[limonade]

Sorry if this seems like low level chemistry, but why is it that para trifluoromethylphenol is  more acidic than para nitrophenol. I thought that resonance usually wins out over inductive effects. Is it just because there are three fluorines and the inductive effects are additive?

[limonade]

Also, my answer key says that para methoxyphenol is more acidic than para nitrophenol. I thought it should be the other way around since the methoxy group is electron donating, and through resonance decreases the positive charge character on the hydroxyl oxygen, making it less acidic than paranitrophenol which is electron withdrawing and increases positive charge character on hydroxyl oxygen, making it more likely to give up its hydrogen and thus be more acidic.

Am I just missing something?

[orgopete]

It may just be me, but whenever I begin to think how chemicals or atoms should behave, I am frequently wrong, so I take a different approach. Oh, p-trifluoromethylphenol is more acidic than p-nitrophenol. I didn't anticipate that. I guess I didn't know how effective an electron withdrawing group a trifluoromethyl would be.

[sario]

Para-trifluoromethylphenol is more acidic than para-nitrophenol because when an H is lost from OH group, the negative charge on the Oxygen atom is more delocalized through the ring in the case of Para-trifluoromethylphenol. This is because the inductive effect of the Flourine ( the most electronegative atom in the periodic table) is greater than the inductive and resonance effect of NO2 group. The real problem in these cases is that there isn't a way to establish what it is more acidic for example by a formula; you have to compare two things in a relative way.
I hope you will understand me, and sorry for my English

[Dan]

I am very skeptical of your source for the order of acidity of the phenols p-CF₃ > p-OMe > p-NO₂ in the answer key

Bordwell pKa (in DMSO) table lists the order of acidity as p-NO₂ > p-CF₃ > p-OMe
http://www.chem.wisc.edu/areas/reich/pkatable/

Wiliams pKa (in water) table lists p-NO₂ > p-OMe (no data for p-CF₃)
http://research.chem.psu.edu/brpgroup/pKa_compilation.pdf