[Fireredburn1]

The hydration of an alkyne gives you an enol which will tautomerise to give you a ketone.

Here, 5-hexynoic acid is treated with HgSO4 and acid in an alkyne hydration reaction, however it cyclises to give you a cyclic ester (pyrone)

What was the mechanism of this reaction?

[zsinger]

You must first show us an attempt.  Hint:  Think about what might happen when the carbocation is formed.
          -Zack

[AlphaScent]

Wouldnt the hydroxyl of the acid or water attack the atom that is more substituted ?

[orgopete]

Wouldnt the hydroxyl of the acid or water attack the atom that is more substituted ?

That would be one possibility, but that will give a different product. I think I'd look at the Barton rules for cyclization.

[AlphaScent]

Baldwins rules??

[AlphaScent]

6-endo-tet is what he wants.

Though I postulate you will also see 5-exo-tet closure. 

Which reaction is more favored? Which rate is faster?

My argu

[AlphaScent]

Sorry, clicked enter on accident..

My argument is that the 5 carbon is better at stabilizing the arbocation, therefore is more apt to attack.

I think I am missing something with the closure rules. 

Why is it that 6-endo-tet is favored?

[orgopete]

Baldwins rules??

Baldin, Barton, what's the diff. Yes, I knew it didn't sound right but I just couldn't remember, so thanks for correcting me.

I was only guessing, obviously I didn't look it up. I don't know whether this is an instance of Baldwin's rules applied or not. None the less, if it gives the pryone indicated, it still doesn't answer the mechanism question.

[AlphaScent]

Ha ha, I knew what you meant. 

To be completely honest, I am not an expert in these rules either.  It seems to me that both are possible, but figureing out which rate is faster will tell which is more probable.  A problem unto itself.

I do not see how one gets pyrone from that.  Where does the enol-ether like double bond come from?

[AlphaScent]

Even if we get the 6-endo-tet.  This is what I see happening.  Water would have to be omitted to stop competition of the cyclization.  I do not see where the other double bond comes from.  There is nothing to eliminate...

Please correct me if I am wrong.  I do not see the process.

[orgopete]

No NaBH4 and different product.

[Archer]

You would reduce to the lactol or diol depending on conditions.

[Archer]

I thought the Hg was replaced by a proton, via a carbocation at the oxygen bearing carbon.

That gives the same product though.

My only guess without thinking too hard would be autoxidation to the more congugated system. It's pretty weak as suggestion

It's going to bugg me all night, when I can sit down with a pencil and paper I will look in more detail.

[Archer]

What about this

http://www.sciencedirect.com/science/article/pii/S0022328X00850548

Followed by dehydration?

Not too sure of the mechanism though, still working on that.