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inductive effect and hydrogen bonding


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As you see in the first question -NO2 on ortho causes inductive effect and creates a hydrogen bonding with H in COOH and is considered to be stronger acid than others. In the second question, the -NO2 again shows I- and H bonding but is less acidic. I don't get how the latter one isnt the strongest. 

IMG_20210607_173005.jpg

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Posted (edited)

First are you sure you have the correct order of acidities ?

These are similar questions to the ones you asked last July but you are correct there is an additional effect with the possibility of H bonding.

Please check your acidities then study this reference and then come back with any further questions.

https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Chemistry_(McMurry)/20%3A_Carboxylic_Acids_and_Nitriles/20.04%3A_Substituent_Effects_on_Acidity

Edited by studiot
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1 hour ago, mundane said:

As you see in the first question -NO2 on ortho causes inductive effect and creates a hydrogen bonding with H in COOH and is considered to be stronger acid than others. In the second question, the -NO2 again shows I- and H bonding but is less acidic. I don't get how the latter one isnt the strongest. 

IMG_20210607_173005.jpg

I think this is quite tricky, at least for somebody as rusty as I am. 🙂

If it were not for the H bonding, I think one would expect the ortho-  substituted nitro group to increase the acidity the most, being closer.

The H bonding will, I should have thought, tend to weaken the acidity, by providing additional binding of the H atom. 

In orthonitrophenol the H bond setup is close to ideal, as it forms a 6 membered ring, so it can be a strong H-bond. So that could explain why the acidity is anomalously lower than the para version.

In orthobenzoic acid however, you have 7 atoms jostling to form the H-bonded ring, which will bend it out of planar and may make the H bond weaker. So perhaps the H bonding effect is not strong enough to disturb the usual order of effectiveness of the electron-withdrawing group. 

But I have to say, this looks like one of those things in chemistry that can be rationalised either way after the event, but can't really be reliably predicted just from theory. All a bit hand-wavy. 

But maybe @John Cuthber will have something to add (or tell me I'm wrong). 

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A further thought on this: H-bonds seem to need to be linear, i.e. Do-H............Ac, in a line. Also, they are directional, requiring the participation of a lone pair on the Ac atom.

In o-nitrobenzoic acid, it seems to be geometrically impossible to get (CO)O-H........O(-ON-Ph) in a line, with one of the nitro group oxygen lone pairs lined up the right way. So I wonder if, in fact, there is any H-bond at all in this molecule! 

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