E-Factor and Atom Economy

Two of the most used indicators to assess green chemistry and evaluate the potential environmental acceptability of chemical processes are the following [31-34]  

It is a simple calculation based on the stoichiometry of the reaction, but does not account for solvents, reagents, reaction yield and reactant molar excess. Atom economy is one of the 12 principles of green chemistry [36]. The larger the number, the higher the percent of all reactants appearing in the product. 

The Efactor, introduced by Sheldon [31, 32], is defined as the mass ratio of waste to desired product  

Notably, the E-factor depends on the definition of waste, and may include process use only, or chemicals needed for scrubbing, for example. The E-factor can be split into different sub-categories (i) organic waste and (ii) aqueous waste. 

The E-factor takes the chemical yield into account and includes reagents, solvents losses, all process aids and, in principle, even fuel (although this is often difficult to quantify and it is usually not included). An exception is water as solvent, which is generally not included in the E factor. 

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Quantifying Environmental Impact Efficiency, E-factors, and Atom Economy... 

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. 

The E-factor and atom economy can be used to compare reaction alternatives, but the problem remains that there are different types of waste. Obviously, water is good waste, while, for example, heavy metal salts are bad waste. To solve this problem, Sheldon introduced the environmental quotient EQ (Sheldon, 1994), which takes the amount as well as the nature of the waste by multiplying the E-factor by an arbitrarily assigned hazard factor Q. Assigning absolute Q-values to waste streams is difficult, because cases differ according to location and type of waste. Nevertheless, the EQ gives a better measure of the environmental impact of a process than the E-factor or the atom economy alone. 

In Chapter 1 the concept of atom economy was discussed as a design tool. Similarly in Chapter 2 the term E-factor was introduced as a measure of the amount of by-products formed per unit weight of product. Unlike atom economy the E-factor is determined from an actual process or can be extrapolated from laboratory work. As a valuable extension to the E-factor concept Sheldon has proposed an Environmental Quotient which is the product of the E-factor and a by-product unfriendliness ... 

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. 

Iu search for efficieut aud greeuer processes over the past few years various heterogeneous catalysts such as titanium incorporated mesoporous molecular sieves [45,46], Schiff-base complexes supported on zeolite [47] and Zn(II)-Al(III) layered double hydroxide (LDH) [48], oxomolybdenum(VI) complexes supported on MCM-41 and MCM-48 [49], polyoxometallate supported materials [50], Co and Mn-AlPO s [51] etc. have been developed and studied for the catalytic epoxidatiou of a-pinene. Many of these processes suffer from drawbacks and limited applicability due to the poor conversion and also the selectivities. Sacrificial aldehydes are often used as an oxygen acceptor in such processes to achieve reasonable yield and selectivities but, this procedure leads to an increase in the E-factors and decrease in the atom economy [51]. 

Examine the list of the 12 principles of green chemistry shown at the beginning of this chapter. Which of these principles relate to the concepts of atom economy, the E-factor, and the environmental quotient Q ... 

A discussion of biocatalysis and green chemistry covers the principles of green chemistry. The concepts of sustainable development and green chemistry are introduced and defined. Green chemistry metrics, such as E factors and the atom economy, together with environmental impact and sustainability metrics, are discussed. The many attractive features of biocatalysis in the context of green chemistry are also discussed. 

There are some ways to evaluate environmental burden in chemical synthetic processes. The atom economy is the fraction of the atoms in the product to the fraction of all atoms in the reactants. The E factor is the ratio of the mass of all waste to the mass of product. For the following reaction concerning the production of propylene oxide, calculate the theoretical atom economy and E factor. 

The concept of atom economy, introduced by Barry Trost in 1991, is similar to that of the. E-factor [12]. Here one considers how many and which atoms of the reactants are incorporated into the products. With these two concepts, we can evaluate chemical reactions to get a quantitative result. 

Quantitative evaluation of chemical processes in terms of environmental impact and eco-friendliness has gradually become a topic of great interest since the original introduction of the atom economy (AE) by Trost [1], and the E-factor by Sheldon [2]. Since then, other indexes have been proposed for the green metrics of chemical processes, such as effective mass yield (EMY) [3], reaction mass efficiency (RME) [4] and mass intensity (MI) [5], along with unification efforts [6, 7] and comparisons among these indexes [8]. 

Early pioneers in green chemistry included Trost (who developed the atom economy principle) and Sheldon (who developed the E-Factor). These measures were introduced to encourage the use of more sustainable chemistry and to provide some benchmarking data to encourage scientists to aspire to more benign synthesis. 

The synthesis of adipic acid in the laboratory can be carried out by the oxidation of cyclohexene with potassium permanganate (Equation 4.6). The E-factor of this reaction is 2.61, which means that for 1kg of adipic acid 2.61kg waste (mainly Mn02 and KOH) is produced. The atom economy is 27.8%, indicating that only 27.8% of the atoms in the reactants will be incorporated into the product. 

A greener method has been developed using hydrogen peroxide as the oxidant, with catalytic amounts of sodium tungstate and a quaternary ammonium phase-transfer catalyst (Equation 4.7) [16]. Since the solvent and the by-product are water, the reaction is indeed much greener (E-factor = 0.49, atom economy 67%). 

However, the true greenness of this reaction remained far from being ideal, as the necessity to prepare initially the arylboronic acids (or their derivatives) as nucleophilic starting material and to recycle (or to eliminate) the associated waste thereafter violate several of the TPGC. Hence this not only contradicts the concept of atom economy [35], but also increased Sheldon s environmental impact factor E (E = kgwaste/kgproduct) [36]. As a consequence, this resulted in a decrease in the value of the reaction mass efficiency (RME) forthe Suzuki-Miyaura reaction. The value RME = 1 characterizes an absolutely green reaction, but all reactions with RME >0.618,... 

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