I'm pretty sure you cannot say they are diasteromers of each other. Meso compounds are identical to each other whereas diasteromers are not identical. I'm not a 100% sure on this, so maybe someone else can weigh in.
I think you have it a bit wrong. Let me explain a bit, then I'll come back to what you've said and try to explain where you're off a little.
Let's say I have a 6-carbon compound and that C2, C3, C4, and C5 are chiral centers. Further, suppose that the compound is symmetric, sort of like a six-carbon sugar, but with the carbonyl group removed. So, since there are 4 chiral centers, there are 16 possible stereoisomers. You could generate them all by starting with the Fisher projection for one with all the same configuration (e.g.,
S) and then systematically changing one at a time.
2S, 3S, 4S, 5S
2R, 3S, 4S, 5S
2S, 3R, 4S, 5S
2S, 3S, 4R, 5S
.....
2R, 3R, 4R, 5R
This gives you all the possible stereoisomers for this compound. Now comes the part of identifying the relationships within that set. Any
pair of compounds are enantiomers if and only if they are non-superimposable mirror images. Here is an example of a possible enantiomeric pair:
2S, 3S, 4S, 5S and
2R, 3R, 4R, 5R
Notice that every chiral center has been inverted. When we talk about a mirror image, that's what we're doing - the mirror inverts every chiral center. Now, I said these were
possible enantiomeric pairs because its entirely possible that when you perform this inversion, the two compounds end up being identical. So, you need to actually check to see.
Now, if I only invert some (but not all) of the chiral centers, I get something like the following:
2S, 3S, 4S, 5S and
2S, 3R, 4R, 5R
Here, I've inverted all but one of the chiral centers on the compound. Clearly, these two can't be enantiomers because they aren't mirror images of each other. If they were, all four of their chiral centers would have been inverted and they aren't. These two stereoisomers which are not enantiomers are referred to as diastereomers. As before though, notice that the term only makes sense when referring to a relationship between two different molecules. To say that something is a diastereomer or an enantiomer is silly. The fact that those terms refer to pairs of compounds is often missed by students and they tend to get confused by it. Make sure you understand the distinction.
Notice that
cis- and
trans- aren't mentioned in this discussion. This is part of my complaint with the way stereochemistry is often taught. Diastereoisomers are almost invariably introduced with compounds like 1,2-dichloroethene and professors use this example to teach diastereotopic pairs. It is true that the
cis- / trans- isomers are diastereomers of each other, but it is misleading because it gets students thinking that those are the only types of diastereomers. The true definition is much more general - diastereomers are any pair of stereoisomers that are
not enantiomers.
Now, let's talk
meso. Let's look at the compound with configuration
2S, 3S, 4R, 5R. Imagine placing a mirror plane between the
3S and
4R carbons. Do you see how the left hand side of the compound is reflected on the right hand side of the compound? This is true because I specifically selected a compound that was symmetric. Compounds like this which have an internal mirror plane, where one half is reflected to yield the other half, are referred to as
meso compounds. The term
meso refers to the compound, not to a pair. This makes it a fundamentally different definition than other words. Let me try to summarize the difference:
diastereomer and
enantionomer refer to a relationship within a pair of compounds.
meso refers to an individual type of compound which has an internal mirror plane or plane of symmetry.
Diastereomers are really different molecules - they have different boiling and melting points, they behave differently in reactions, they rotate light in a polarimeter differently, and they often have different NMR spectra as well. The real question is what is special about enantiomers and
meso compounds. The simplest distinction is this:
enantiomers rotate plane polarized light the same, but in different directions. The physics behind how it does this, while interesting, is not important.
Meso compounds do not rotate plane polarized light at all, even though it has the same number of chiral centers as any other of that set of stereoisomers. The reason light is unaffected is rather subtle, but makes sense if you think about it. Looking at our example earlier, we see that the left hand side of the compound will rotate light some particular angle because the left hand side has two chiral centers. The right hand side does the same thing, but in an opposite direction. It turns out that the two halves of the molecule cancel out and the net rotation of the light is zero. The same reasoning explains why racemic mixtures of two enantiomers do not rotate light either. A
meso has chiral centers but is overall achiral because the two halves negate each other.
Now, let's look at your statement.
Meso compounds are identical to each other whereas diasteromers are not identical. I'm not a 100% sure on this, so maybe someone else can weigh in.
You are using the term
meso to refer to a relationship between two compounds, which isn't correct. Now, an interesting question that your statement raises is whether or not a
meso compound can have a diastereomer or enantiomer. If we look at the earlier example of the
meso compound, we see that we can easily form the following pair of diastereomers:
2S, 3S, 4R, 5R and
2S, 3R, 4R, 5R
These two compounds are clearly not enantiomers and they remain stereoisomers, so they are indeed diastereomers of each other. Additionally, the one on the left is a
meso compound.
Now, to see if the
meso compound can have an enantiomer, we determine its mirror image:
2S, 3S, 4R, 5R and
2R, 3R, 4S, 5S
But, since the compound was symmetric, it turns out that the mirror image is superimposable, so the
meso compound cannot have an enantiomer.
OP: In hindsight, I think this is what you were saying, but I wanted to leave the explanation, long-winded that it might be, behind for others that might not understand it too well to read.
Hope this helps.
