[orgo814]

What is the relationship between the two benzylic hydrogens in meso-2,5-diphenylhexane? What is the relationship between the two benzylic hydrogens in (R,R)-2,5-diphenylhexane?

This is confusing me. My answer for the meso is homotopic since if you replace a hydrogen with something like Cl you'll get the same molecule as if you replaced it on other side. Is this correct or am I totally off?

For the second one (R,R), my answer is enantiotopic since if you replace a hydrogen with something like Cl you'll get the opposing configuration (i.e. if replace first hydrogen you would get SR, if replace second hydrogen you would get RS). Is this correct?

[mjc123]

I think it's the other way round. Instead of Cl, try replacing the H with D, so you don't change the priority of the substituents, which may confuse the R-S naming. Then with the meso you get (2R, 5S) 2-deutero-2,5-diphenylhexane and (2S, 5R) 2-deutero... With the RR isomer you get (2R, 5R) 2-deutero... either way.
Or draw them out as below. Then you see that in the meso, the Hs are related by reflection symmetry, therefore replacing one or other gives nonsuperimposable mirror images. In the RR they are related by rotation symmetry, therefore the products of replacement are superimposable.

[orgo814]

If you replace let's say carbon 2 hydrogen with D it becomes 2S, 5R. If you replace carbon 5  hydrogen with D it still is 2S,5R since it isn't changing the stereochemistry...so I believe this would be homotopic..?

[mjc123]

Are you talking about the meso isomer? No, you don't change the stereochemistry, but you change which carbon is numbered 2! With everything else being symmetrical, the carbon with the D is numbered 2. Then you get the compounds I named, what might be alternatively called, say, (2S, 5R) 2-deutero.. and (2S, 5R) 5-deutero.... But if you put the original molecule the other way round - call it (2R, 5S) 2,5-diphenylhexane, rather than (2S, 5R)... (but you can see that these are the same molecule) you get (2R, 5S) 2-deutero... and (2R, 5S) 5-deutero... But (2S, 5R) 5-deutero.. is the same as (2R, 5S) 2-deutero... So either way you get the same pair of enantiomers.
These hydrogens in a meso compound are enantiotopic. In the RR they are homotopic. General rule.