[leticiamc08]

The question is, which of the following is/are meso compound(s)?

The answer choices are:

A. Both I and III
B. II
C. Both II and III
D. III
E. I

I answered D. III, but I got it wrong. Can someone explain, please?

[Schrödinger]

Did you try drawing the Fischer projection formulae?
They always help.

[KritikalMass]

The correct answer is C, correct? Meso stuff can get tricky because you have to consider multiple chiral centers but you also have to consider rotation about those centers. Are you good with molecular models? If you are then I suggest you build a model of each molecule and see if you can transform (rotate) them into their mirror image. The tricky part is sometimes you have to rotate the entire molecule 180 degrees left or right or up or down, but in this case you don't have to do that.

Whichever molecule you can't rotate into its mirror image is the non-meso molecule.

If you don't have molecular models I strongly suggest you get some and learn how to use them. They can bring a molecule to life for you. In this problem you presented you are looking at these drawings and they seem flat and static like they are just stuck there so it's hard to visualize them moving and rotating so unless you are really good at rotating and twisting things around in your mind it is almost impossible to see this stuff. If you get a model set and use it to build say 5 molecules per day for a month you will start to be able to get a feel for them in your mind. This sounds lame but it actually gets to be fun and you will probably start trying to build things like ATP! But even if it isn't fun for you it is amazing the difference the model making makes.

Good luck!

[Jay]

Hey I think the easiest way to find a meso-compound, they are optically non-active, is to see if they have a mirror-plane. If the molecule has a mirror-plane it's optically non-active and if it has chiral centres nevertheless, it's a meso compound. So with III you can clearly see there is a mirror plane, so it's a meso-compound. With 1 you have to rotate the molecule to see that it too has a mirror plane, so I and III are mes-compounds. II does not have a mirror plane, so is optically active and not meso.

So the correct answer should be A.

[KritikalMass]

Hey I think the easiest way to find a meso-compound, they are optically non-active, is to see if they have a mirror-plane. If the molecule has a mirror-plane it's optically non-active and if it has chiral centres nevertheless, it's a meso compound. So with III you can clearly see there is a mirror plane, so it's a meso-compound. With 1 you have to rotate the molecule to see that it too has a mirror plane, so I and III are mes-compounds. II does not have a mirror plane, so is optically active and not meso.

So the correct answer should be A.

I think you should take a closer look at I. How would you rotate I into its mirror image?

[Jay]

i do apologies my drawing is so primitive. I am at home and dont have any software here.

[KritikalMass]

i do apologies my drawing is so primitive. I am at home and dont have any software here.

Hmm, good job. I should have built the models myself before posting my other post. I gave you a snack for that by the way, enjoy it!