# 4.2.2: Atom Efficiency and the 2006 Presidential Green Chemistry Award

The Presidential Green Chemistry Challenge Awards [1] began in 1995 as an effort to recognize individuals and businesses for innovations in green chemistry.

The Presidential Green Chemistry Award Sculpture

IWC replica Réplica de reloj Typically five awards are given each year, one in each of five categories: Academic, Small Business, Greener Synthetic Pathways, Greener Reaction Conditions, and Designing Greener Chemicals. A committee of the American Chemical Society selects the awardees. Through 2006, a total of 57 technologies have been recognized for the award, and over 1000 nominations have been submitted.

Propylene glycol is a potential replacement for toxic ethylene glycol in antifreeze used in cars, solar heating systems, and deicing airplane wings. It is also used as a solvent in pharmaceuticals, or as a moisturizer in cosmetics, as well as the main ingredient in deodorant sticks, in fog machines, and a multitude of other uses.

It's an important enough industrial chemical that the 2006 Presidential Green Chemistry Award was given to Professor Galen J. Suppes of the University of Missouri/Columbia for an improved synthesis.

Propylene glycol is synthesized in the laboratory by oxidation of propylene (CH2CHCH3, see Figure above) with potassium permanganate:

$3 \ce{CH_2CHCH_3} + 2 \ce{KMnO_4} + 4 \ce{H_2O} \rightarrow 3 \ce{HO-CH_2-CHOH-CH_3} + 2 \ce{MnO_2} + 2 \ce{KOH} \label{1}$

This is considered a "good" synthesis, because it converts nearly all of the starting material to the desired product. But environmentally (and economically) it is extremely undesirable, for several reasons. It uses a non-renewable petrochemical as a reactant, uses an expensive oxidizing agent (KMnO4), produces a strong caustic (KOH) as a product and an undesirable byproduct (MnO2). It does have one advantage: It proceeds at low temperatures.

## Atom Economy

Notice that there seems to be an aweful lot of extraneous stuff in this synthesis. It requires 387 g of KMnO4, but that results only in the addition of two -OH groups to each propylene molecule (and they may come,in part, from H2O).

As a measure of this inefficiency, Barry trost developed the concept of atom economy, and for this work he received the Presidential Green Chemistry Challenge Award in 1998[2]. The percentage atom economy is defined as

% atom economy =

So we see that the mass of reactants that actually end up in products is 155g (3.68 mol) of propylene and 125.2 g of OH groups (2 x 3.68 mol, or 7.36 mol, with a molar mass of 17.01 g/mol OH), for a total of 280 g. The mass of all reactants is 155 g + 387 g + 88.5 g = 630 g, so the percent atom efficiency is

% atom economy =

We see that a lot of atoms are wasted in this synthesis. For comparison, let's look at several industrial processes that have been developed.