EK Organic Chap 1

[BobbyDylanFan]

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Hi folks,

Just studying some organic from EK and came across a question i can't seem to visualize. Also, it looks like Orsay is backed up since his courses started, so I thought that I would pose the question to you guys for a little help. Pg 105 #12 where to draw the line of symmetry to see the meso compounds (explanation is on pg 132) Thanks !

 

[Bostonienses]

Hey there! You can rotate the A and the bromine about the axis of the carbon. Really hard to see without a model set or something of the like. I still don't see it until I build a model myself...

 

[DrLady]

Bostonienses,
I was thinking the same thing, rotating, but then I thought that maybe I am wrong so I didn't post. There will always be anti-addition, so well, will rotation about the axis of carbon help in putting a line of symmetry?

BDF,
When I saw your post, I instantly checked my desk drawers for the plastic model set that I keep, but it seems that I left it at home and I don't have it here with me at school. I work best when I make models if i can't visualize in 3-D. Do you have any plastic models? That really helps!

 

[Bostonienses]

Ok, let me explain this in a less crappy manner. The carbon-carbon single bond can "rotate", and at one point in time, the "rotation" will give you a meso compound. It's hard to see because that drrawing is only a "snapshot" of the molecule, and it's drawn in two dimensions. Try drawing a dash-line-wedge formula or a ball and stick drawing... It might make more sense. Anyone else have a more eloquent explanation?

 

[Bostonienses]

aaah, DrLady, I was posting as you posted! My last post wasn't a reply to what you said... Yes, plastic models are the best shot. Does anyone have a "formula" or something of the sort that can help us see these things without them?

 

[BobbyDylanFan]

To my organic chemistry lady friends: Thank you for your help on the orgo problem 🙂 You would both make great miss saltys. I got by without the model set in my orgo classes, thought I would be okay with mcat stuff...maybe not😱 Now back to studying for my pathophysiology exam...heee haaa 😉

 

[Jayhawker]

Let me have a shot. A meso compound is a superposable mirror image. We know that there are maximum 2^n (n = # of stereocenters) stereoisomers possible or in general, 2^n-1 enantiomers though it may have less if any stereoisomers are meso.

What I do is rotate the WHOLE molocule 180. If you get the same molocule as the one you are trying to imitate, it is meso. This is assuming that you have held one carbon and rotated to get the identical groups in the same position.
Many times, when the molocule is the same on the top carbon as the one below, i.e. the bottom mirrors the top, it is meso. One of the favorites is Pasteur's discovery of optical activity with tartaric acid. It will be in your organic book. Anyway, 2R,3S and 2S, 3R are identical. There are 2 stereocenters. 2^2 = 4 maximum possible sterioisomers. However, there are only 3 (two enantiomers and one meso) because of 2R,3Sand 2S,3R.

Say it with me......"Stereo Chem s...ks!"

Good Luck😱

Rock Chalk Jayhawk
Darin

 

[Mudd]

A couple of thoughts here:

• 1) Alkene reactions are no longer on the MCAT, so the crux of this question has been neutered.
  
  2) Meso compounds can be viewed as having either a mirror plane or an inversion point. The structure with a mirror plane is a 180 degree rotation from the structure with an inversion point.

Trans-2-butene has two methyl groups trans to one another and two hydrogens trans to one another. Addition of Br adds two more identical groups (Br atoms) across from one another. The result is a structure with an inversion point, making it meso.

.....H. CH3
......|. |
Br?C?C?Br
......|. |
...H3C. H

If you happen to have the BR orgo books, look on page 158 of book 1, there is a great drawing and explanation of this concept. {My copy is about six years old, so it might be on a different page by now.}