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February 25, 2012

Carbon is the child prodigy of the periodic family. Imagine Leonardo da Vinci, Marie Curie, Thomas Edison and Stephen Fry all rolled in to one person and you are starting to get a glimpse of the greatness. Approximately one third of the discipline of Chemistry is devoted to the compounds of carbon – ‘Organic Chemistry’ is loved and loathed by chemists and volume after volume can be found on the subject in any even vaguely scientifically inclined university library. To give a comprehensive overview of this extraordinary element is work for generations of  scientists and writers far better than me. Instead I am going to try and pick on some of the features that make carbon so interesting.

As an element carbon can be dull coal or glittering diamonds (there are stories of girls of the Moulin Rouge throwing diamonds in to the fire to watch them burn). Carbon can also take more exotic forms as in Bucky balls (nano-scale footballs of carbon), graphite (in your pencil lead) or graphene (a single sheet of graphite).

Carbon has been known about since prehistoric times but the most recent form, or allotrope, was discovered in 1985. Considering all these different forms are made of only one element the range of their properties is staggering. Crystal clear diamonds are the hardest known naturally occurring material, its name is Greek for unalterable or unbreakable, and pure diamonds are excellent insulators. Opaque graphite is soft and greasy and conducts electricity (if you have a potato clock at home you can swap one of the metal connectors for a sharpened pencil and it will work just as well. If you have a diamond ring you can try the same trick but the potato clock won’t work. People may also look at you strangely as you twist your jewellery in to a raw potato).

The compounds of carbon are even more numerous and diverse. From sugars to proteins to DNA there are more compounds of carbon than you can poke a stick at – even the stick will have a huge number of different carbon compounds within it. All of this comes from the fact that carbon has four electrons in its outer shell (in between the complete and perfect shells of helium and neon) and all capable of forming a pair with an electron from another atom. You can stick four different atoms to each carbon and then you can form long chains, circles, cages, sheets etc. etc.. The mind boggles at the versatility.

Perhaps the most curious result of this friendly and all inclusive attitude to bonding with other atoms is the ability of carbon to form ‘handed’ or chiral molecules. If you take a good look at your hands (perhaps in the privacy of your own home to avoid awkward moments at work or on the tube). You will notice that they are mirror images of each other and no amount of twisting and turning can make a left hand look like a right hand. The four major components of your hands (fingers, thumb, palm and back) all point in different directions and are like the four different atoms, or groups of atoms, bonded to a chiral carbon atom. So what I hear you bellow.

Chiral carbon would be an interesting aside in the chemistry of this element if it wasn’t for the fact that every living thing on this planet has a preference for one hand. The everyday examples of this are the 90% of humans who are predominantly right-handed. 90% of sea shells twist in a right-handed (dextral) orientation. Left-handed coiling shells (sinistral – the origin of the word sinister) are highly collectable. On the supra-molecular level 100% of all living organisms have one hand of DNA. Many molecules in your body are handed and rarely occur naturally with both ‘hands’ present.

With so many of the molecules in your body being ‘one-handed’ this can have implications when introducing new handed molecules in your food or as therapeutic drugs. To illustrate the problem find someone you know reasonably well and shake hands with them. Ignoring the social awkwardness of the situation as long as you shake right to right or left to left the hands should fit together comfortably. Shaking a left hand with a right hand is not so easy and the hands won’t fit together so well. The same can be true on the molecular level causing significant problems.

A classic example of a simple chiral compound we eat in food is carvone. The right handed molecule smells like caraway but the left smells of spearmint. Another deceptively simple chiral molecule is thalidomide – a drug given to hundreds of pregnant women in the late 1950s to treat morning sickness with tragic consequences. One hand of thalidomide is indeed effective against against morning sickness but the other hand causes severe birth defects. Both forms are chemically identical and no-one at the time considered they would have any significantly different biological function – hindsight is a wonderful thing. Unfortunately you cannot give just one form of thalidomide to a patient as it will convert to both forms inside the body. All chiral drugs are now rigorously tested in both forms before being released on to the market.

There is still hope for thalidomide though this will be of little comfort to those living with the consequences of its previous use in the 1950s. It has proved an effective treatment for leprosy and, with careful warnings for any women taking the drug not to start a family whilst receiving treatment, thalidomide can still do some good.

Next week is nitrogen – bland and innocuous or creepy identical twin?


Images by @SciCommStudios

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