Illustrated Glossary of Organic Chemistry

Green chemistry: The branch of chemistry that seeks to reduce or eliminate the production of waste (especially hazardous waste) and/or use of hazardous chemicals in chemical processes. Green processes often include high atom economy. For example, a traditional organic synthesis route might require chlorinated solvents (generally considered hazardous to both people and the environment) and several steps (each of which generates some amount of waste). A greener version of this same process might avoid solvents altogether, and require just one step to complete.

In this synthesis of benzamide, benzoic acid is first reacted with thionyl chloride (SOCl₂) to produce benzoyl chloride (an acid chloride), using dichloromethane as the solvent. A byproduct of this step is sulfur dioxide (SO₂), a hazardous gas. In a second step the benzoyl chloride is reacted with an excess of ammonia (NH₃) to form benzamide. Excess ammonia is needed because some ammonia acts as a base (it produces ammonium chloride, a waste product), and does not become part of the final product. Compared to the benzamide synthesis show below, this route is not very green, and has poorer atom economy.

	+	NH3			+	H2O
Benzoic acid		Ammonia		Benzamide		Water Nonhazardous waste product

In this route to benzamide, benzoic acid is heated with one equivalent of ammonia to give the final product. The waste product is water, a substance generally considered nonharmful to people and to the environment. No solvent is needed. This process is greener than the route shown above because it involves less steps, avoids thionyl chloride (a hazardous chemical), and has better atom economy.