[sundrops]

Water makes a fairly poor nucleophile.
but why?

is it because of it's size? or are there other factors as well?
does polarity come in to play here aswell?

any input guys would be fabulous!  

[Laborduck]

This is a rule of thumb offered by my organic professor to guage a molecule as a nucleophile-

Weak Nucleophiles
*Tend to have an attached hydrogen
:Nuc--H
Ex. H20, R0H, NH2--R (R= H, C-chain, nitrogen derivatives)

Strong Nucleophiles
*Tend to have a negative charge
:Nuc-
Ex. -OH, -OR, -S

Nucleophilic attack follows Coulomb Laws- opposite charges attract. Nucleophiles, being partially or fully negatively charged, will be atrracted to electrophiles, being partially or fully postively charged. The greater the differences in charges the greater the attraction, which allow for nucleophilic attack.  Water, being partially charge, hence, would not want to attack a non-charged carbon. Not enought attraction there... Water, though, can attack a carbon that is fully postively charged. There is enough attraction in this case.

[Laborduck]

Nucleophiles are beginning to be called donor (electron donor), while electrophiles as acceptor (electron acceptor). I like the donor and acceptor terms b/c i'm learning chemistry w/ more ease by using these terms

[Donaldson Tan]

orbital 1b1 is the HOMO of the water molecule. I think the reason why water is such a weak nucleophile is that its HOMO is so comparatively lower in energy level than the HOMO of other nucleophiles, that it constitutes as a weak nucleophile.

Do correct me if I'm wrong. I am not too sure about this.

[Hbond]

The high electronegativity of oxygen stabilizes the electrons of oxygen making it a poor nucleophile. The hydrogen of water on the other hand, is induced electrophilic, due to the highly electronegative oxygen. The electrophilic potential of hydrogen is strong enough to withdraw electron density from the poor nucleophile oxygen aspect of water to form a hydrogen bond.