[Cdncory]

How can you tell whether or not a structure is a Nucleophile or Electrophile? I know how to ID it when it is a simple ion (ie Br-) but when it is a compound like phenol how do you know?

[movies]

A very basic definition would be species with a lot of electon density tend to be nucleophilic, species that are electron deficient tend to be electrophilic.

[Cdncory]

Would this be right then?

[movies]

You're very close.

First of, you should account for the loss of the bromide in the first equilibrium.  You can show this by putting something like - Br^- above the arrow, and + Br^- below the arrow (for the reverse reaction).

Secondly, the way you have drawn the loss of the hydrogen from the second to last species is incorrect.  The second water molecule plucks the H off of the R-OH₂⁺ (you have it drawn as the water attacking the oxygen).  The electrons from the O-H bond you are breaking in that step end up on the oxygen, not on the hydrogen.

Other than those two things, I think it looks okay.



I think I see where you got confused.  The oxygen, although it has a positive charge, is actually not a very good electrophile because there is still a lone pair of electron on the oxygen and all the bonds between oxygen and H (or C) are polarized towards oxygen (since it is very electronegative).  The hydrogens are more electrophilic because of the bond polarization (electrons are "closer" to oxygen than hydrogen) and the hydrogen is less encumbered by sterics.