What are Aromatic Compounds?

At the same dinner that I mentioned in my previous blog article on Einstein’s beliefs, I was quizzed on the nature of carcinogenic substances and how the body handles them.  I have found that most lay people are profoundly ignorant of basic chemical and biochemical concepts.  I was asked what are “aromatic” compounds and are they carcinogenic because they are volatile and because of their smell?

It is interesting that the first example of cancer discovered in the early 1800’s was that of chimney sweeps and especially their young helpers, who actually went into the chimneys to clean the soot and carried sacks of soot on their back.  They developed growths that were later diagnosed as cancers, that clearly resulted from contact with soot.  But, how and why?

Actually “aromatic” describes a whole class of organic compounds (i.e. compounds made up of carbon) that are based on benzene.  Starting in the early 19th century, carbon compounds were characterized in which carbon has four valencies, i.e. makes four bonds with other atoms, such as hydrogen.  Thus the simplest such compound (known as hydrocarbons) is methane CH4 (known as natural gas).  Derivatives of these are the well-known alcohols, such as methanol CH3-OH and ethanol C2H5-OH.  But, then the stable substance benzene was discovered to have the composition C6H6.  No one could then figure out what its structure might be.

The problem was solved by a German chemist named Kekule who related that in 1858 he fell asleep on the upper deck of a London bus, and dreamed of a snake swallowing its tail.  When he awoke he realized that benzene must have a cyclic structure, in fact a six-membered carbon ring with one hydrogen attached to each carbon.   It was subsequently discovered that there are many aromatic compounds with benzene rings fused to each other (called poly-aromatic hydrocarbons).  Soot is an excellent source of such compounds, and it was eventually shown that benzo-a-pyrene is the active ingredient in soot that is highly carcinogenic and mutagenic and causes cancer.  The next question is why?