[rlabomba]

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.

[Babcock_Hall]

What mechanism (what reaction conditions) do you envision for the first step?  I think what you wrote is dubious.

[rlabomba]

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.

[Babcock_Hall]

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.

[OrganicDan96]

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 

[wildfyr]

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??

[rlabomba]

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?

[wildfyr]

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.

[OrganicDan96]

how about something like this?

[wildfyr]

Can't go from alkyl halide to alcohol with NaOH.

[sjb]

...
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

[Babcock_Hall]

@OP, What can you make with bromocyclohexane?

[rlabomba]

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?

[OrganicDan96]

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.

[OrganicDan96]

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.