In the 1990s, an alternative process was developed by the BHC Company (now BASF Corporation) that requires only three steps and has an atom economy of ~80%, nearly twice that of the original process. First marketed in the early 1960s, ibuprofen was produced using a six-step synthesis that required 514 g of reactants to generate each mole (206 g) of ibuprofen, an atom economy of 40%. The synthesis of the common nonprescription pain medication, ibuprofen, nicely illustrates the success of a green chemistry approach ( Figure 4.15). The percent yield of a given chemical process, on the other hand, evaluates the efficiency of a process by comparing the yield of product actually obtained to the maximum yield predicted by stoichiometry.
Though the definition of atom economy at first glance appears very similar to that for percent yield, be aware that this property represents a difference in the theoretical efficiencies of different chemical processes. For the example in the previous paragraph, complete reaction of the hydrogen would yieldĪtom economy = mass of product mass of reactants × 100 % atom economy = mass of product mass of reactants × 100 % The reactant yielding the lesser amount of product is the limiting reactant. Each reactant amount is used to separately calculate the amount of product that would be formed per the reaction’s stoichiometry.
Reaction of all the provided chlorine (2 mol) will consume 2 mol of the 3 mol of hydrogen provided, leaving 1 mol of hydrogen unreacted.Īn alternative approach to identifying the limiting reactant involves comparing the amount of product expected for the complete reaction of each reactant. Hydrogen, therefore, is present in excess, and chlorine is the limiting reactant. This represents a 3:2 (or 1.5:1) ratio of hydrogen to chlorine present for reaction, which is greater than the stoichiometric ratio of 1:1. For example, imagine combining 3 moles of H 2 and 2 moles of Cl 2. Identifying the limiting and excess reactants for a given situation requires computing the molar amounts of each reactant provided and comparing them to the stoichiometric amounts represented in the balanced chemical equation. This substance is the limiting reactant, and the other substance is the excess reactant. If these reactants are provided in any other amounts, one of the reactants will nearly always be entirely consumed, thus limiting the amount of product that may be generated. The balanced equation shows the hydrogen and chlorine react in a 1:1 stoichiometric ratio. H 2 ( g ) + Cl 2 ( g ) ⟶ 2HCl ( g ) H 2 ( g ) + Cl 2 ( g ) ⟶ 2HCl ( g )
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