Since you still don't get how tetrahedral stereogenic centers work, I made a video to help explain it. If that doesn't work, I don't know what to tell you.
Thursday, March 25, 2010
Sunday, March 7, 2010
Introduction to Determining Chirality
Stereoisomerism can be a lot trickier to identify than constitutional isomerism. With this in mind, and to some extent because I am too busy to make really good posts right now but also want to keep this project moving, there will be a series of short posts on the subject, starting with this one.
In order for any of this to make sense, you need to understand what it means for a particular atom to be a tetrahedral stereogenic center. Don't panic. Just peruse the previous entry and make sure you grasp the concept I am describing. The pictures are probably best for this, but what we're basically dealing with are atoms attached four different groups. This is because when an atom (usually carbon) is attached to four different groups, it is not superimposable on its mirror image. And, if it helps any, this concept can be extended to macroscopic things in our everyday lives. The textbook contrasts gloves and socks. In a pair of socks, the two individuals are identical (usually). But in a pair of gloves, the right glove and the left glove are not interchangeable.
Chiral molecules are like gloves (or shoes, for that matter). Even though the properties of the isomers are virtually identical, they are, in principle different from each other and these differences can manifest in ways that are relevant to us. An obvious demonstration of this is in pharmaceuticals, where often only one of the isomers has the desired effect, but the drug is sold and administered as a mixture of both versions. I should do a post on the thalidomide incident. Not right now, though. But maybe later.
Anyway, this isomerism can show up in other types of situations and hopefully I'll soon get to some of them, but for now, we shall focus on chirality that arises from tetrahedral stereogenic centers. Here are some points to keep in mind about these types of chiral molecules.
1. A molecule for which the mirror image is superimposable is achiral. A molecule for which the mirror image is not superimposable is chiral.
2. A carbon that is bonded to four groups, none of which are identical to each other, is a stereogenic center (aka chiral center). This does not necessarily mean that the molecule itself is chiral as we shall see.
3. A molecule that contains exactly one stereogenic center is chiral. Always. No exceptions.
4. A molecule that contains more than one stereogenic center might be chiral, but it might not. More on this later.
In order for any of this to make sense, you need to understand what it means for a particular atom to be a tetrahedral stereogenic center. Don't panic. Just peruse the previous entry and make sure you grasp the concept I am describing. The pictures are probably best for this, but what we're basically dealing with are atoms attached four different groups. This is because when an atom (usually carbon) is attached to four different groups, it is not superimposable on its mirror image. And, if it helps any, this concept can be extended to macroscopic things in our everyday lives. The textbook contrasts gloves and socks. In a pair of socks, the two individuals are identical (usually). But in a pair of gloves, the right glove and the left glove are not interchangeable.
Chiral molecules are like gloves (or shoes, for that matter). Even though the properties of the isomers are virtually identical, they are, in principle different from each other and these differences can manifest in ways that are relevant to us. An obvious demonstration of this is in pharmaceuticals, where often only one of the isomers has the desired effect, but the drug is sold and administered as a mixture of both versions. I should do a post on the thalidomide incident. Not right now, though. But maybe later.
Anyway, this isomerism can show up in other types of situations and hopefully I'll soon get to some of them, but for now, we shall focus on chirality that arises from tetrahedral stereogenic centers. Here are some points to keep in mind about these types of chiral molecules.
1. A molecule for which the mirror image is superimposable is achiral. A molecule for which the mirror image is not superimposable is chiral.
2. A carbon that is bonded to four groups, none of which are identical to each other, is a stereogenic center (aka chiral center). This does not necessarily mean that the molecule itself is chiral as we shall see.
3. A molecule that contains exactly one stereogenic center is chiral. Always. No exceptions.
4. A molecule that contains more than one stereogenic center might be chiral, but it might not. More on this later.
Monday, March 1, 2010
Stereogenic Centers
The fifth chapter in this textbook is all about stereochemistry. I considered skipping it, but decided against it. However, for now I am skipping a lot of the fourth chapter. It's not that I'm tired of alkanes, it's just that the remaining sections dealt with conformations and I'd rather get back to that stuff later.
