The 3 rules of meso compounds

The 3 rules of meso compounds


But I know what you guys are interested in
is these rules, right? So let’s talk about these rules. There’s three rules that will help us predict
if a molecule is meso. Believe me, this is going to help you so much
because in your textbook and according to your professor, it’s very difficult to tell
when something’s meso or not. A lot of times they just expect you to recognize
it, but I prefer to have rules. Here are the criteria. First of all, it has to have two or more chiral
centers. It can’t have less than two. So if it has one, that’s not meso. If it has two, maybe it’s meso. Three, maybe it’s meso. Two or more. The second criteria is that it has to be atomically
symmetrical. Now this is actually really tricky to communicate
because that doesn’t mean perfectly symmetrical it just means that everything has to be connected
in the same way on both sides even if wedges and dashes don’t match up. That’s okay. I just need the atoms to be in place. Symmetrically. Then finally, I need an even number of the
chiral centers to be opposite to each other. That means that if that I have two chiral
centers, I need two of them to be opposite to each other. That means that I need my first one if it’s
R, I need the other one to be S. If this first one is S, I need the other one to be R. Why
is this? Because this has to do with the part about
canceling out that I was talking about. I need for these chiral centers to cancel
out so that overall the molecule, even though it has two chiral centers, it will overall
be achiral because they’ll perfectly cancel each other out. Let me go ahead and give you a really simple
version of a meso compound and it would be something like this. Let’s say that I had a cyclohexane and I had
a wedge here and I had a wedge here. This is an example. There’s lot of different meso compounds out
there, but this is an example of a meso compound. Why is that? First of all, do I have any chiral centers? Yeah, I happen to have two. This is a chiral center and this is a chiral
center. So both of these are chiral centers because
if you count the groups, if you go around, the directions are different depending on
which side you take, so the R groups are different so these are two different chiral centers. Is this molecule chiral? No, it’s not. Because if you look, it actually has an internal
line of symmetry. If it has an internal line of symmetry, that
means it’s achiral. But another way you could tell is by looking
at the actual configurations of the chiral centers. What I would see is if I went ahead and figured
out the chiral centers like the priority one, two and three, what I would find out is that
this one goes around this way. And then I would find that this one goes around
this way. So what I would find is that one of these
is an R and one of these is an S and they’re symmetrical to each other. They are both the same distance from the middle. So what that means is that this is another
way of proving that it’s meso by looking at the configurations and saying, hey, they’re
opposite and they’re symmetrical and there’s two or more of them, so this is meso. Now you guys might be wondering, “Johnny,
that looks so much more complicated than just doing the internal line of symmetry. Why don’t I just do the test one, internal
line of symmetry test?” You can if it’s a ring. But remember, that only really works for rings. For other kinds of compounds, it doesn’t work. So for example, (a) and (b), these compounds
that I’m going to give you down here, you really should use that test because that test
is not going to work very well for these. So what I want you guys to do is go ahead
and start on (a) and try to figure out the three things, how many chiral centers there
are, what they’re configurations are and if it’s symmetrical. Then once you figure that out, tell me if
you think it’s meso or you think it would just be a normal chiral compound. All right, so go ahead and try to solve (a)
and then I’ll show you guys how to do it.

14 Replies to “The 3 rules of meso compounds”

  1. Hey, you stated if there are 3 chiral centers it might be Meso, but then you say it must be an even number of chiral centers opposite each other.. So should we say rule #1 is there must be an even amount of chiral centers?

  2. GOT IT ….and after all i have watched this video ! i concluded that…your t shirt is awesome …pleas tell me where to buy it ?? or you gonna gift me one 🙂
    Thanks

  3. This is the link for the answers to part a and b. If nothing shows up try making a clutch account and trying again. Its free to make an account. Hope this helps 🙂

    https://www.clutchprep.com/organic-chemistry/meso-compound/12014/learn/1108

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