Ring expansions occur as a carbocation rearrangement when an unstable cycloalkane is near a carbocation. You will come across ring expansion mechanisms during alkene reactions and later in SN1 and E1 reactions.
They key is to recognize when a ring is unstable, and the type of rearrangement that will help it become more stable. But don't stop there. Ring expansions may be followed by a 1,2-hydride or 1,2-methyl shift.
The video below takes you through a tricky reaction mechanism taken from a students actual exam.
(Watch this video on YouTube or read the transcript coming soon)
Watch the previous video: Hydride Shift Methyl shift mechanism
Watch next video: Oxymercuration-Demercuration Reduction
Return to: Alkene Reaction Mechanisms Home Page
i seriously don’t understand why my professor do not acknowledge the fact that a hydride shift can follow a ring expansion to become a more stable carboncation along with a less strained ring. we had the same example with chloroethyl cyclopentane and professor said ring expansion wont occur because it wont yield a better carboncation but it can following a hydride shift, then he said only ” 1 type of carbon rearrangement occurs per reaction ” … and i see alot of people online doing the multiple rearrangement and according to my professor that is wrong ‘-.- can you please clarify leah ?
when a ring is stable?
Hey Leah, I’m confused about the numbering part of the ring. I don’t understand the whole numbering randomly. Can’t you assume you’re going to have a pentane?
I don’t know if that makes sense. I’m confused on figuring the numbering out
Can ring expansion and contraction also be called as 1,2 shifts ? Or is that term reserved only for hydride ,methyl,ethyl etc shift ?
Hey Leah, is there any specific reason why you chose to break the top bond for the alkyl shift as apposed to the bond on the side? If so, is one intermediate more stable than the other?
This is by far, the most elaborate and amazing explanation for ring expansion. I absolutely love it and didn’t think i would understand the concept so well. You’ve explained it in a terrific manner. Thanks a lot 🙂 just one doubt, when you were expanding the ring and the bond of the ring attacked the carbocation of the side chain, why did you choose the bond to the left of the side chain for attack, why not the bond to the right? And even if you did choose the bond to the right of the carbocation side chain, would that give a different product?
It’s an amazing explanation but i just can’t seem to get this one point right.
Hi Leah, does a hydride shift always take priority over a methyl/alkyl shift?
I believe so Jen. It’s smaller and easier to move