# Which ring expansion in cyclobutyl(cyclopropyl)methanol is favourable?

Predict major product:

In this question first I've protonated the OH group and then water is removed to form a carbocation. Now I have to expand the ring, but I'm confused which ring to choose: 3-membered ring or 4-membered ring?

Note: I can make an alkene (which would be the final product of the reaction) by removing an alpha hydrogen if it is known which ring to expand.

• Perhaps there is more than one rearrangement? Could both rings expand? – Michael Lautman May 30 at 12:09
• I think there will be two separate ring expansion products, but the major product will be the expansion of the cyclopropyl group. The reason ring expansions occur at all, is to relieve ring strain. The farther away you are from a hexagon, the more strain there will be. – Argento May 30 at 13:23

It is clear to me that no matter which ring you open the first, you'd get the same product as A. Michael Lautman gave the correct product , first opening the cyclopropyl ring. Michael's mechanism is fair, but I'm reluctant to say it is the most preferable path because it gives you bicyclobutyl $$2^\circ$$ carbocation as an intermediate, which needed extra 1,2-hydride shift to get stabilized as $$3^\circ$$ carbocation (total of 5 steps). However, if you have opened the cyclobutyl ring first, you'd get the same final product A, through a relatively low energy cyclopropylcyclopentane carbocation (total of 5 steps) even though it is also needed a 1,2-hydride shift (in relatively more stable cyclopentyl $$2^\circ$$ carbocation when compared to $$2^\circ$$ carbocation in cyclobytyl ring in Michael's mechanism) to get stabilized as $$3^\circ$$ carbocation. Complete mechanism is depicted below:

This mechanism is supported by following reference:

• G. K. Surya Prakash, V. Prakash Reddy, G. Rasul, J. Casanova, G. A. Olah, “The Search for Persistent Cyclobutylmethyl Cations in Superacidic Media and Observation of the Cyclobutyldicyclopropylmethyl Cation,” J. Am. Chem. Soc. 1998, 120(51), 13362–13365 (https://doi.org/10.1021/ja9828962).

1. Ring expansion of the cyclopropyl group
2. 1,2-hydride shift
3. Expansion of the cyclobutyl group
4. Expansion of the "new" cyclobutyl group
5. Elimination

• The first two structures aren't really distinct. – Zhe May 31 at 1:02
• @Zhe can you elaborate please? The first structure is the initial carbocation formed in the reaction. The second has undergone a ring expansion. If you are going to downvote an answer, it would be nice if you provided some constructive feedback. – Michael Lautman May 31 at 1:33
• Cyclopropylcarbinyl, aka homoallyl, aka cyclobutyl cations are generally thought to be non-classical instead of distinct species. – Zhe May 31 at 13:03

The $$\ce{C4H7^+}$$cation shown as the first entry in Scheme 1 is described as a bridged structure that is isomeric with cyclopropylmethyl and cyclobutyl ions.1

Scheme 1

The product following this pathway is shown in the post by Michael Lautman.

Second pathway is shown here in Scheme 2 with 2 in ring expansion to give2a .Next is elemination to give 3 and 4 .

Scheme 2

2a could undergo a hydride shift to 2c ,that on elemination gives third product 5.

Referance

1 Advanced Organic Chemistry FIFTH EDITION

Part A: Structure and Mechanisms

FRANCIS A. CAREY and RICHARD J. SUNDBERG

• Cyclopropyl carbinyl is generally thought to be a nonclassical cation and not two isomeric forms. – Zhe May 31 at 1:01
• @Zhe i thought so too ,but its a direct quote from advanced organic chemistry by francis carey and sundberg – Chakravarthy Kalyan May 31 at 3:13
• Historically, there are opposing view points on this subject, which Carey/Sundberg does not capture. Cf. the excellent work of George Olah. – Zhe May 31 at 13:06
• @Zhe, I understand that, but I would have appreciated some insight into what the concept of the non classical carbocation would change about the product. – Michael Lautman May 31 at 16:45
• @MichaelLautman For one thing, you don't ever have to consider ring expansion of the cyclopropyl carbinyl cation; the non-classical view would be that it exists in an intermediate form. On first principle, the unknown regarding this mechanism is which steps are under thermodynamic versus kinetic control? – Zhe May 31 at 17:43

The cycloproyl ring may expand as it is more angle strained. So after ring expansion a comparatively more stable carbocation will be formed.