Less waste, safer chemicals

Every reaction turns starting materials into products. Green chemistry is the work of doing that with less waste, less energy and safer chemicals.

A factory pays for every atom it buys. Atoms that end up in the product are sold, and atoms that end up anywhere else become waste to be cleaned up and thrown away. In 1998 the chemists Paul Anastas and John Warner set out twelve principles of green chemistry. The first one is that the best waste to deal with is waste that is never made.

Atom economy

Atom economy measures how many of the starting atoms end up in the product you want. Work it out from the balanced equation: the mass of the wanted product, divided by the mass of everything that reacts, times 100.

An addition reaction puts everything together, so its atom economy is 100%. Ethene and water join into ethanol, the addition across a C=C you met earlier, and every atom of both ends up in the ethanol. Factories make ethanol this way at about 300 °C, with phosphoric acid as a catalyst.

Yeast makes ethanol too, by fermenting glucose: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂. Carbon dioxide is given off as a by-product, so only part of the glucose ends up as ethanol. Using masses from C = 12, H = 1 and O = 16, 180 g of glucose makes 92 g of ethanol and 88 g of carbon dioxide. The atom economy is 92 ÷ 180 × 100, about 51%.

Atom economy comes from the equation alone. The yield chemists measure when they make a molecule is a different measure: how much product a real batch gave, out of the most it could give. A reaction with a 100% yield still makes waste when its equation has a by-product, and a reaction with a 100% atom economy still makes waste when its yield is low.