OChem: Simple resonance question

[cartman1980]

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Hello all,

Going back to one of the basics of OChem, i am stuck on something i can't wrap my head around.

Its regarding protonation site for a Carboxylic acid. Carboxylic acid has 2 oxygen sites (lets say site (a) is C==O, site (b) is -O-H). In determining which of the two sites will actually encourage protonation, resonance comes into play for site (a).

The question i have is regarding site (a). When H+ tries to attach to (a), oxygen will donate one of its electron pair, thereby developing a positive charge.

TBR OChem lists the following 3 resonance rules:
1. best structure allows for filled octets for all atoms (except H)
2. best structure reduces overall formal charge
3. (-)ve charge gets assigned to most electronegative atom
4. (+)ve charge gets assigned to least electronegative atom

So, when (a) develops (+)ve charge, AND
because C is less electronegative than O,
the C=O(+) will be converted into (+)C-O. This I get.

However, the resonance stabilization apparently doesn't stop there. O from site (b) apparently donates an electron pair to form C=O(+)ve at site (b), giving rise to the resonance hybrid #2. Per rule#(4) above, shouldn't the resonance hybrid allow C instead of O to carry the positive charge?

Hope the question makes sense.

 

[Rabolisk]

There are three resonance structures for protonated carboxylic acids. One with a positive charge on an oxygen, one with a positive charge on the other oxygen, and one with a positive charge on the carbon. You can see that the first two two structures are equivalent, because there is no difference between the two oxygens once both are protonated. Nevertheless you have to show both of them to show electron delocalization. This is no different than the two resonance structures of benzene. The third resonance structure is the most favorable structure because it follows all the rules that TBR gives. It is the most important one contributing to the overall structure of the molecule, which shows why the carbon has a greater positive charge than it did before the protonation. The first two structures alone would only suggest that the two oxygens are partially positive, rather than the carbon. Hope that makes sense.

 

[cartman1980]

crystal clear..
thank you sir..
what threw me off was TBR only depicted two (which were the same) of the three; not the third carbocation...🙂