Acidity strength upon adding hydroxyl group

[mourinho]

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Is acidity strength increased or decreased when a hydroxyl group is added adjacent to the OH group in 2-hydroxyphenyl acetic acid? In this case the acid functional group is not attached directly to the ring.

 

[aldol16]

Well, I doubt it will have much effect on pKa1 because the carboxylic acid functionality is attached at the benzylic and not at a phenyl position. Therefore, it is one carbon removed from being able to be stabilized/destabilized by anything on the ring and so even an electron-rich ring won't do much to destabilize the charge.

Even if you look at a comparable example - hydroxybenzoic acid as compared to dihydroxybenzoic acid - where the carboxylic acid is attached at a phenyl position, the pKa's are comparable. This is because the phenyl ring also does not really stabilize the negative charge developed on the oxygen - that charge stays on the carboxylic acid.

 

[mourinho]

Well, I doubt it will have much effect on pKa1 because the carboxylic acid functionality is attached at the benzylic and not at a phenyl position. Therefore, it is one carbon removed from being able to be stabilized/destabilized by anything on the ring and so even an electron-rich ring won't do much to destabilize the charge.

Even if you look at a comparable example - hydroxybenzoic acid as compared to dihydroxybenzoic acid - where the carboxylic acid is attached at a phenyl position, the pKa's are comparable. This is because the phenyl ring also does not really stabilize the negative charge developed on the oxygen - that charge stays on the carboxylic acid.

The pKas of the above you mentioned are comparable but since I am concerned about the minor details, dihydroxybenzoic acid has a slightly higher pKa because the hydroxyl group tends to destabilise the ring and this might as well occur for the 2-hydroxylphenylacetic acid?

 

[aldol16]

The pKas of the above you mentioned are comparable but since I am concerned about the minor details, dihydroxybenzoic acid has a slightly higher pKa because the hydroxyl group tends to destabilise the ring and this might as well occur for the 2-hydroxylphenylacetic acid?

Yes - the hydroxy group is net donating when on a phenyl ring and so pushes electron density onto the ring. This partial negative charge tends to make losing the carboxylic acid proton less likely since that would put two negative charges close to each other, which is unfavorable due to Coulombic considerations. But when you're talking about the acetic acid analog, this effect is much less pronounced because the carboxylic acid is not on the phenyl ring but rather on a benzylic site. Since the pKas for the benzoic acids are so similar, I doubt you would be able to measure an effect for the acetic acid.

 

[7331poas]

Why make the questions debate able like this? Obviously it will increase the pka, the question is whether it will do it "much"

 

[aldol16]

Why make the questions debate able like this? Obviously it will increase the pka, the question is whether it will do it "much"

Well, MCAT questions will generally not be debateable like this. They go through to make sure it's not debateable. Because if you know pre-meds, anything debateable would cause an uproar. In this case, it's not obvious at all that it will increase pKa. In fact, it likely does not. What I mean is that you would be not be able to measure a difference in pKa that cannot also be fully accounted for by error in your measurement.

 

[7331poas]

Well, MCAT questions will generally not be debateable like this. They go through to make sure it's not debateable. Because if you know pre-meds, anything debateable would cause an uproar. In this case, it's not obvious at all that it will increase pKa. In fact, it likely does not. What I mean is that you would be not be able to measure a difference in pKa that cannot also be fully accounted for by error in your measurement.

This issue is what instrument you are using. I dunno, I just really dont like the question

It will definitely raise the pka a finite amount. I agree that it probably wouldnt be detectable. But....

 

[aldol16]

This issue is what instrument you are using. I dunno, I just really dont like the question

It will definitely raise the pka a finite amount. I agree that it probably wouldnt be detectable. But....

I definitely agree that I don't like the question. The problem is that at some point, the effect is negligible or non-existent. For instance, one could imagine that a planet on the other side of the universe has an infinitesimally small but finite gravitational effect on Earth. We just never take it into account and say it has no effect because it's essentially nothing and couldn't be measured.

To use a more relevant example, most HIV treatments don't usually result in a "cure." But the effect of many good ones is to reduce viral load to the point where it's no longer detectable. So while one could still very well have the virus, it's just not at a detectable level - to the patient, he/she is effectively cured.