Diasteromers, Enantiomers, Meso, Chiral, etc. Got a 3/23 on Examkrackers

[luctoretemergo]

Ok, so I never quite mastered this material in intro chem, and now it's coming back to haunt me. I took the practice exam for section 1 of orgo chem in Examkrackers and got a 3/23! I am very confused on recognizing these compounds. I get that
Chiral molecules have no super imposable mirror image (c attached to 4 diff substit's)
Enantiomers have a mirror image
Meso have a plane of symmetry
Diasteromers have no mirror image

That all came from what I knew, yet I got a 3/23 on this section. If someone could be so kind as to walk me through recognizing each and things such as, is a diasteromer chiral? is a chiral molecules always a diasteromer? etc. I'd really appreciate it!

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[NickNaylor]

A compound is chiral at an atom if that atom has four different groups attached to it. If there aren't four different groups attached to it, then it's achiral and can't be an enantiomer/diasteromer/etc..

Two compounds are enantiomers if they are mirror images of each other. They're also enantiomers if, at every chiral atom, the configuration is switched (e.g., from R in one compound to S in the other).

If two compounds are NOT mirror images, they are diastereomers. Two compounds are also diastereomers if the configuration is not changed at every atom. For example, if you have a compound that is S at one atom and R at another (we'll call it SR), a compound with a SS or RR configuration will be a diastereomer (notice the configuration changes at only one atom). If you were to draw the compounds, they wouldn't be mirror images. The enantiomer would have a configuration of RS (it changes at both atoms) and would look like the mirror image if drawn.

I always struggle with meso compounds because I can't recognize planes of symmetry unless the compound is drawn in a very straightforward way. As you said, though, look for planes of symmetry. If a compound is symmetrical, then at least two groups are the same and the atom is achiral (so notice that meso compounds are NOT chiral and, thus, can't be diastereomers or enantiomers).

That's a fairly quick run-down, but hopefully it helps.

 

[PiBond]

just a few things to add on:

enantiomers have opposite absolute configurations at all chiral centers and thus have opposite optical rotations. ie if one rotates +41.5 its enantiomer must rotate -41.5. enantiomers also have the same physical properties and thus can only be separated via resolution and not distillation

diasteromers need at least 2 chiral centers. in essence, at least one but not all chiral centers are changed in diasteromers. thus they don't have an predictable optical rotation relationship. diasteromers have slightly different physical properties and are thus easier to separate

meso compounds have an internal plane of symmetry and also have an even number of chiral centers. thus they have the exact same amount of R and S chiral centers. meso compounds also have zero optical activity because the chiral centers effectively cancel each other out in the molecule.