When Carbon Bonds to Metals: Organometallic compounds
A typical organoarsenic compound
I love organometallic compounds. I find they are so exciting. There’s so much potential in learning about them. I would even argue a lot of research is being put into them to find easier ways to perform reactions.
But I’m getting ahead of myself. What is an Organometallic compound? Well, simply speaking, it’s when a metal bonds to at least one metal ion covalently. If your high school chemistry ever said carbon only bonds to other commonly covalent elements, well they’re wrong.
The most famous, and maybe the more boring ones are the Grignard Reagents. This is when a carbon compound is covalently bonded to magnesium. They are used in a lot of different processes, from moving an oxygen around, to polymerizing to making a resonance structure.
However, I’m going to talk about some weird chemicals involving arsenic, tin, and lead.
Firstly, lead. Most of you may own a car, and most of you probably put gas in that car and drive it around. Assuming you buy the gas here and not Algeria or Myanmar, then you’ve probably seen signs that say “unleaded gas” or “lead free.” This is because we used to put lead in gasoline.
The structure of Tetraethyl lead
There’s an interesting little story to why this was. In the early part of the 20th century, when vehicles were becoming more and more abundant, it was discovered that adding certain chemicals to gasoline could not only increase fuel efficiency, but also help in antiknocking and as an octane booster. In the 1920’s, It was discovered that Tetraethyl lead was great at all these.
A typical gas pump; modern day
Tetraethyl Lead is exactly that. It’s 4 ethyl groups covalently bonded to a lead atom. Also at this time we discovered that ethanol was a good additive as well, and worked just as well. However, the patent holders of tetraethyl lead ran a massive campaign to encourage to use of leaded gas, and it worked. If you know anything about lead, then you probably know why we stopped using it. Something about lead being in the air and destroying catalytic converters, or so the story goes.
The common structures of antimicrobial arsenic medications
Now, for arsenic. Arsenic is a fun element really. If it wasn’t so toxic and had such a nasty reputation, I feel a lot of interesting chemistry could be done with it. We used to use arsenic for a lot of different applications. Paint, dye, medicines, alloys. Of course we don’t do any of that anymore. What a lot of people may not know, is there’s this bizarre chemical that we used to use as an antibiotic some time ago.
Organoarsenic medications have an interesting back story, one fitting of my other class, The Darker Side of Science. When Europe, primarily Britain was colonizing Africa, there arose a problem of African diseases affecting livestock on a detrimental scale. This made colonization hard for European settlers, used to eating foods revolving around the use of cattle, sheep, and general european farm animals.
In 1905, two British scientists reported that an organoarsenic compound discovered a few years before was very effective at treating trypanosomiasis in livestock and humans. It was very difficult to store however, and unless an exact dose was given, the patient risked losing limbs or even life. After further development, the drug was put to the test in the German East Africa. The results weren’t promising, as 2% of all patients reported blindness, but it did help reduce the disease.
[Fun Fact: Colonial medicine was so crucial to European expansion, that in 1922, the German chemical company Bayer offered the British Government the formula to it’s new antimicrobial compound Bayer-205 in exchange for the return of colonies Germany lost after WWI]
Eventually, a new drug (with less side effects, including death) was developed using arsenic, called Salvarsan, or Compound 606. It was a drug that was not only effective at trypanosomiasis, but also effective at treating patients with syphilis. The drug was very volatile, and still had many side effects like nausea(you got to give them credit, they were giving people mercury salts before this) but was regardless effective. It remained in use until the 1940, when penicillin was developed.
Organoarsenic compounds are still of interest, because arsenic is a metal that bonds relatively easy to carbon. In some ways it behaves like it’s cousin nitrogen, forming bonds in 3,4, and 5 configurations. It was even thought that a bacteria using arsenic in it’s DNA was found, but that was disproven.
Finally, organomercury compounds. Mercury is one of those elements that has a good and bad side. I personally think besides it’s ability to bond with carbon, it’s chemistry is on the dull side. It’s organo compounds are used in applications where it’s toxicity is useful, like as a pesticide, fungicide, and preservative.
Merbromin structure
Most people over 30 probably remember using a product called Merbromin. It’s aa brominated aromatic structure covalently bonded to a mercury, used as a topical antiseptic. It’s falling out of favor in more civilized nations, but is still in use in a lot of developing parts of the world, where a cheap, easy to make antiseptic is an integral part of a budding health care system.
Merbromin anticeptic
Nitromersol, another aromatic mercuric compound, was used for vaginal contraception, diaper rash, and as a preservative for vaccines. Alkylated mercury compounds are used a fungicides and pesticides.
Structure of Nitromersol
You may have noticed an interesting trend in this lecture. The metals I picked to talk about all have a bad rap about them. This was intentional. I wanted to show that even in the stereotypical bad reaches of the periodic table, lie some interesting gems. A good chemist knows that stereotypes about elements are often skin deep. An element is toxic, how can we refine that property? How can we turn something that is bad all the time, and instead make it bad at one specific thing(like a bacteria, or pest)? An element bonds a specific way, how can that be useful?
