Carcinogenic Hydrocarbons

Once in the body, they are converted into arene oxides by cytochrome P???. An arene oxide is a compound in which one of the double bonds of the aromatic ring has been converted into an epoxide. The changing of the aromatic hydrocarbon into an epoxide forms a more water-soluble compound that can be eliminated from the body. Arene oxides can react in two ways. They can undergo attack by a nucleophile to form addition products or rearrange to form a phenol. Some aromatic hydrocarbons are carcinogens, meaning they can cause cancer.

Investigations have shown that hydrocarbons themselves are not carcinogenic, but the carcinogens are the oxidation products of the aromatic molecules. Nucleophiles react with epoxides to form addition products. 2’-Deoxyguanosine, a component of DNA, has a nucleophilic NH? group that is known to react with certain arene oxides. Once it attaches to an arene oxide, the 2’-deoxyguanosine can no longer fit into the DNA. This results in the genetic code not being properly transcribed, which can lead to mutations that cause cancer.

Arene oxides’ becoming carcinogenic depends on the rates of its two reaction pathways: rearrangement and reaction with a nucleophile. When an arene oxide rearranges, it forms phenols which are not carcinogenic. However, the formation of two addition products from nucleophilic attack by DNA can be carcinogenic. If the rate of arene oxide rearrangement is faster the nucleophilic attack by DNA, the arene oxide is harmless. If the rates are the other way around the arene oxide is likely to be carcinogenic.

The rate of arene oxide rearrangement depends on the stability of the carbocation formed in the rate determining step. So, an arene oxide’s cancer causing ability depends on the stability of the carbocation. If the carbocation is stable, the rate will be faster and therefore the arene oxide is less likely to be carcinogenic. If the carbocation is unstable, the rate will be relatively slow and the arene oxide will more likely be attacked by nucleophiles. This leads to the possibility of carcinogens.

After reading thoroughly through the Organic Chemistry 5th Edition book by Paula Bruice, I must agree with the investigations that have proven aromatic hydrocarbons to be non-carcinogenic. It is kind of interesting that arene oxides become carcinogenic depending on the reaction pathway it takes. I would like to know what other, if any, factors may contribute to carcinogens being formed once aromatic hydrocarbons enter the body