Formation of tosylates from alcohols
Description: Treatment of alcohols with p-toluenesulfonyl chloride (TsCl) results in the formation of tosylates, which is an excellent leaving group.
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In the last three examples, it seems like pyridine is used as a weak base (proton acceptor). Is there a particular reason it is not used in the first three examples? Why can’t the Cl just act as the proton acceptor in the last three examples?
On paper (e.g. for our purposes) sometimes pyridine is shown, and sometimes it is not. I think what happens is that instructors don’t want to over-complicate things, so not putting pyridine in is one of those little details that can sometimes be omitted and doesn’t ultimately change the key concepts of what’s being taught.
[Probably too much information – feel free to ignore]
In practice, pyridine is necessary for removing the HCl that forms as the reaction progresses. Using pyridine results in higher yields because 1) it drives the equilibrium to the right (preventing HCl from protonating OTs) and 2) pyridine can make TsCl more reactive towards substitution since it attacks TsCl to give Ts[pyr]+ which has a better leaving group that is more readily attacked by the alcohol.
Why does Cl- in step 3 act as a base, not a nucleophile that attacks C in the C-O bond, because after the pronation step, the O(+)Ts becomes a good leaving group?
Good question! With primary alkyl tosylates, that reaction is plausible, but slow.
However, it turns out that with *allylic* toslyates (e.g. H2C=CHCH2OTs) the reaction is a lot faster and will often result in allylic chlorides. You likely won’t see any examples in your course, but it is something to watch out for in the lab.
Is this reaction possible if Methyl group were not there on the benzene ring?
Yes, except it wouldn’t be tosylate, it benzenesulfonyl.
I thought it was TosCl instead of TsCl???
Either can be used.
Excellent. I was reading Kaplan MCAT review and messed up with this reaction. This page gives clarity of thought that may be needed to do good on MCAT….
it’s excellent……..