Stereochemistry of Bromine Addition to trans-Cinnamic Acid

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Discussion Overview

The discussion revolves around the stereochemistry of bromine addition to trans-cinnamic acid, specifically focusing on whether the addition is syn or anti. Participants are analyzing melting point data and exploring the implications of molecular rotation and reaction mechanisms.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Nautica reports a melting point of 199°C, suggesting it corresponds to specific dibromo products, and seeks clarification on whether the addition is syn or anti.
  • Another participant questions the melting point of the other product, suggesting inconclusiveness if the measured melting point does not clearly align with known values.
  • Nautica mentions that repeated experiments yielded melting points between 197°C and 198°C, raising concerns about the significance of the difference in melting points.
  • Some participants discuss the possibility of a 50/50 attack from both sides of the double bond, with Nautica speculating on a potential 80/20 split.
  • One participant clarifies that a 1:1 mix of enantiomers is expected, as the starting material is achiral, and questions the feasibility of an 80/20 mix.
  • Nautica expresses confusion about molecular rotation after the double bond is broken, questioning whether the bromine addition could lead to a 50/50 product distribution.
  • Another participant explains that the single bond formed after bromine addition is not free to rotate due to the stability of the bromonium ion, emphasizing that bromine addition results in the anti product.
  • Nautica and another participant agree on the melting point of 200°C indicating a specific product, with Nautica questioning the relationship between erythro pairs and syn/anti products.
  • Clarifications are made regarding the definitions of anti and erythro products, with participants discussing the implications of the trans starting material on the product outcome.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the addition (syn vs. anti) and the implications of melting point data. There is no consensus on the exact mechanism or product distribution, and the discussion remains unresolved regarding the stereochemical outcome.

Contextual Notes

Participants note limitations in their understanding of the reaction mechanism and the implications of molecular structure on product formation. The discussion highlights dependencies on definitions and the stability of intermediates, which remain unresolved.

nautica
We determined our melting point to be 199 C. Which would correspond to (2R,2S) and (2S,3R) 2,3-dibromo-3-phenylpropanoic acid (mp 202-204 C) but we are trying to determine whether this is a syn or an anti addition.

Any suggestions?

Thanks
Nautica
 
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Which is the mp of the former? If it's not apparent which mp the measured is closer to than you have to say that it was inconclusive. You know it is anti based on the mechanism but you can tell that from this experiment.
 
The other has a mp of 98 C. The melting points from 5 different experiments all came up between 197 and 198.

thanks
nautica
 
Originally posted by nautica
The other has a mp of 98 C. The melting points from 5 different experiments all came up between 197 and 198.

thanks
nautica

Well if I had to pick a difference of 100 degrees or 4 degrees I'd go with the latter. That's acceptable experimental error.
 
Yes, but from looking at the models I do not understand, why it is not a 50/50 attack from both sides of the double bond. And, was also thinking that it might be a 80/20 split between the 2.

thanks
nautica
 
Originally posted by nautica
Yes, but from looking at the models I do not understand, why it is not a 50/50 attack from both sides of the double bond. And, was also thinking that it might be a 80/20 split between the 2.

thanks
nautica

I'm not sure I understand the question. You do get a 1:1 mix of the enantiomers, they have the same melting point. There's no way you would get a 80:20 mix because the cinnamic acid is achiral, the bromine can react initially to either face just fine.

The alternative to the anti (2S, 3R) and (2R, 3S) product would be the cis product (2S, 3S + ent.) and you would see any of that from this reaction.
 
We got melting of the 200 C. So we know that it is either the syn or the anti addition.

The problem I am having is that after the double bond is broken the molecule is free to rotate, which should allow for a 50/50 split. This did not appear to happen, So maybe there is something in the solution that keeps the bond from rotating. I know that when the Br joines, it joins between the 2 carbons which would cause a trigonal planar situation, but when that is broke the bond should be free to rotate?

Nautica
 
You stated earlier that the other option has a mp of 98. Or was that a typo? If so than you can't say anything based on mp.


When the double bond is broken the single bond is NOT free to rotate. It's stuck in the three membered ring bromonium ion, which is quite stable. The ring does not open until the bromide attacks on the opposite face. Bromine addition only gives the anti product.
 
I agree, our melting poing is the 200 and that would be the product.

Based on this mp it would be the erythro pairs, which I thought was the syn version of this product.

Does the anti addition not result in the anti product?
 
  • #10
Originally posted by nautica
I agree, our melting poing is the 200 and that would be the product.

Based on this mp it would be the erythro pairs, which I thought was the syn version of this product.

Does the anti addition not result in the anti product?

No, the anti product is the erythro product. Remember that the starting material is trans, make the anti product model and then rotate the bond, you'll get the erythro product.

Yes, anti product = anti addition.
 
  • #11
That clears it up. I was confused by looking at the fisher projection with the Br's on one side.

Thanks
Nautica
 

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