Chemistry · Ch 16 — Green Chemistry and Nanochemistry
Atom Economy
Atom Economy
Atom economy measures how much of the mass of the starting materials ends up in the useful, desired product at the end of a chemical process, rather than being lost to unwanted byproducts. A reaction with good atom economy incorporates most of the atoms of its reactants into the desired product, generates only small amounts of unwanted byproduct, and therefore creates fewer problems of waste disposal downstream. The percentage atom economy of a reaction is calculated as: % atom economy = (formula weight of the desired product / sum of the formula weights of all the reactants used in the reaction) x 100. The chapter works through this with the conversion of butan-1-ol to 1-bromobutane: CH3CH2CH2CH2OH + NaBr + H2SO4 -> CH3CH2CH2CH2Br + NaHSO4 + H2O. Only the mass of the atoms that actually end up in the desired product, 1-bromobutane (4C + 9H + 1Br = 137 u), is counted in the numerator, while the denominator sums the formula weights of every reactant used (4C + 12H + 5O + 1Br + 1Na + 1S = 275 u), givi …
Worked out. A fully worked numeric example applying the atom-economy formula to the conversion CH3CH2CH2CH2OH + NaBr + H2SO4 -> CH3CH2CH2CH2Br + NaHSO4 + H2O. The desired product, 1-bromobutane, contributes 4 carbon, 9 hydrogen and 1 bromine atom for a formula weight of 137 u; every reactant used together contributes 4 carbon, 12 hydrogen, 5 oxygen, 1 bromine, 1 sodium and 1 sulfur atom for a combined formula weight of 275 u. Dividing 137 u by 275 u and multiplying by 100 gives an atom economy of 49.81%, meaning just under half of the total mass fed into the reaction ends up in the desired 1-bromobutane product, with the remainder going …