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).