NUcleophile help!!

[starpaste]

I learned about what makes a good nucleophile.

But take a sample case

RCO2H

is the carbonyl oxygen more nucleophilic or the oxygen attached to the proton?

 

- they both have resonance (??)

[big314mp]

The carbonyl oxygen would technically be more nucleophilic, as it has more electron density. However, it really isn't a very good nucleophile at all because the negative charge is distributed via resonance.

 

Something like an alkoxide (RO-, the conjugate base of an alcohol) is a much better nucleophile as the negative charge is all concentrated on one atom.

[starpaste]

but doesnt the double bond pull in the electrons from oxygen making it less polarizable ?

 

 

thanks!

[big314mp]

In carbonyl groups, the oxygen is double bonded to the carbon. There are two resonance structures for this. The first has a double bond, and no formal charges. The second has a single bond, a formal minus on the oxygen, and a formal plus on the carbon. This second structure is less favored than the first, due to the presence of the formal charges.

 

What those two resonance structures tell, is that the pi bond between the carbon and the oxygen is polarized towards the oxygen, thereby putting somewhat of a minus charge on the oxygen. Since this is only a partial minus charge, adding another bond will put a formal plus charge on the oxygen (or on the carbon if you move the pi electrons in the double bond to the oxygen). This type of structure is unfavorable, so the carbonyl oxygen is not nucleophilic.

[qwerty123qw]

Wait starpaste i think you mean in RCOOH?

if it is HO--C=O, i thought it would be the OH bc it is more basic than c==o. yet it the O in c=o is still the better nucleophile?

[big314mp]

If you look at the mechanism of a fischer esterification, the carbonyl is actually protonated first.

 

http://en.wikipedia.org/wiki/Fischer_esterification

[Tartaglia]

You really need the terms "soft" and "hard" when determining what is the best nucleophile.

 

Good nucleophiles are usually ( though not always) "soft". This means the electrons are in a diffuse orbital where the charge is spread out. Bad nucleophiles are usually "hard" where the orbital is tightly bound to the atom and thus can not "reach" easily into the electrophilic centre.

Since the OH oxygen has two sp3 lone pairs, which are more diffuse than the two sp2 orbitals on the carbonyl oxygen, I would expect the OH oxygen to be more nucleophilic.

 

The answer does depend to some extent on the electrophilic centre as some centres like hard nucleophiles.