[rleung]

Thanks so much.  I think I get it now.  Is the following another way to explain why H2S is a better nucleophile: basically, water SHOULD be the better nucleophile in "in theory", but since water by itself as a nucleophile would interact with itself via H-bonds (thereby reducing its own nucleophilic character), H2S would be a better nucleophile for SN2.  Hopefully I didn't screw that up...

Ryan

[kkjc2]

Not quite true.

Nucleophilic attacks can either be favoured by a charge interaction (for a polarised substrate - eg. a carbonyl  (+)C=O(-)) or an orbital interaction (for a not polarised subtarte - eg. your MeBr)

Charge-interaction type nucleophilic attack is favoured by a charged/ strongly polarised nucleophile; eg. OH-
Orbital-interaction type nucleophilic attack is favoured by a good orbital interaction. (good energy match, large orbitals, etc) eg. H₂S:

Hydrogen sulfide is a better nucleophile in this particular case, because the C-Br bond is not polarised, so a good orbital interaction is needed for a good nucleophilic attack. Water is not as good a nucleophile because its lone pairs are more contracted in smaller orbitals which do not overlap well with the C-Br sigma* orbital in the S_N2 reaction.

In other cases, depending on the subtrate, water may be a better Nu- than H₂S.

Basicity do not relate well with nucleophilicity.
H-bonding of the nucleophile doesn't affect its nucleophilicity.
There are no 'theory' to say that water is a better nucleophile than hydorgen sulfide; but Water is a stronger base than H₂S as a fact. (pK_a H₂O 16, H₂S 7)

You need to think in terms of Orbital interactions (relates to polarizability), geometry of the subtrates, favourable charge interactions, effect of solvents etc.
for deciding on nucleophilicity of your nucleophile in an S_N2 mechanism.

[movies]

Water is a stronger base than H₂S as a fact. (pK_a H₂O 16, H₂S 7)

These pK_a numbers are for the equillibrium where water or hydrogen sulfide are acting as the acid, so they don't really reflect the basicity of water.  The pK_a you should look at is that of hydronium, H₃O⁺, which is -1.74.  I can't find an exact number for H₃S⁺, but I would guess it is about -3.  This is based on pK_a's of protonated THF (-2.05) and the sulfur analog (-4.4).

Other than that, great post!