Thalidomide?

Question

Thalidomide has two distinct forms. One form (1) acts as sedative and sleep aid whereas the other (2) causes terrible birth defects. Strangely, both have the same chemical formula, C13H10N2O4. The differences between the two are entirely due to the different arrangement of the atoms in space.

Why two molecules with the same chemical formula may still have different connectivity and geometries?

Answer 1

The structures are mirror images. Although the formulae are the same one is "left" handed and the other "right" handed. The handedness can affect how chemicals interact with biological receptors such as on cell surfaces. One version may "click in" to a particular receptor while the other may not.

Answer 2

Thalidomide, like a lot of molecules, is chiral. This means that it has two forms that, while identical in chemical make-up, are different in their 3-demensional structure. The best way to think of "chirality" is to think about your right and left hands. Both look the same at first glance, right? Except they're not quite the same. They're mirror images of each other. You can't superimpose one of them on the other. Try it - try to put one hand over top of the other. It doesn't quite work, because they're not the same. Mirror images, but not identical.
Okay, so let's try to explain this in terms of chemistry, and thalidomide in particular. A good start would be to look up thalidomide on Wikipedia.org so you can see what its structure is. Notice that the picture shows a squiggly line attached to the letter N. This is the nature of the problem you're asking about. The squiggly line means that the molecule is not flat, like your computer screen, but 3-dimensional. That squiggly part either sticks out of the computer screen at you, or away from you.
Think of the picture of thalidomide like your hand. If you had two versions of it: one sticking out, and the other sticking in, they'd look the same, but would be a little bit different. You couldn't superimpose them on each other.
Thalidomide can exist in these two different forms, and this is why it can look the same, but have two entirely different sets of properties. While it may not seem like a big deal, in nature, this difference is very big. Just as you cannot shake somebody's right hand with your left hand, nature cannot "shake" certain molecules with others if they don't fit. Just like hand-shakes.
Good luck!

Answer 3

Thalidomide is a racemic molecule. It has one chiral center and therefore consists of two enantiomers (nonsuperimposable mirror images). The two enantiomers readily interconvert. One isomer is useful for morning sickness the other is teratogenic (causes birth defects). It two isomers are identical in structure and molecular formula but they differ only in the arrangement of the atoms around the chiral center. Hope this helps. Good luck.