[inash524]

Hi all,

On the attached screenshot, I'm given the reagents of a chemical reaction and asked to predict the product. I'm quite clear on the oxymercuration process and what happens there, so I initially predicted a product following the general Markovnikov addition rules. However, the answer key (also attached) states that a ring is formed during this process. Ring formation is something that I'm struggling with since I have trouble identifying when it might take place. What aspects of this reaction indicate that a ring might form? My first thought was that since water is not included in the list of reagents, only the alcohol group could relieve the strain from the HgOAc compound. However, my professor has been quite inconsistent on whether or not she includes water as a solvent. Do I typically assume water is present if no solvent is specified?
Thanks so much!

[clarkstill]

However, my professor has been quite inconsistent on whether or not she includes water as a solvent. Do I typically assume water is present if no solvent is specified?

I think many of us are familiar with this particular complaint, especially in SN1/SN2 E1/E2 type questions.  Sadly you often have to read the examiner's mind when it comes to solvents... Generally I would assume water is not present in a given reaction.

Broadly speaking, forming 5- and 6- membered rings is favourable. 3-rings form quickly (since entropically the ends are always close together) but are strained. 4-rings are pretty hard to form. 'Medium sized rings' (7-13-membered) are hard to form - entropically the ends are rarely in close proximity since there are many rotatable bonds between them, and these ring sizes experience 'transannular strain'.

As a rule of thumb, I would generally look out for reactions that form 3-, 5- and 6- membered rings; other sizes tend to be much less common.

Also, your product looks good, but how about stereochemistry?

[inash524]

Hi Clarkstill,

Thanks so much for your response. I agree that for these types of problems, I should probably avoid assuming the presence of water. However, I'm still unsure of why twisting the long hydrocarbon + alcohol would be favorable in this problem. Does a conformation that twisted not seem extremely energetically unfavorable?

[Alwin Kristen]

That is 5-exo-tet ringclosure reaction and it is favorable. Check Baldwins rules for more information. Intramolecular reactions are very favorable. That long hydrocarbon chain has great entropy when it swings around in space. When you see possibilities for intramolecular reactions, ringclosure ect., it would propably happen. Here the hydroxylgroup is the one closest of carbon where HgOAc is attached. Trough lose Sn2 stage HgOAc ends to the less substituted end.