You'd have seen few twins as unlike each other as the boys in the Suite Life of Zack and Cody. In chemistry too, there are twins, like the chemicals that give oranges and lemons their flavours. They're called mirror compounds.
What are mirror compounds?
Mirror compounds are just like Zack and Cody. Cody is more studious and less fashionable, and Zack is just the opposite. In any pair of mirror compounds, the chemical behaviour of one will be quite different from the other. Let's see some examples.
Limonene is a compound present in citrus fruits. One form is S-limonene, which is found in lemons and gives the lemony smell. The mirror version is called R-limonene, and is found in oranges and gives an orange-y smell! Another compound is carvone. R-carvone gives the spearmint flavour in chewing gum, while S-carvone gives a caraway (siya-jeera) flavour (chemistry twins are always called R and S, or D and L).
Why are they called that?
Mirror compounds are all organic compounds, that is, they are made of carbon and other elements. Now a carbon atom can attach to four different atoms, often to other carbon atoms. This helps them form a chain, like this:
(H = hydrogen, C = carbon, X, Y & Z = any other atoms)
Now exchange the Y and Z, and you get another compound, that looks like the mirror image of the first:
Copy one out and hold it to a mirror. You'll see the other one!
Mirror compounds were discovered by Louis Pasteur. He was watching crystals of tartaric acid under a microscope, when he saw that they were of two distinct shapes. Using a pair of tiny pincers, he separated them into two groups. You can try this yourself, just ask your teacher to help make tartaric (or citric) acid crystals!
So is that important?
Limonene and carvone were mild examples. Sometimes, the differences are more sinister. Thalidomide is a drug that was used to treat morning sickness in pregnant women. However, only S-thalidomide is good. Its twin, R-thalidomide is evil - for it causes terrible deformities in children. In many other drugs, only the R- or S- twin is of any use, the other is often useless.
Aspartame is a sweetener used in place of sugar by people who have diabetes. It has 200 times the sweetness of sugar. However, only L-aspartame is sweet. Its twin D-aspartame is really, really bitter. Hence people who make aspartame have to make sure that they include only L-aspartame in the final product, and filter out all the D-aspartame.
Amino acids are very important for us, as they are the building blocks of proteins. However, for most of them only the L twin has any use, the D twin is not used. When it comes to sugars like glucose and fructose, only the D-twin is useful. Our bodies have a way of creating amino acids and sugars such that only the useful twins are ever made!
How do we know L from R?
It is a curious method. You dissolve the purified powder in water or alcohol and put it in a special glass tube called a polarimeter. You pass light from one end of the tube. At the other end is an electronic detector. You keep rotating the tube till the detector reading is maximum. If you had to rotate to the right, the compound is the R- or D- twin. If you rotated left, it is the S- or L- twin.
If you have a polarimeter in your school lab, ask your teacher to show you how. You can use citric acid or tartaric acid, which are commonly available preservatives.
P.S. R stands for Rectus and S for Sinister. D is for dextrorotatory and L is for laevorotatory. These are Greek and Latin names, and they simply mean right or left.