Atom economy green metric

Green Metrics Atom Economy is Not the Whole Story... 

Yield and other mass-related metrics such as atom economy, reaction mass efficiency and mass intensity have been examined by Constable et al with regard to their significance concerning greenness and costs. The importance of using a (product) concentration term, which can be mass intensity or mass index, is additionally emphasized by Laird et al This is in compliance with Winterton, who in his twelve more green chemistry principles demands the establishment of full mass balances. 

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] ... 

At least one of the 12 principles—number 2, design safe products—is a must for pharmaceutical synthesis. Indeed, pharmaceutical products are regulated and tested for safety. Principle number 7, maximize atom economy, refers to a metric against which the greenness of a process may be evaluated. Green metrics are covered in the next section. 

Barry Trost of Stanford University published the first formal green metric in 1991 in Science. Trost s idea, atom economy, measures the percentage ratio of the total mass of the products to starting materials (Equation 13.2).26... 

Shortly after Trost introduced the idea of atom economy, Roger Sheldon of Delft University of Technology in the Netherlands reported another green metric called E-factor.30 31 E-Factor is the ratio of the mass of the total waste from a process to the mass of the product generated in that same process (Equation 13.5). 

Solvents are not included in the calculation. Like effective mass yield, carbon efficiency strives to make a value judgment on the relative importance of various wastes. Elimination of water from a molecule would not count against the carbon efficiency of a reaction, but it would give a decreased atom economy. By not including all wastes, carbon efficiency is not as strict as most green metrics. Of course, all metrics have their shortcomings and carry their own assumptions. 

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]. 

These figures clearly show how a greenness evaluation of a process based only on "visual inspection of synthetic schemes can be misleading a very large part of the waste comes from isolation steps, which do not appear in synthetic schemes. Moreover, some green metrics, such as atom economy do not consider at all this portion of waste. 

Extensive statistical analysis of the mass intensity, yield, atom economy, and stoichiometry show that these data do not correlate with each other in any meaningful way. Because these metrics appear to be of discretely different types, following one metric in isolation from others may not drive the best behavior for greening reactions. This is illustrated in Table 2.2, which contains an example of three different chemistries of similar mass intensity that have generally different and conflicting data trends for the other metrics. 

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. 

Atom economy is a simple metric for the initial comparison between different reactions that is receiving considerable attention. In an ideal scenario, all the atoms in the starting materials are incorporated into the product with anything else needed in only catalytic amounts, and from this viewpoint, addition reactions are ideal green reactions. Many types of addition reactions are known, catalytic hydrogenation and hydroformylation are examples that are... 

There are certain metrics used in Green Chemistry. One is associated with principle two Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product. That is the metric of atom economy [14]. Atom economy is the mass of the product divided by the sum of the mass of all the reactants. Ideally this number is one and the closer to one the better. Atom economy does not take into account solvent or other reagents such as bases. Another metric is the Environmental factor or E-factor [15], The E-factor is the mass of the total waste divided by the mass of the product. Ideally the E-factor should be zero and the smaller the number, the better. However, some fine chemical or pharmaceutical syntheses have E-factors of about one hundred. Using micelles, the Lipshutz group reduced the E-factor of a Heck reaction from 137 to 7.5 and then even lower with recycle [16], Consider the following experimental procedure [17],... 

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