Atom economy

Atom economy (atom efficiency/percentage) is the <a href="/facts/Conversion_(chemistry)/153nYK4B">conversion</a> efficiency of a chemical process in terms of all <a href="/facts/Atom/FoQ4kGN5">atoms</a> involved and the desired products produced. The simplest definition was introduced by <a href="/facts/Barry_Trost/m78NHIpU">Barry Trost</a> in 1991 and is equal to the ratio between the mass of desired product to the total mass of reactants, expressed as a percentage. The concept of atom economy (AE) and the idea of making it a primary criterion for improvement in chemistry, is a part of the <a href="/facts/Green_chemistry/7csVY28w">green chemistry</a> movement that was championed by <a href="/facts/Paul_Anastas/VtUUFbeb">Paul Anastas</a> from the early 1990s. Atom economy is an important concept of green chemistry philosophy, and one of the most widely used <a href="/facts/Green_chemistry_metrics/8cDWC7Vw">metrics for measuring the "greenness"</a> of a process or synthesis.
Good atom economy means most of the atoms of the reactants are incorporated in the desired products and only small amounts of unwanted byproducts are formed, reducing the economic and environmental impact of waste disposal.
Atom economy can be written as:

atom economy
        
        =
        
          
            molecular weight of desired product
            molecular weight of all reactants
          
        
        ×
        100
        %
      
    
    {\displaystyle {\text{atom economy}}={\frac {\text{molecular weight of desired product}}{\text{molecular weight of all reactants}}}\times 100\%}

For example, if we consider the reaction

A
        +
        B
        →
        C
        +
        D
        ,
      
    
    {\displaystyle A+B\rightarrow C+D,}

where C is the desired product, then

atom economy
        
        =
        
          
            
              
                Molecular weight 
              
              C
            
            
              
                Molecular weight of 
              
              A
              +
              B
            
          
        
        %
      
    
    {\displaystyle {\text{atom economy}}={\frac {{\text{Molecular weight }}C}{{\text{Molecular weight of }}A+B}}\%}

Optimal atom economy is 100%.
Atom economy is a different concern than <a href="/facts/Chemical_yield/fkBhd9dH">chemical yield</a>, because a high-yielding process can still result in substantial byproducts. Examples include the <a href="/facts/Cannizzaro_reaction/WjXQxh40">Cannizzaro reaction</a>, in which approximately 50% of the reactant aldehyde becomes the other oxidation state of the target; the <a href="/facts/Wittig_reaction/7Cjao2HI">Wittig</a> and <a href="/facts/Suzuki_reaction/aRISucCH">Suzuki reactions</a> which use high-mass reagents that ultimately become waste; and the <a href="/facts/Gabriel_synthesis/vYN14AFG">Gabriel synthesis</a>, which produces a <a href="/facts/Stoichiometry/I5p9cJzV">stoichiometric</a> quantity of <a href="/facts/Phthalic_acid/bh1keeDw">phthalic acid</a> salts.
If the desired product has an <a href="/facts/Enantiomer/wjPv5J3y">enantiomer</a> the reaction needs to be sufficiently <a href="/facts/Stereoselective/WlhPhqXH">stereoselective</a> even when atom economy is 100%. A <a href="/facts/Diels%25E2%2580%2593Alder_reaction/zc6dVpKl">Diels–Alder reaction</a> is an example of a potentially very atom efficient reaction that also can be chemo-, regio-, diastereo- and enantioselective. Catalytic hydrogenation comes the closest to being an ideal reaction that is extensively practiced both industrially and academically.
Atom economy can also be adjusted if a pendant group is recoverable, for example <a href="/facts/Evans_auxiliary/L194Bx0g">Evans auxiliary</a> groups. However, if this can be avoided it is more desirable, as recovery processes will never be 100%. Atom economy can be improved upon by careful selection of starting materials and a <a href="/facts/Catalyst/LPBS7TQC">catalyst</a> system.
Poor atom economy is common in <a href="/facts/Fine_chemicals/TuwYNbpp">fine chemicals</a> or <a href="/facts/Pharmaceuticals/h9mhwWP2">pharmaceuticals</a> synthesis, and especially in research, where the aim to readily and reliably produce a wide range of complex compounds leads to the use of versatile and dependable, but poorly atom-economical reactions. For example, synthesis of an alcohol is readily accomplished by reduction of an ester with <a href="/facts/Lithium_aluminium_hydride/QMMhdqEy">lithium aluminium hydride</a>, but the reaction necessarily produces a voluminous floc of aluminum salts, which have to be separated from the product alcohol and disposed of. The cost of such hazardous material disposal can be considerable. Catalytic hydrogenolysis of an ester is the analogous reaction with a high atom economy, but it requires catalyst optimization, is a much slower reaction and is not applicable universally.

Atom economy open-in-new

Atom economy