Organic Chemistry questions

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Folks, some organic chemistry questions I’m trying to answer.

  1. Why does a reaction of 1,3-cyclopentadiene with Bromine go much faster than a reaction of 1,3-butadiene with HCl?

  2. Why would ether be used in either of the above reactions?
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  • In reply to:
Folks, some organic chemistry questions I'm trying to answer.

1) Why does a reaction of 1,3-cyclopentadiene with Bromine go much faster than a reaction of 1,3-butadiene with HCl?

2) Why would ether be used in either of the above reactions?



If I remember correctly, the reason that 1,3-cyclopentadiene reacts quicker is because it is not conjugated like 1,3-butadiene is. Since conjugated molecules are more stable, they are less reactive.

I don't remember why they are done in ether.
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  • hawkeye Said:
  • In reply to:
Folks, some organic chemistry questions I'm trying to answer.

1) Why does a reaction of 1,3-cyclopentadiene with Bromine go much faster than a reaction of 1,3-butadiene with HCl?

2) Why would ether be used in either of the above reactions?



If I remember correctly, the reason that 1,3-cyclopentadiene reacts quicker is because it is not conjugated like 1,3-butadiene is. Since conjugated molecules are more stable, they are less reactive.

I don't remember why they are done in ether.



I don't think that's it, 1,3-cyclopentadiene IS conjugated.
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Geofff,


I can’t picture it. What is the expected product for each? If you draw a reaction mechanism, is there a clear difference? Does the fixed relative position of the double bonds in the cyclopentadiene help?


When in doubt, when someone says “why ether?” a good default answer is “non-polar solvent”.

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Also, please give me some context by telling me the title of the chapter(s) you’re studying.

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I’ll have to check my notes when I get home as to what product I get.


As for the ether, if you have the following options, which would you choose?

  1. It stabilized the carbonium ion (carbocation)

  2. It enhances the regioselectivity of the reaction.

  3. It is the universal solvent.

  4. It does not participate in the reaction.

  5. It is necessary for the formation of the bromonium ion.



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Would potential carbocation rearrangement influence the speed of the reaction? That may happen w/ the butadiene but obviously not with the cyclopenta~. I also remember something specific about 1,3s that I learned late in organic 2, where you end up with a different arrangement than you’d expect, but I can’t recall it well enough to say whether it’s applicable here.

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Of those options, stabilizing the intermediate step would be the main thing I’d think of the ether solvent doing.


Is this organic 1 or 2, and what are the topics you’re studying? I think Denise was right in saying that the context could help. Are you covering e.g. SN1 vs SN2 type reactions right now?

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Is the cyclopentadiene reaction with Br2/light, or just Br2? If it’s Br2/light, it’s a radical reaction and would be much faster just because radicals are more reactive.


I bet it’s about the stability of the leftover anion; HCl is a weaker acid than HBR, which means that Cl- is less stable than Br-, so it’s probably reacting faster because the Chlorine anion reacts more violently to get an electron back. Just guessing.

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I’m going for option 4 on the ether.


Added info from a short chat with Geoff:

  1. The chapter is on SN1/SN2 reactions.

  2. Both these reactions are SN1.


    Based on this additional information, I’m suggesting when he gets home, he draw the mechanism and see which has the more stable carbocation. That’s the key to reaction rate in SN1, if I recall. Resonance stabilization may be involved here. I am too lazy and have forgotten too much ochem to draw this out for my own self.
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I am particularly curious on this now - taking OChemI right now and not being able to answer the above is just ticking me off!

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Denise, you amaze me! I think I purged all organic chemistry out of my brain as soon as I got my MCAT score back and knew I wouldn’t have to take it again!

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Asked an organic chemist I know. His answers:


ring opening releases strain and increase the speed of the reaction. Ether complexes with the product reactants to bring them in closer proximity for reaction to occur.


Is ring opening something you’ve discussed in class, Geoff?

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Ok, I went ahead and submitted my answers and got one right, one wrong.


For the first question… why did the reaction occur faster? I got this right… the carbonium ion forms faster since you have two halogen ions which are more reactive than the HCL.


The second question… why was ether used? I answered because it stabilized the carbonium ion. That’s wrong, it’s because it does not participate in the reaction.


On to the next chapter!!

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On to the next chapter!


I’ve got to admit, I’m not overly thrilled with the ether question. If it were “why is ether used and not [some specific highly reactive chemical],” then sure. But, say, octane wouldn’t react with those things, either. We use ether because it does other things than just not react.


I’ve also got to admit, this organic stuff was surprisingly fun to come back to

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  • Emergency! Said:
Denise, you amaze me! I think I purged all organic chemistry out of my brain as soon as I got my MCAT score back and knew I wouldn't have to take it again!



So it turns out I was amazing, but incorrect.

Please note I am tutoring someone who's taking ochem lab right now, so I get refreshers all the time.
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Ok, I got another answer from my instructor in regards to these questions. Here are his comments


“The carbonium ion forms faster in the case of the first reaction above. The bromonium ion, which forms as an intermediate, has octets of electrons which make it more stable than any carbonium ion (see the last paragraph in Sec 5.6 on p 128. The more stable the cation the faster it forms. The second reaction reacts to form conventional carbocations.”


Also,


“Solvents may or may not participate in the reaction. In many instances, although the solvent doesn’t participate, it’s chemical characteristics are essential to the success of the reaction (e.g., aprotic solvents vs. protic solvents; polar solvents vs. nonpolar solvents, etc). In the present instance, ether, an aprotic, non-polar solvent is very inert and is not involved in any way in the reaction except as a “carrying” medium for the reactants.”

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Can I just say, I DID take organic chemistry, got a 10 on BS, so I must have known stuff like this at one time, but now I have no idea what y’all are talking about.