o-nitrophenol vs m-nitrophenol? Who's more acidic?

[drzakisadiq]

Can you guys tell me the reasoning why o-nitrophenol is more acidic than m-nitrophenol? I had a feeling that it was o-nitrophenol simply because I thought it would have a greater effect on the phenol from the ortho position than the meta, but I don't know if that's a valid reason. Am looking for the mechanics behind this. Thanks guys.

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[PooyaH]

I think o-nitrophenol is the least acidic due to strong intramolecular hydrogen bonding. The hydrogen of OH would be affected by its interaction with with the oxygen on NO2 which is electronegative. Basically, ortho can't donate H+ easily. I think the order of increasing acidity should be ortho<meta<para

 

[PooyaH]

haha this is funny! Now that I thought about it more I think it should be m<o<p because only ortho and para can make resonance! So I think ortho is more acidic than meta but definitely less acidic than para!
Somebody help! 😀

 

[AdaAda87]

Ortho Nitro is more acidic due to resonance, as meta nitro would have no resonance. If it was para and orth, Para would be more acidic.

 

[doc3232]

ya, ortho is more acidic definitely
however, m-nitrophenol is more acidic than phenol also.

 

[mddang]

Hmmm.. I always thought that ortho would be more acidic due to the stronger inductive effect (The ortho position is closer to the OH than the para position.)

 

[doc3232]

Hmmm.. I always thought that ortho would be more acidic due to the stronger inductive effect (The ortho position is closer to the OH than the para position.)

*Pulling out by old O-chem book*
I got nothing...
What you said makes sense, but also I think if the nitro was ortho then the Oxygen (from the NO2) may stabalize and hold the Hydrogen in not letting it deprotonate.
Basically, I think ortho is less acidic because intramolecular Hydrogen bonding holds the proton.
Anyone have pH values...my book didn't.

 

[PooyaH]

*Pulling out by old O-chem book*
I got nothing...
What you said makes sense, but also I think if the nitro was ortho then the Oxygen (from the NO2) may stabalize and hold the Hydrogen in not letting it deprotonate.
Basically, I think ortho is less acidic because intramolecular Hydrogen bonding holds the proton.
Anyone have pH values...my book didn't.

7.14 (para),
7.23 (ortho)
8.35 (meta)

 

[klutzy1987]

To understand the asnwer to this question, you have to understand how a nitro group work and why it increases acidity. First, a nitro group will inductively pull electros away from a molecule. This would stabilize a negative charge and destabilize a positive charge. Now if you think about it, the charge is when using a nitro group is always on the carbon that is 1 away from the carbon where the substituent resides (the charge is in the ortho osition while the substituent attacks in the meta position). This is because the nitro will destabilize the positive charge and if a substituent would attack ortho the charge will fall directly under the NO2 group. However a negative charge will be welcomed on the carbon under the NO2. Therefore a positive charge 2 carbons away is better than 1 carbon away because trhrough resonance the charge 2 carbons away can shift directly under the NO2. So when you have a negative charge on the ortho or Para OH group, it is similar to a poitive charge that is 2 carbons away, which is stabilized much better than a poisitive charge one carbon away. Basically the O- takes the place of a carbon when considering resonance.

Any questions just post them and ill try and help. This is not an easy topic to understand or explain but illl do my best.

 

[klutzy1987]

I spent to much time thinking this through and typing it for noone to read it.

 

[klutzy1987]

Someone at least tell me if what i said here makes sense.

 

[td4azklz]

Someone at least tell me if what i said here makes sense.

I understood you klutzy. So, the para positions IS more acidic, right? because of the pull......

 

[futuredent22]

Someone at least tell me if what i said here makes sense.

I understand what you are trying to say, but its hard to explain something like this without being able to draw the diagram and show it.

If anyone still doesnt understand, just take the -H off the phenol and draw the resonance structures. You'll see that you need the nitro group in ortho position to be able to send electrons to the N and form a C=N bond.

 

[td4azklz]

I understand what you are trying to say, but its hard to explain something like this without being able to draw the diagram and show it.

If anyone still doesnt understand, just take the -H off the phenol and draw the resonance structures. You'll see that you need the nitro group in ortho position to be able to send electrons to the N and form a C=N bond.

Ahhh, i see it now!! Thanks

 

[klutzy1987]

I understand what you are trying to say, but its hard to explain something like this without being able to draw the diagram and show it.

If anyone still doesnt understand, just take the -H off the phenol and draw the resonance structures. You'll see that you need the nitro group in ortho position to be able to send electrons to the N and form a C=N bond.

Yup, I was just trying to explain how the resonance works, but it is really difficult without verbally explaining it to someone lol.

 

[doc3232]

I understood you klutzy. So, the para positions IS more acidic, right? because of the pull......

No, para is more acidic because when the nitro group is ortho, there is Hydrogen bonding. I explained it earlier...
Anyway, the fact that ortho is closer than para means nothing. Resonance is not at all affected by this and either will acidity.
You can't say one is more acidic than the other because one is para and the other is ortho.
It will depend each time on steric hindrance and in this case intramolecular bonding (the O from nitro group bonding to the H and not "releasing" the Hydrogen to the solution, which would have lowered the pH).