Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: rlabomba on December 19, 2017, 09:07:12 AM
-
Hi guys,
Had my final yesterday and I had a problem that I want to verify with you.
I did bromination they KOH to double eliminate.
Only reservation I have is did I put the BR's in the wrong place? should the BR's be on adjacent carbons or the same carbon like I did? Thanks.
-
What mechanism (what reaction conditions) do you envision for the first step? I think what you wrote is dubious.
-
My thinking was radical bromination followed up by E2 elimiation with a small base to form the alkyne. Not sure if I executed the bromination correctly though.
-
Ordinarily one thinks of radical bromination adding a single bromine atom. If one let's the reaction continue, bromocyclohexane could continue to add bromine atoms. This might create a mixture of products, if I am not mistaken.
-
could you do a monobromination then convert that to an alcohol then to a ketone then deprotonate and trap as an enolester (such as an enoltriflate) then eliminate using a strong base?
edit:it may be possible to convert cyclohexane directly to cyclohexanone
-
Its is poor chemistry to just randomly brominate and expect to get the dibromo species in the right spot. Such a product, while possible to produce with random radical bromination, would be impossible to separate, and would yield very low. Monobromination, is a good way to give yourself a handle on an alkane, especially a very symmetrical one like this.
I think youre on the right pathway, but need some help. Can you think of a non-alkane substrate that will allow you to add two bromines either gem or vicinal??
-
Its is poor chemistry to just randomly brominate and expect to get the dibromo species in the right spot. Such a product, while possible to produce with random radical bromination, would be impossible to separate, and would yield very low. Monobromination, is a good way to give yourself a handle on an alkane, especially a very symmetrical one like this.
I think youre on the right pathway, but need some help. Can you think of a non-alkane substrate that will allow you to add two bromines either gem or vicinal??
You could do Vicinal dibromide to an alkene, but how could you create an alkene from this cyclohexane?
Couldn't you do BR2/light, KOH, BR2/Light, KOH?? wouldn't that accomplish this?
-
You're on the right track, your reaction sequence is close, but I think you should draw it out so that you can see how to have the right number of bromines in the right spots. That second Br2/light isnt quite what you want. You want to go from cyclohexene to 1,2 dibromocyclohexane, then hit it with KOH.
Also, please have care. BR2 is sloppy, we write Br2 or at least Br2.
-
how about something like this?
-
Can't go from alkyl halide to alcohol with NaOH.
-
...
Also, please have care. BR2 is sloppy, we write Br2 or at least Br2.
Especially in this case, where alkyl boranes may well also be involved. I don't like Pauson-Khand reactions for a similar reason :)
-
@OP, What can you make with bromocyclohexane?
-
You're on the right track, your reaction sequence is close, but I think you should draw it out so that you can see how to have the right number of bromines in the right spots. That second Br2/light isnt quite what you want. You want to go from cyclohexene to 1,2 dibromocyclohexane, then hit it with KOH.
Also, please have care. BR2 is sloppy, we write Br2 or at least Br2.
Does this look more like the correct mechanism?
-
i would think this would be unlikely as the dibrimide would be anti to each other. elimination can only occure if a proton is anti to the bromide which they would not be in this case. that is if it goes via the E2 elimination pathway.
-
Can't go from alkyl halide to alcohol with NaOH.
why not?
of course the SN2 reaction would be in competition with the elimination reaction but you can control the conditions to give you the desired product.
-
rlabomba yep, that is exactly what I had in mind! I'm fairly sure that was the test answer expected. Although, once again, Don't capitalize the "R" in "Br". It implies you are using a weird boronic ester, not diatomic bromine. Just to be specific, I would say the first Br2 is in the presence of light, the second one isnt, or uses NBS as a bromine source.
Organicdan,
I had this discussion with a professor a long time ago, the reaction simply doesn't work in real life. OH-'s basic properties simply overcome its nucleophilicity, even under cold temps. I challenge you to find a scholarly source where OH- will displace an alkyl halide directly, even a primary one. I used scifinder for this cyclohexane substitution and was unable to find an example. This is a case where paper chemistry should bow to what we observe in the real world.
I did find a couple examples where a primary alkyl halide in mixed aqueous/organic solutions could be cooked to give an alcohol (like this http://pubs.acs.org/doi/abs/10.1021/jo00156a045), but that is water, not NaOH. Another exception is that some alkyl iodides will slowly hydrolyze in neutral water.
-
How long would cyclohexyne exist?
I have no opinion whatsoever, just saw that Wiki tells
"only exist as transient reaction intermediates or as ligands coordinating to a metal center"
https://en.wikipedia.org/wiki/Cycloalkyne
because of the strain.
-
Enthalpy,
I dont think OP is proposing to make this on the bench. Its from a test.
-
does it matter? Why would you put such a question on a test? While one might miss some examples that wont exist in real life, this one seems unnecessary because you can just make it linear alkyne and be fine..
Not to mention, the double elimination might be tricky as well even for larger rings (8 membered)
-
does it matter? Why would you put such a question on a test? While one might miss some examples that wont exist in real life, this one seems unnecessary because you can just make it linear alkyne and be fine..
Not to mention, the double elimination might be tricky as well even for larger rings (8 membered)
and why would you start with such a molecule that contains no functional groups?
-
Why not?
As a TA, I would say that making molecules from stupid starting materials that you would never use in a lab is fine as a learning tool but I would avoid compounds that are not reasonably stable (such as the cyclohexyne)
-
Supposedly, the professor will be happy with a synthesis applicable to linear alkynes.
Wiki gives syntheses for the cycloalkynes
https://en.wikipedia.org/wiki/Cycloalkyne#Synthesis
and they differ from linear alkynes, at least to my untrained eye. Possibly beyond the scope of the course.
If I grab it properly, cyclohexyne would only serve in situ and immediately for subsequent reaction steps.
-
When one has dibromocyclohexane as the substrate for an elimination, what would prevent its becoming cyclohexadiene?
-
Very good point Babcock. This may in fact be more likely since allylic protons are 2 pKa units lower than alkenic ones. Perhaps we NEED the gem dibromo to force the alkyne. I doubt it will double eliminate in two directions to give an internal ketene from the gem dibromo cyclohexane.