While constitutional isomerism is interesting, most of the time we'll be discussing constitutional isomers in terms that they are completely different compounds. It's just important to keep in mind that they are made up of the same atoms in the same proportions. In case you've forgotten, the thing that makes compounds constitutional isomers is that the atoms are connected to each other in different ways for each molecule.
Constitutional isomers often have dramatically different chemical properties. Their physical properties differ too. They might have different functional groups. In contrast, stereoisomers don't exhibit such bold differences. Two compounds that are stereoisomers of each other not only have the same atoms, but the atoms are connected in the same way. Their properties are almost identical. But the spatial positions of the atoms are different.
There are multiple ways for molecules to exhibit stereoisomerism. But we won't go over all of them at once. Instead, we'll take this slowly. It seems only natural to start with the case of stereogenic centers, specifically tetrahedral chiral centers, but don't worry about those terms right now. The important thing to grasp is the concept.
I am a big fan of the written word. I strive to be as good as I can at communicating concepts verbally. However, this concept is just so much easier to convey using a picture. So here you go.
I made this in ChemSketch and it's supposed to be CHBrClF (a carbon attached to a hydrogen, a bromine, a chlorine, and a fluorine). It doesn't really matter what the things attached to the central carbon are, though, so long as they are all different things. They could be other atoms or even organic groups like methyl, ethyl, and so on. A carbon (or another atom) attached to four groups, none of them identical, is a stereogenic center. It is chiral because it is non-superimposable on its mirror image. Here's the mirror image.
I had to mess around with the program a bit to get this to work, but other than that, does it look exactly the same as the previous molecule? Yes? Look again. With the white ball (representing hydrogen, but whatever) on top, we can look down and starting from brown and going clockwise, we will necessarily have a different order for each of these. It's unavoidable. They're almost the same, but they're different in this one respect. A classic example is the difference between a right hand and a left hand. But for this type of stereoisomerism, all that we need is one central atom with four different groups attached to it. Usually, the central atom is carbon and one or more of the groups are part of an organic molecule (rather than just the single atoms used in my example).
While constitutional isomerism is interesting, most of the time we'll be discussing constitutional isomers in terms that they are completely different compounds. It's just important to keep in mind that they are made up of the same atoms in the same proportions. In case you've forgotten, the thing that makes compounds constitutional isomers is that the atoms are connected to each other in different ways for each molecule.
Constitutional isomers often have dramatically different chemical properties. Their physical properties differ too. They might have different functional groups. In contrast, stereoisomers don't exhibit such bold differences. Two compounds that are stereoisomers of each other not only have the same atoms, but the atoms are connected in the same way. Their properties are almost identical. But the spatial positions of the atoms are different.
There are multiple ways for molecules to exhibit stereoisomerism. But we won't go over all of them at once. Instead, we'll take this slowly. It seems only natural to start with the case of stereogenic centers, specifically tetrahedral chiral centers, but don't worry about those terms right now. The important thing to grasp is the concept.
I am a big fan of the written word. I strive to be as good as I can at communicating concepts verbally. However, this concept is just so much easier to convey using a picture. So here you go.
I made this in ChemSketch and it's supposed to be CHBrClF (a carbon attached to a hydrogen, a bromine, a chlorine, and a fluorine). It doesn't really matter what the things attached to the central carbon are, though, so long as they are all different things. They could be other atoms or even organic groups like methyl, ethyl, and so on. A carbon (or another atom) attached to four groups, none of them identical, is a stereogenic center. It is chiral because it is non-superimposable on its mirror image. Here's the mirror image.
I had to mess around with the program a bit to get this to work, but other than that, does it look exactly the same as the previous molecule? Yes? Look again. With the white ball (representing hydrogen, but whatever) on top, we can look down and starting from brown and going clockwise, we will necessarily have a different order for each of these. It's unavoidable. They're almost the same, but they're different in this one respect. A classic example is the difference between a right hand and a left hand. But for this type of stereoisomerism, all that we need is one central atom with four different groups attached to it. Usually, the central atom is carbon and one or more of the groups are part of an organic molecule (rather than just the single atoms used in my example).
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