Sheldon’s E-factor

It is a truism that the simplest concepts are often the most effective, and this can be said of Sheldon s E factor [19, 20] which was developed by Sheldon in order to highlight the amount of waste generated to produce 1kg of chemical product across different branches of the chemical industry. The E factor is defined as the mass ratio of waste to product. E factor is now widely quoted across many different chemical industries in many different fora, as Sheldon provided a simple benchmark guide for different sectors of the chemical industry and this has been widely pubhshed and presented in multiple venues. For examples of processes described in this book where E factors have been calculated see Sechons 5.4 and 8.4. 

In this chapter, we will summarize and critically discuss the different regeneration approaches outlined in Table 8.1. As a structuring principle, we have chosen Sheldon s E-factor [6], starting from the (seemingly) most environmentally benign (since waste-free) to the most waste-intensive (yet most widespread) methods. Finally, a glimpse at NAD(P)H-independent regeneration methods is taken. 

This can be demonstrated by various criteria and proved very convincingly by means of Sheldon s environmental factor E [7], which is far more suitable and constructive than Trost s atom effiency [8]. Sheldon defined the E factor as the amoimt of waste produced per kilogram of product, and specified the E factor for every segment of industry (Table 1). 

In 2002, Eissen and Metzger proposed EATOS (an environmental assessment tool for organic syntheses), an environmental performance metrics for daily use in synthetic chemistry [11]. This tool allows rapid quantitative assessment of both the E-factor and the potential environmental impact (PEI, Sheldon s Q) of a process. They also provided a PC software application to perform this calculation, which is available from them [12]. With this tool, Sheldon s Q can be quantitatively assessed for both the feedstock and the output (product and wastes) of a multistep synthesis. The assessment is made on the basis of the available substance s ecotoxicological and human toxicological data. 

A comparative assessment between two alternative routes to Elliot s alcohol, an industrial intermediate in the preparation of pyrethroids of the resmethrin family, is presented in this chapter. This assessment is made with the aid of the EATOS tool and takes into consideration both the masses (E-factor) as well as environmental impact of the substances employed and released by the processes (Sheldon s Q). 

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,... 

The low E factor (< 0.04) indicates that the utilization of material resources is improved more than tenfold according to Sheldon s assessment [19] production of the bulk chemical w-butyraldehyde is classified alongside the highly efficient mineral-oil refining processes. 

Moreover, examination of the Sheldon E-factors convincingly demonstrates the practicability of this concept as compared with Trosf s [ 30] enigmatic atom economy which is concerned with esoteric examples of little relevance. Some people believe that the newer version of the atom economy , justifiably emphasizing homogeneous catalysis with exotic examples of the myriad of substances that are required to serve the needs of society ( ) [30b], are better founded. 

Strictly speaking, this observation is included in Sheldon s wider statement, according to which the E factor is refined and becomes the environmental quotient EQ, depending on the nature of the waste. Since such quotients are debatable and will vary from one company to another and even from one production to another [7], they will not be discussed here. The crucial point is that, on the same basis (taking into account all the byproducts, including those used in ligand manufacture. 

Cohen Hubal, E.A., L.S. Sheldon, J.M. Burke, T.R. McCurdy, M.R. Berry, M.L. Rigas, V.G. Zartarian and N.C.G. Freeman (2000). Children s Exposure Assessment A Review of Factors Influencing Children s Exposure, and the Data Available to Characterize and Assess that Exposure, Environ. Health Perspect., 108, 475-486. 

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