In the not so distant past of organic chemistry, it was commonly accepted practice to use whatever reagents were necessary for a particular transformation – never mind how toxic or how environmentally polluting the reagents might be. Chemists weren’t particularly concerned about the hazards of the reagents they used. This wasn’t so much a willful disregard of things as it was a lack of knowledge. Adding tetraethyllead to gasoline seemed like a fantastic idea; it took years for public awareness of lead poisoning to curb the use of the additive. Freon seemed like a great invention – it saved thousands of lives by introducing refrigerated storage to vaccines and foodstuffs. We didn’t understand until years later that Freon is one of the worst greenhouse gases and that it was also setting up a catalytic cascade in our upper atmosphere, destroying ozone.
One of the most important synthetic reactions introduced in textbooks of organic chemistry is the addition of alcohols to alkenes. Alkenes are compounds which contain an extra set of electrons in the form of a carbon-carbon double bond. Alcohols contain oxygen atoms, which are electron rich. Normally, a bond is formed when a atom that is rich in electron density “attacks” an atom which is deficient in electron density. It’s the old “opposites attract” idea. An excess of electrons gives one atom a partial negative charge, which attacks the atom with a deficiency of electrons (partial positive charge). In the case of alkenes and alcohols, both atoms are electron rich and so the reaction between the two doesn’t take place to any appreciable extent unless a catalyst is used.
Catalysts are neither reactants nor products, but rather they’re additional molecules present in the flask that are not consumed during the reaction. Their presence helps the reaction proceed faster. An analogy you can draw is washing a load of laundry. It is possible to wash everything by hand in a bathtub, but things will go faster if you use a washing machine. The machine isn’t consumed by this action, and the end product (clean clothes) is the same. Catalysts fill a similar function. In the case of this alkene-alcohol reaction, early organic chemists determined that a toxic mercury chloride salt would allow this reaction to proceed at a much faster rate. Unfortunately, mercury is an environmental pollutant and mercury poisoning is a very real danger for chemists handling the material. This means that the mercury-catalyzed reaction is a great teaching tool as it introduces some theoretical concepts for students to learn, but in our current legal and regulatory environment it’s not a very important industrial reaction. No chemical plant is going to use tons of mercury chloride catalyst for a reaction when they know they’re going to have to pay for its disposal. It’s simply not economically feasible.
That’s why I was excited to read an article in the journal Chemical Communications that outlined other, non-polluting, catalysts for this reaction. A group of chemists from Germany tried a wide variety of metal salts and they discovered that gold chloride could catalyze the reaction with high efficiency. A trace amount of the gold salt is added to the reaction flask and the entire setup is heated to 120 degrees Celsius for several hours. The product (an ether) is obtained in high yield. Unfortunately, early attempts with gold chloride indicated that the catalytic activity was dying out after just a few hours. The German scientists discovered that the gold chloride was being transformed into gold metal particles during the course of the reaction. Gold chloride catalyzes the reaction, but gold metal does not. To help keep the gold in its proper form, the researchers modified their procedure and added a small amount of copper chloride along with the gold salt. The copper chloride acts as an oxidizing agent, which means that the gold ions never get transformed into gold metal and they stay in the form necessary to catalyze the reaction.
Organic chemistry hasn’t always gotten things right the first time around. However, anyone who works in the field is aware of the pressing need to keep things “green”, and to avoid producing hazardous waste. Even chemists who simply don’t care about the environment are kept in line by the economic penalties associated with “dirty” chemical processes. I’m confident that as the years go on, the continual process of reaction refinement will gradually phase out some of the largest offenders amongst our known reactions.
The source of this article can be found at: http://www.rsc.org/Publishing/Journals/CC/article.asp?doi=b706961h