Having looked at the conformational level of organic structure in detail, we're now going to transition to talking about the configurational level, which is the province of a field called stereochemistry. The way we're going to approach this is to first begin by looking at a single molecule property called chirality, or handedness, that tells us that, really, that stereochemistry is relevant to a molecule. Chiral molecules are stereochemically active, we might say.
Then we're going to transition into looking at relationships between molecules in a stereochemical sense and look at types of isomerism where connectivity is the same but configuration differs. And finally we're going to zoom in within a particular molecule containing two groups of the same type and ask about their spatial relationship. The essential question there is, are the groups in the same spatial environment or different spatial environments? And if the answer is different, how are they different?
Let's jump right into talking about chirality and stereocenters. One of the points that this slide makes is that chirality is a property of any object, not just a molecule. So we can think about everyday objects in thinking about chirality. But this shows you the basic idea of the molecular picture, that a right-handed and left-handed screw are fundamentally different in the same way a right-handed and a left-handed molecule are different.
So we're going to look at the criteria for a molecule to be handed essentially, how to determine whether a molecule is handed, and a structural unit that is often associated with chirality, the stereocenter, which is a very important structural unit to recognize for stereochemistry in general anyway. Before all that, I want to address the question, why is chirality important? One historical answer for this is that ignoring chirality has potentially deadly consequences.
In the late 1950s, the drug thalidomide was marketed to pregnant women as a cure for morning sickness. Thalidomide forms a mixture of the two molecules you see here when it's placed in the body. And these two molecules are isomers.
More specifically, we call them enantiomers. One of these enantiomers, the one you see on the left, is the bioactive form that cures morning sickness. The other, which is formed in the body even if we start with a pure sample of the compound on the left, causes birth defects.
Hopefully you can see the problem here. By ignoring the stereochemical behavior of this molecule inside the body, we've given a drug to pregnant women that cures one affliction but causes one far worse. The broader point that this suggests is that biochemistry is chiral chemistry.
By a wide margin, the vast majority of important biochemical molecules are chiral. And so to properly understand the behavior of these molecules, it's not sufficient just to know what's connected to what. To really appreciate how they behave.
how they react and their properties, we also have to understand something about the spatial orientations of groups and the spatial relationships between different molecules. This slide shows us examples of chiral biochemical molecules both at the small molecule level and at the polymeric or macromolecular level. Here is the sugar molecule glucose and here's glucose wound up as an amylose polymer. Both of these structures are chiral.
Here's the familiar double helix of DNA and if we look at an individual monomer within the DNA backbone, this one in particular is adenosine, we see chirality as well. And finally, here we have an amino acid, this is the amino acid tryptophan, and the amino acids are bound up into proteins which are very large structures that are asymmetric, that are chiral and have a handedness associated with them. Another reason we study stereochemistry is just that it's intrinsically fascinating. Stereochemistry is one of my favorite areas of organic chemistry as a whole. And one of the reasons why is that we can connect spatial properties of molecules to spatial properties of everyday objects and look at interactions between everyday objects to understand stereochemistry on a deep level.
For example, each and every one of us is carrying around a pair of enantiomers in our hands. So even if you don't know what enantiomers are yet, take my word for it that you're carrying around a pair of enantiomers and your hands have an enantiomeric relationship. The differences between enantiomers are subtle.
In many ways, enantiomers are just a pair of enantiomers. they act identically. For example, let's assume that you're perfectly ambidextrous. Whether you use a spoon with your left hand or your right hand is irrelevant.
This is because the spoon lacks a handedness. We'll see what this means a little bit later. And for that reason, it doesn't really matter whether you use a spoon with your left or right hand.
Both interactions feel the same, again assuming you're perfectly ambidextrous. The same is not true of an object that has a handedness. like this coffee mug.
Whether I hold this coffee mug with my right or left hand matters if my goal is to get liquid into my mouth. So the opening of the coffee cup is here, and if I hold it with my right hand and drink, it looks like this. But if I hold the mug with my left hand and drink, now all of a sudden I have to do a goofy motion like this.
There's something fundamentally different about interacting with this mug with my right and left hands, and that's because the mug has a handedness. We'll formalize this and look at this in the molecular level in the remainder of the videos in this series.