Is this rearrangement possible in carbocations? enter image description here

  • $\begingroup$ chemistry.meta.stackexchange.com/questions/141/… see that link once. $\endgroup$ – Avyansh Katiyar Jan 17 '18 at 17:14
  • $\begingroup$ Not a homework question. Was learning about rearrangement , just thought if it is possible for something like this to happen $\endgroup$ – SR810 Jan 17 '18 at 17:17
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    $\begingroup$ @SR810 its fine whether it is or isn't a literal homework question, but you should include you thoughts on why you think it should/shouldn't occur and what research you have attempted to support that conclusion. $\endgroup$ – Tyberius Jan 17 '18 at 18:00
  • $\begingroup$ Oh! I was learning about rearrangements and saw 5 membered to 6 membered ring expansions. So I thought if 4 membered to 6 membered was possible. $\endgroup$ – SR810 Jan 17 '18 at 18:03
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    $\begingroup$ Possible duplicate of Carbocation Rearrangement - Till which neighbouring carbon is it possible? $\endgroup$ – Mithoron Jan 17 '18 at 20:22

I'd say it's almost surely impossible.

First of all, a carbocation on primary carbon is extremely unstable, so the question becomes how would you generate that. It is possible under some very extreme conditions, but then those conditions and the surroundings of the newly formed carbocation would be more controlling the reaction than the carbocation on its own. This newly generated carbocation would probably instantly react with solvent or with the leaving group thus reversing to the compound it was formed from.

Secondly, if we assume that the primary carbocation formed and this is some idealized situation where there are no other molecules around then the more probable scenario is that you will observe hydrogen transfer from the neighbouring secondary carbon, which would then undergo another hydrogen transfer, eventually forming a relatively stable tertiary carbocation.

  • $\begingroup$ What I was thinking was that formation of a 6 membered ring will reduce ring strain to a great extent hence making the carbocation more stable. So maybe in a reaction where the 4membered ring carbocation cation is formed as an intermediate the product is formed as a result of the ring expansion into a 6 membered ring. Like it happens for 5 membered ring Carbocations. $\endgroup$ – SR810 Jan 17 '18 at 17:22
  • $\begingroup$ When it occur in five-membered rings the carbocation the original carbocation is just next to where expansion take place - you can imagine it as if the carbon forming bond slid along the bond of carbocation and tertiary carbon; or imagine it with a transition state where all three carbons are connected by partial bonds. When it comes to rearrangements remember that the bonds don't actually exist like sticks holding atoms - a line is just a metaphor for two atoms being held close by the cloud of two electrons. But when there are more atoms around (and charges), this cloud can deform. $\endgroup$ – Jan Rzymkowski Jan 17 '18 at 19:18
  • $\begingroup$ Anyway, if that is for your own knowledge then I would recommend you to forget about carbocation rearrangements and move on. There are beloved by chemistry professors because they can trip students, but they have almost none relevance in actual chemistry. All those primary carbocations from exams and tests exist almost only on paper. Only carbocations to actually care about are the stabilized ones - like tertiary, allylic, benzylic. $\endgroup$ – Jan Rzymkowski Jan 17 '18 at 19:25

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