benzenes

[qwerty123qw]

I cant seem to figure this out. If a benzene ring has 2 substituents 1) OH group 2) COOH group and we are attacking the electrophile carbon in the carbonyl CH3C=OOC=OCH3 (acet. anhydride), why is the O from OH rather than the O from the OH in COOH used? Both cant go through resonance, so what other differences would there be.

[Tartaglia]

Imagine attack by PhO- and RCO2-. The negative charge is more effectively delocalised across the two oxygens in the carboxylic acid, than the -ve charge in PhO- which is less effectively delocalised within the ring. In this case the larger effective negative charge on the phenoxide will be the most important factor in determining nucleophilicity

[lavoisier]

That would be true if you did this reaction in presence of at least 2 Eq of a suitable base; but when both OH and COOH aren't deprotonated, does the same explanation still hold?

 

Sometimes these chemoselective reactions are difficult to rationalise. o-aminophenol, for instance, reacts with some electrophiles at the oxygen in presence of Et3N (because you have O- and NH2 competing), and at the nitrogen in presence of pyridine (because you have OH and NH2 competing).

 

Here, with both groups still in the acid form, I think it's more a matter of electron density.

[Tartaglia]

There is a thermodynamic consideration here too as the ester will be the favoured product over the mixed anhydride