Organic Chemistry in "Real Life": Rings!!

In this course we learn that the 6-membered ring is particularly stable among the cycloalkanes. Later, we will learn why the 6-membered aromatic (benzene) ring is so stable. 6-membered rings of both kinds are so stable that they occur very frequently in organic structures.

Any attempt to describe exactly how widespread 6-membered rings are in organic chemistry is completely hopeless, the task would be just too big. So, here are just a few representitive structures. Cyclohexane is not flat, the chair conformation is shown below. Molecules containing multiple cyclohexane rings make structures that chemists seem to have a fondness for. Adamantane used to be a favorite that has now been "abandoned" for the sexier buckball (soccerball type) molecules.



 how many rings does adamantane contain?


what does Dr. Gould think of much of the current reaseach work on Buckyballs?

Of course, elemental carbon exists in the form of 6-membered rings as graphite or diamond, which is just an adamantane molecule that has grown to the size of a single crystal! The only bonds in diamond are C-C single bonds. Remember, a C-C single bonds is worth around 90 kcal mol-1. Do you have more respect for 90 kcal mol-1 now?

>Multiple benzene rings fused together are called polyaromatic hydrocarbons (PAH's). You may have heard of PAH's in the context of polluting components of soots and incomplete combustion, and indeed many are known carcinogens, including benzene itself. This is unfortunate since I used to have to purify 5 liters of benzene each week by shaking it with conc. sulfuric acid in a seperatory funnel. Releasing the pressure in the seperatory funnel released a fine mist of conc. sulfuric and benzene vapor into the atmosphere that made tiny holes in all my clothes and when inhaled, probably accumulated in my liver!

6-Membered rings are an important part of the fundamantal structures of many many important biological molecules. Here are just a couple of examples.

How about this example?

And why is tetracycline so called?

Before organic chemists came up with synthetic dyes, indigo was once as important a cash crop in the US as cotton

OK, you get the idea. Let's end this with a look at the four bases that form the ATCG genetic code in DNA.

6-Membered rings are not only important in organic systems. Look at this wonderful STM image of an array of individual gallium atoms on a graphite surface.

It is interesting to see how an atomic 6-membered ring motif can repeat on the macro scale. An extreme example is found in Fingal's cave, on the tiny island of Staffa off the northwest coast of Scotland (Fingal's cave inspired Mendelssohn's Hebridean overture). You can only get there by boat (from personal experience it can be a very rough ride!) from the island of Iona (where Saint Patrick landed to bring christianity to the British Isles, and the burial place of Macbeth, am I getting off track here?). The cave contains massive volcanic basalt hexagonal columns. I don't know the composition of basalt, but clearly the hexagonal packing of the ions is reflected in the macro structure of these giant crystals, each a couple of feet in diameter and many meters tall.

6-Membered rings abound in nature, some obvious examples include this honeycomb and wasp nest, and these amazing pictures of snowflakes!

And no really chemistry, but hexagons are commonly used in tiling and mosaics, as in these Islamic designs (left) and this design found at Pompeii (right).

A hexagram is a 6-pointed figure composed of two equilateral triangles superimposed on each other. Can you see the Star of David hexagram in this Silver bowl from Damascus?