meso compounds – achiral molecules with chiral atoms

meso compounds – achiral molecules with chiral atoms


Welcome to Chem Help ASAP. Sometimes when you are dealing with diastereomers,
you run into this idea of meso structures. We need to discuss what this means and how
do we recognize it. I have a molecule on the left, but we’re really
going to start on the right. In a different playlist, I mentioned that
the total number of stereoisomers that you can draw for a molecule is equal to 2^n, where
n is equal to the number of stereocenters. This molecule on the left has two stereocenters. Two stereocenters, 2 to the 2nd power would
be 4. We would expect to be able to draw four stereoisomers
for this molecule. Let’s go ahead and just draw them out. We’ll keep drawing this same backbone for
every structure. For our first one, let’s invert both of the
stereocenters. So let’s swap the hydrogen and the bromine. That will now be S. The other one, let’s swap
the hydrogen, the bromine. Assuming did the first stereochemical assignments
properly, that will be R. Let’s go down here, and let’s draw two more. Let’s swap the left-hand stereocenter – the
H and the Br. That will now be S. We’ll leave the right-hand
one alone. That will be S. Let’s draw one more. So, let’s swap… What do we need? We have R/S. We need the R/R. So, we’re going
to swap both of them off of this bottom left structures. That will be R. (and H… Br) That will be R. So, we have all four possibilities
– R/S, S/R, S/S, and R/R. Ta dah! We’ve drawn our four stereoisomers for this
particular structure. However, remember, for all these things, to
be stereoisomers of each other, they need to be non-superimposable. The molecules themselves need to be chiral. We have a problem because this molecule… If we cut it perfectly in half, we would find
that both halves are equal. This has a plane of symmetry. Some people call this a mirror plane. So, this is an achiral molecule. Achiral molecules can’t have an enantiomer. So, the way we drew this, we said, “Oh. Well. That’s the R and that’s the S. If we reverse
both of them, we’ll make it the S and the R.” These should be enantiomers. They are not enantiomers. They are identical structures, because this
has that same plane of symmetry. If you can imagine taking this molecule and
rotating around… (use this as an axis now)… by 180 degrees, you would rotate both these
bromines, which are forward, to being back. The hydrogens, which are back in the left-hand
structure, would be forward on the right. These are identical. We thought we were drawing enantiomers. We were just drawing identical structures. These are superimposable – not non-superimposable. Therefore, we have one less structure. This is an example of a meso molecule – a
molecule that has two symmetrically placed, identical stereocenters that cancel out. The human body is actually meso. We have a right and left hand. Those are chiral objects, but the body itself
has a plane of symmetry. It is achiral. It’s because our two stereocenters, our right
and left hands, reflect into each other. So do our right and left ears, and our right
and left feet. When you have a molecule that has identical…
for different groups on two carbons – the same four different groups on both, you run
into this possibility of a meso structure. Meso molecules are molecules that contain
stereocenters and yet retain a plane of symmetry. It reduces your possible number of total stereoisomers. So, just be aware of this, that it’s going
to reduce the possible…. There are actually just three possible stereoisomers
for this particular molecule with its two stereocenters.

Add a Comment

Your email address will not be published. Required fields are marked *