[Kate]

Hello.

I need to write the mechanism for the first step of this reaction: http://en.wikipedia.org/wiki/File:THP_ether.png

Ok, I'm having trouble doing it. Also, keep in mind that there's a catalyst: p-toluenesulfonic acid; and that the alcohol is methanol.

I'm not sure if the first step of the mechanism is the addition of the H of the OH group from methanol, because I think that the carbocation would be formed in the second carbon counting from the O atom of the ether (because this carbon is the more substituted one).

Input would be much appreciated. 

[Arctic-Nation]

Vinyl ethers are always protonated on the second carbon atom from oxygen to give a stabilized oxonium cation. This is the first step of the reaction.

I don't understand how you can call the second carbon atom from oxygen the 'more substituted one,' though, as all carbon atoms in dihydropyran are secondary.

[orgopete]

Ok, I'm having trouble doing it. Also, keep in mind that there's a catalyst: p-toluenesulfonic acid; and that the alcohol is methanol.

I'm not sure if the first step of the mechanism is the addition of the H of the OH group from methanol, because I think that the carbocation would be formed in the second carbon counting from the O atom of the ether (because this carbon is the more substituted one).

CH3OH + p-TsOH  CH3OH(+) + pTsO(-) (but not completely, MeOH -2.2, pTsOH -2.8 )

So the proton probably would be coming from a CH3OH2(+). I would also guess the oxygen of the tetrahydropyran would be more basic than the carbon. However, it would not lead to any further reaction. If the electrons of the oxygen can participate or resonate with the electrons of the double bond, then protonation can occur on the more basic carbon atom.

Hint, except for the pTsO(-), your mechanism should not have any other anions.

[Kate]

Thanks for your responses.

I figured it out already: the acid gives the proton to the double bond, a carbocation forms on the 1st carbon counting from the oxygen atom and then the oxygen of the methanol binds to the carbocation and finally there's the removal of the hydrogen atom from the oxonium by the conjugated base of the p-toluenesulfonic acid.