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I understand how and that the R,S and R,R stereoisomers are different.

But I don’t see why the R,R and S,S compounds are stereoisomers and not meso compounds with each other.

2D drawing of two stereoisomers

Online it says that these two compoundsare different stereoisomers, but I don’t see why they are not identical.

I went onto a 3D molecule visualiser website, and still cannot see how they are different.

3D structure of molecules

It does say that the visualiser website is not always accurate, but I can’t see why these molecules are not identical.

Furthermore, what specific type of stereoisomers are these two structures. Would it be correct to say they’re a member of the set of ‘conformers and rotamers’, which itself is a subset of the set of diastereomers, or are they something else.

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Probably a good way to see this is with a Newman projection. Imagine looking at the R,R isomer molecule down the middle carbon-carbon bond axis. Then you have one of the chiral carbons (the proximal atom) facing you and the substituents $\ce{Cl, CH3, H}$ will be arranged around it in clockwise order*. The other (distal) chiral carbon along the viewing axis will have the same substituents in counterclockwise order. If you turn the molecule around to switch the locations of the chiral carbons, you still get the clockwise-front and counterclockwise-back outcome; the mirror image, corresponding to the S,S-isomer, is not superposable on the R,R molecule we have described.

With the meso (R,S) arrangement, two mirror images can be superposed: the Newman projection looking one way has $\ce{Cl, CH3, H}$ in clockwise order at both the proximal and distal locations, but turning the molecule around renders both orders counterclockwise.

Thus our molecule has a pair of enantiomers and one meso isomer. This is a total of three counting the pair of mirror images separately.

*The ordering of substituents ranked 1,3,4 when the rank-2 substituent is directed to the back matches the 1,2,3 order when 4 is directed to the back.

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