Say acetone, and what comes to a chemist’s mind? Washing laboratory glassware is a good bet. Actually, the prime use of acetone is to produce polymethyl methacrylate, better known as Plexiglas. Much of the five million tons of acetone produced annually in the world goes towards satisfying the hunger for Plexiglas. Skylights, light fixtures, automobile parts, furniture, pediatric incubators, hockey rinks, fish tanks, medical implants, household appliances, and of course, all those dividers we have seen during the COVID pandemic are all made of this material.
Plexiglas was introduced in 1935 by the Rohm and Haas Company. The actual process for making polymethyl methacrylate starts by reacting acetone with hydrogen cyanide to produce acetone cyanohydrin, which is then converted to methyl methacrylate, which in turn is then polymerized to yield polymethylmethacrylate. Plexiglas quickly proved strong enough to be used in the cockpits of military aircraft and was also used in the bomber noses of airplanes. Then came dentures, automobile tail lights, and a host of consumer items. Other companies were quick to get into the polymethylmethacrylate business. In 1937, DuPont introduced its own version under the name Lucite which eventually would even be incorporated into paints that drip less than other paints, dry quickly, and rarely blister.
Where did all the acetone needed to make the large amounts of plastic come from? The traditional method was based on the “destructive distillation of wood” which involves heating wood to a temperature of 450°–550°C in the absence of air. This yields a variety of gases such as carbon dioxide, carbon monoxide, and methane as well as a liquid composed of many compounds with acetic acid and methanol being dominant. The solid residue that remains is essentially charcoal. For acetone production the important compound was acetic acid which was readily converted to calcium acetate, which in turn could be heated to yield calcium carbonate and acetone.
The name acetone derives from the Latin root acetum for acetic acid by adding “one” meaning “daughter of” because it was obtained from the acid. Up to the First World War most of the calcium acetate required to make acetone was produced in Germany where the Black Forest provided ample supplies for the destructive distillation of wood. And in those days the main use of acetone was in the production of cordite, the propellant used in the standard British rifle cartridge. Cordite was made by extruding a mixture of nitrocellulose and nitroglycerin dissolved in acetone through small holes. Evaporation of the acetone resulted in a spaghetti-like cord, hence the name “cordite.”
The British shortage of acetone, a consequence of being cut off from German supplies, resulted in a crisis that was addressed by Chaim Weizmann, a professor at Manchester University who had been engaged in trying to find a way to make butyl alcohol for the production of synthetic rubber, also needed for the war effort. He discovered that the bacterium Clostridium acetobutylicum could be used to ferment sugars into a mixture of butyl alcohol and acetone. This discovery led to the building of large industrial plants that produced acetone through fermentation and allayed the shortage of cordite, contributing to the Allies victory over Germany. Weizmann was an ardent Zionist and became the first President of Israel when the country was established in 1948.
Today, fermentation has been supplanted by a method that involves the reaction of benzene with propylene, both derived from petroleum, to form a compound called cumene which is then treated with acid to yield acetone and phenol. It is interesting that both acetone and phenol are produced in this reaction because these two compounds are used to make bisphenol A, the controversial chemical that has been targeted by some activists for elimination because of its purported endocrine disrupting properties. Police have even had to break up some demonstrations, ironically protecting themselves with polycarbonate shields made from bisphenol A. Polycarbonate is also used in a myriad of other items ranging from electronics parts and DVDs to hockey helmets and the safety glasses we wear in labs. Talk of eliminating the production of bisphenol A is unscientific folly.
