8
$\begingroup$

How will the cyclobutane ring behave in the case of cyclobutylmethylium (cyclobutylmethyl cation)?

enter image description here

I initially thought there would be ring expansion to a five membered ring so that there may be less angle strain and a secondary carbocation instead of a primary one. But I have also been told that there will be ring contraction for stability purposes. I would be highly obliged if someone could explain the mechanism involved here.

$\endgroup$
8
$\begingroup$

I think your friend is thinking of the cyclobutyl carbocation which does ring contract to the cycylopropyl carbinyl carbocation (and also equilibrates with the methallyl carbocation).

However, just as you thought, the cyclobutyl carbinyl carbocation does ring open to the cyclopentyl carbocation (ref_1, ref_2, ref_3). This rearrangement is driven by carbocation stability (primary to secondary) and relief of the steric strain present in the 4-membered ring, again, just as you suggested.

enter image description here

Note: this question has been asked previously on SE Chem, however I believe the accepted answer is incorrect, as it seems to primarily address the cyclopropyl carbinyl case, and what is said about the cyclobutyl carbinyl carbocation (little to no ring expansion) is incorrect.

$\endgroup$
  • $\begingroup$ What would you think is more stable from the species $\ce{C4H7+}$, the cyclopropylmethylium (that you drew) or the methylcyclopropan-1-ylium? I tend to think the latter, but I am absolutely not certain. $\endgroup$ – Martin - マーチン Apr 30 '15 at 6:51
  • $\begingroup$ @Martin Good question, the cyclopropylmethylium is primary and resonance stabilized, while the 1-ylium is tertiary with a bit more ring strain. I'd vote for the former, but I dunno. Somebody must have done a good calculation. $\endgroup$ – ron Apr 30 '15 at 12:59
  • $\begingroup$ @ron in general, there's probably some level of delocalized charge between both carbons. $\endgroup$ – Geoff Hutchison Apr 30 '15 at 16:00
  • $\begingroup$ @GeoffHutchison How would you describe the delocalization in the methylcyclopropan-1-ylium case? $\endgroup$ – ron Apr 30 '15 at 17:47
  • $\begingroup$ The point would be that under QM, in either case, the charge will not wholly be on one carbon. That said, I ran a high-level CCSD calculation and it's pretty localized on the primary (as you drew): i.imgur.com/MhTrSzu.png $\endgroup$ – Geoff Hutchison Apr 30 '15 at 18:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.