Isomerism: some different arrangements of atoms within a molecule

Remember the athlete who lost his bronze medal? Keep reading.
Carbon is one of the most versatile elements. It forms millions of different molecules. Most of these occur naturally in plants and animals. The carbon bonds point at specific angles. This leads to some very interesting molecular variations which are essential to us as humans. Consider a hypothetical carbon molecule that has our different groups attached to a central carbon atom. We will call them A, B, D and E all joined to carbon. Remember that carbon covalent bonds are directed at 109.5 to each other. This molecule can be arranged in two ways.
These can best be seen if the molecule is drawn in its actual three-dimensional shape.
You will then see that, although all the groups are the same, the two forms cannot be superimposed on top of each other; they are mirror images of each other, just as your hands are mirror images of each other. You can put them together, palm to palm, but you cannot put them one on top of another so that they match. We say the two forms are asymmetric. Note that there must be four different groups attached to the carbon atom. If any two are the same, then there is only one structure and the forms can be superimposed; there is thus no asymmetric carbon atom.
It might be worth trying this idea out with, let’s say, a tangerine as the carbon, four cocktail sticks as the bonds, and the different groups, A, B, D and E represented by, say, a cherry, a chunk of cheese, a small pickled onion and a piece of pineapple.
Now just make sure that the ‘bonds’ (the cocktail sticks) are as evenly arranged around the tangerine as possible. It is like the carbon at the centre of a triangular prism with its bonds pointing to the corners. If it is not exactly right, don’t worry, even an approximate value will demonstrate the principle. Now do the same with another tangerine and another four cocktail sticks.
Sit the two tangerines facing each other on a coffee table (having first cleared off all the magazines and mugs with a deft swipe of the forearm), so that their front two ‘feet’ are exactly opposite each other. Now stick a cherry on the top cocktail stick of each structure. Stick a chunk of cheese on the ‘tail’ of each. Stick a small pickled onion on each of the ‘legs’ closest to you. Stick a chunk of pineapple on each of the ‘legs’ furthest

away from you. Get a hand-mirror and place it between the two models, until you are convinced that each is a ‘mirror image’ of the other. The two forms cannot be put one on top of the other so that all the pieces match up. Try it and see!

Some ‘molecules’ can be arranged in this way. They are isomers of each other.
‘Isomers’ mean that the molecules have the same overall formula and contain the same atoms, but in a different arrangement. There are a number of different types of isomerism. The mirror images (Figure 2.16) are examples of a type called ‘stereo isomerism’ because they are different in their arrangement in three-dimensional space. These molecules are further called ‘optical isomers’ because it was found that the two isomers twist a beam of polarized light  in opposite directions (polarized light is when the light beams are only in one plane and not from all directions). Molecules which have this property have the same ‘chemical formula’, but when a beam of polarized light is passed through a container of each of the two different isomers, one will twist the light beam clockwise (we call this positive or dextro rotatory, D) and the other isomer will twist it anti-clockwise (or negative or laevo rotatory, L). The D and L are often included in the names of such compounds.

All the molecules of sugars, amino acids, proteins will have optical isomers, because there are many asymmetric carbon atoms in their structures that have four different groups joined to a central carbon atom, but the body is very selective of the type of isomer it can use: D-glucose is sweet but its L-isomer is not. The body cells like D-glucose and uses them for cell building and energy sources, but they hate L-glucose.
Consider another actual molecule where this property occurs, i.e. lactic acid, CH3 C*H(OH) COOH. The asterisked carbon atom has four different groups arranged around it (CH3, H, OH and COOH) and so this molecule must be ‘optically active’ and have two different isomers. We say the starred carbon atom is ‘asymmetric’, i.e. not symmetrical. You can draw out the two forms. It is impossible to say which is the D or L form by simply looking at them; they must be tested for their ‘twisting properties’ with polarized light beams.