[briteyellowness]

question: given that the dehydration of pinacol (2,3-dimethyl-2,3-butanediol) with strong acid and heat is neither an oxidation nor a reduction, which of the following best describes the product?

answer: ketone

i have no idea why?  isn't the carbon tertiary and forming a ketone would mean having five bonds with it, which isn't possible?  the only thing i might be able to see forming is maybe an epoxide?  

[Yggdrasil]

This reaction comes about from what is known as the pinacol rearrangement (at least, this is what my organic prof. refered to it as).  The acid will protonate a hydroxide group to form water.  The water will leave to form a carbocation.  The carbocation will rearrange via a 1,2-methyl shift to form a secondary carbocation connected to the other hydroxide.  This rearrangement is favored because of resonance-stabilization; the lone pair on the oxygen can donate electron density to the empty p-orbital of the carbocation, and in general, resonance-stabilized secondary carbocations are more stable than tertiary carbocations with no resonance-stabilization.  Subsequent deprotonation of the oxygen results in the formation of 3,3-dimethyl-2-butanone (pinacolone[?]).

[movies]

Yep, Yggdrasil has it right.  Mitch has kindly drawn out the mechanism on our mechanism page.

However, I wouldn't call the stabilization of the carbocation a resonance stabilization.  It's really a "neighboring group" effect.  Technically resonance structures must have the same number of sigma bonds.