[trinhn812]

http://i45.photobucket.com/albums/f84/azncrombie/-5195_2130.png

Option 1. constitutional isomers
Option 2. a pair of enantiomers
Option 3. chiral (N) and achiral (M)
Option 4. both chiral
Option 5. meso compounds
Option 6. chiral (M) and achiral (N)
Option 7. both achiral
Option 8. a pair of diastereomers

More than one option can be used.

I think they're both achiral, meso, and disastereomers. Is this correct?

[azmanam]

meso and diastereomers are mutually exclusive - a pair of compounds cannot be both meso and diastereomers at the same time.  Gotta pick one

[azmanam]

ps - you can attach png images directly to the post by using the 'more options' option below your posting box (or click preview to add image to a reply)

[trinhn812]

Oh they can't be diastereomers cause they got mirror images. So they only options are they are meso and achiral right?

[trinhn812]

arg turns out ace organic says its meso, disateriomers, and achiral

[azmanam]

impossible.  two compounds simply cannot be meso and diastereomers at the same time.  not physically possible.  If your book (or software) thinks both meso and diastereomers are correct descriptors, you need to take that question to your instructor and tell your instructor the question is absolutely wrong.

imho, meso and achiral were the right descriptors.

[jj74]

agree,
if two compunds are meso, they're simply the same molecule (which is achiral); two diastereoisomers are different compounds (with different physical properties and different reactivity).
maybe the sense of the answer is that both compounds are meso (achiral), in the sense that they are superimposable to their mirror images despite having two stereocenters, and they are diastereoisomers one to the other.
my 2 cents

[azmanam]

maybe the sense of the answer is that both compounds are meso (achiral), in the sense that they are superimposable to their mirror images despite having two stereocenters, and they are diastereoisomers one to the other.

No.  I'm being blunt not to be mean, but to be unambiguous.  2 compounds will never be both meso and diastereomers.  Diastereomers are defined as stereoisomers that are not mirror images.  Enantiomers are defined as non-superimposable mirror images, and meso compounds are molecules with chiral centers AND a mirror plane - meso compounds are superimposable on their mirror image, thus achiral.  Because they can be oriented as mirror images, they cannot be diastereomers, because they are superimposable, they cannot be enantiomers.  Meso is a separate class unto itself.

[jj74]

probably we are saying the same thing. sorry english is not my mother language (I'm italian) and maybe I'm not clear.
Compound M is a meso compound: it contains two stereocenters but it has a plane of simmetry and it's superimposable to its mirror image (which is not N)
Compound N is a meso compound: it contains two stereocenters but it has a plane of simmetry and it's superimposable to its mirror image (which is not M)

then, the relationship between N and M:
they are stereoisomers, but they're not enantiomers; so they are diastereoisomers: same molecular formula, same connectivity, different spatial arrangements of atoms but they're not mirror images.

or maybe I'm getting confused 

[azmanam]

(sigh) damn.  You're right, I was looking at it too quickly, and thus looking at it wrong (another argument for posting the picture in the thread, so it can be in the same tab as my response

Typically when instructors give 'describe the relationship' questions, the ones that are meso are are meso to each other, that is the two are typically the same compound.

This question is a rare case where a compound has two different meso isomers.  As shown in the image, M and its mirror image are the same compound.  The mirror image of M issuperimposable on M.  Same with N, the mirror image of N is superimposable on N.  M and N are not mirror images of each other (which I assumed they were at first glance).

Because M and N are not mirror images, they are diastereomers.  But because the mirror images of M by itself and N by itself ARE superimposable on themselves, M by itself is meso and N by itself is meso.

Sorry for confusing everyone.  Man, I hate being wrong... but I feel even more like an idiot after going around telling everyone how wrong they were.  My apologies.

The book/software is correct.  The descriptors are meso, achiral, and (in this rare case) diastereomers.

[Dan]

I'd say both compounds are meso and they are related as diastereoisomers. This is entirely physically possible! Consider the two compounds below - both are achiral and they are related to each other as non-superimposable non-mirror images... A pair of meso diastereosimers...

Edit: I was drawing while you were posting Azmanam, I don't think it's that rare, it pops up frequently in undergraduate exam papers I've seen. Another example is galactitol and allitol.