This ochem stereochemistry question makes no sense to me

[phattestlewt]

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Please help..






Correct answer is B.

 

[broski123]

I dont see anything

 

[jayoh]

If you draw it out in a sawhorse, you can see that the bond can be rotated so that there is a plane of symmetry, making it a meso compound. Hope that helps!

 

[FeinMS]

You need to translate those diagrams into 3D shape. It is a difficult transition, but I have a trick.

Consider the upper half and lower half of the molecule separately. If you look at A, C, D, you must switch 2 substituents on the lower half to get the same configuration as the upper half. For example in A, you must switch the location of Br and H, in C, Methyl and H, in D, Br and H. However in B, you must switch the location of all 3 substituents to get the same configuration as the upper half.

What this means is an useful trick in stereochem. If you have to switch 2 substituents of one molecule (or a part of molecule) to get the same looking as the other molecule (or the other part of the molecule), they're non-superimposable mirror images. If you have to switch 3 molecules, they're basically the same molecule with diff conformation. So if you rotate the lower half of B, the two parts will look the same, so there will be a mirror plane -> meso.

 

[phattestlewt]

You need to translate those diagrams into 3D shape. It is a difficult transition, but I have a trick.

Consider the upper half and lower half of the molecule separately. If you look at A, C, D, you must switch 2 substituents on the lower half to get the same configuration as the upper half. For example in A, you must switch the location of Br and H, in C, Methyl and H, in D, Br and H. However in B, you must switch the location of all 3 substituents to get the same configuration as the upper half.

What this means is an useful trick in stereochem. If you have to switch 2 substituents of one molecule (or a part of molecule) to get the same looking as the other molecule (or the other part of the molecule), they're non-superimposable mirror images. If you have to switch 3 molecules, they're basically the same molecule with diff conformation. So if you rotate the lower half of B, the two parts will look the same, so there will be a mirror plane -> meso.

Good trick. I see it now, I'm assuming this only works for fischer projections?

Also, This would mean that if a, c, and d are non-superimposable they are also enantiomers correct?

 

[jayoh]

Good trick. I see it now, I'm assuming this only works for fischer projections?

Also, This would mean that if a, c, and d are non-superimposable they are also enantiomers correct?

I can't answer your question about that trick, because I've never used it, but if you draw them out in a sawhorse you will also see that A/C/D are the same molecule

 

[phattestlewt]

I can't answer your question about that trick, because I've never used it, but if you draw them out in a sawhorse you will also see that A/C/D are the same molecule

What's sawhorse

 

[jayoh]

Here is an example-

For a Fischer projection, the bonds going up/down go downward on a sawhorse, and the bonds going left/right go upwards. Does that make sense?

 

[jayoh]

I always think of a fischer projection as just a flattened sawhorse, so anytime I get those questions I draw them out real quick and it makes it easier to visualize. Hope that helps

What's sawhorse

 

[FeinMS]

it works on other structures too. Most of the times, you will be using on 3d structures.