[Borek]

Thanks borek ...

2 minutes wiki clicking, you really should all learn these tricks.

[spirochete]

Thanks borek ...

Spirochete , then why does O act as an ED group in the pinacole rearrangement ? I couldnt see any resonance there. Incase you need an image , I'll attach one.

There definitely is resonance stabilization in the pinnacol (SP?) rearrangement.  There is one step where a carbocation rearrangement takes place that leads to a resonance stabilized carbocation being formed.  one of the hydroxyl groups is protonated, water leaves, then a 1,2 hydrogen shift takes place which generates the resonance stabilized carbocation. 

Probably the mechanism you've been doesn't show the resonance explicitly.  It's possible to write the mechanism so that the lone pair on oxygen just immediately swoops in and forms the ketone.  In reality there is a second resonance form though, with the protonated carbonyl group.

[spirochete]

oh and btw A is more stable than B , right ?

Yes I'm 99% sure of that, somebody correct me if I'm wrong.

[Hunt]

Thanks borek ...

2 minutes wiki clicking, you really should all learn these tricks.

Yeah I just noticed one can search on wikipedia with molecular formulas...

[Hunt]

Spirochete,

well to make sure I got this , consider the Tetrahydropyran molecule and assume a carbocation exists next to the O. Can I say O stabalizes the C+ by resonance ? ( the O atom give away its electron pair and become positive ... )

[spirochete]

Exactly.  That's why if you start with a double bond adjacent to the oxygen and do acid catalyzed hydration, you end up with the -OH next to the oxygen (see wiki article)

It would also be a good exercise to write out a mechanism for acid catalyzed formation of that straight chain alcohol/aldehyde that forms in the last step in the wiki article.

[Hunt]

Ah I'm finally getting somewhere in this. I'll re-check thak wiki page again. Thanks again for your help spirochete .

[Hunt]

What if I were to compare the ED effect of NO2 and OCH3 ? Our professor considers OCH3 to have a stronger OD effect , and Im assuming that it's the two oxygens attached to the nitrogen that are making NO2 less ED . In this case electronegativity of the attached atoms to the N must be taken into account , am I right ?

[spirochete]

Look at the structure of the nitro (-NO2) group again, notice that it doesn't even have a lone pair to donate. . .

[Hunt]

You're absolutely right spirochete . My mistake ...