Maui3
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Cyclopentanol and 48% Hydrobromic Acid Reacting To Form Cyclopentyl Bromide At Room Temperature?
Hi!
This is not really a write-up or anything, I just wanted to note something, I thought might be helpful for some other people.
Yesterday I planned to make cyclopentyl bromide from cyclopentanol and 48% hydrobromic acid, as well as another project I am working on. I did just
forget that I only have one condenser, and both projects needed refluxing. For that reason, I continued with my other project, while just letting the
brownish cyclopentanol + excess 48% hydrobromic acid mixture stand. I thought that letting it stand might help "kick-start" the reaction or similar.
When I came back to the mixture this morning, I realized that cyclopentanol-HBr mixture from yesterday had seperated into two layers. Normally you'd
reflux the mixture for 6 hours (which is a long time) to get the product, cyclopentyl bromide. But it seemed to already have made the product after
just around 18 hours of standing.
Now according to both cyclopentanols and cyclopentenes density, they would float on top of the HBr-mixture. Only cyclopentyl bromide would sink. So I
suppose it worked at room temperature?
Still a lot of unreacted cyclopentanol, so I'll let it stand longer, and definitely reflux it later to continue the reaction. Still interesting.
[Edited on 24-8-2025 by Maui3]
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Niklas
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Making cyclopentyl bromide from cyclopentanol usually doesn’t has amazing yields because of elimination occurring as a side reaction, by letting it
stand (or more ideally stirring it) at room temperature rather than refluxing this should occur a lot less, so it’s not just useful in terms of the
practicality of not having to set up a reflux.
Would be fairly curious to see how yields of refluxing for 6 h compare to letting it stir (or stand) at room temperature for a week or so.
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Maui3
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I unfortunately do not have any more cyclopentanol to compare letting stand, stir and reflux, but I would be interested in seeing the results if
anybody else has some.
I will update 
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clearly_not_atara
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As a general (but not always correct) rule, the higher MW product is favored at lower temperaters (cyclopentyl bromide) while the lower MW
products are favored at higher temperatures (cyclopentene and hydrogen bromide). In the case of nucleophilic displacement you will usually see more
SN2 (concerted) at lower temperatures and SN1 (carbocation) at higher temperatures. Only SN1 will form cyclopentene.
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