mass efficiency exceeds a threshold value of a for a reaction whose minimum atom economy is zero, (b) Probability that such a reaction can achieve a threshold RME of at least a. i a = 0.61 8, p(a) = 0.08 (8%) as indicated by dotted lines. 

Figure 4.7 (a) Graph showing domains of atom economy, reaction yield and region where reaction... 

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