Exception of Exceptions!
What's the order of acidic strength of the following (most acidic to least )
-
Benzoic acid
-
m-Amino Benzoic acid
-
o-Aminobezoicacid
-
p-Aminobezoicacid
This is a part of my set Aniket's Chemistry Challenges . (BEST OF JEE - ADVANCED)
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2 solutions
This is due to zwitter.ion effect which you must have learnt in amino acids COOH AND NH2 GROUP remain as COO- AND NH3+ AND THEY ARE ATTRACTED BY ELECTROSTATIC FORCE
Is there a wiki for that
Maybe due to H bonding in ortho so acidic H not easily released
Hey puneet, I don't know whether I am completely right but I had studied it somewhere that -COOH is not strong enough to form a Zwitter ion
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I guess you are getting it wrong , you can take the well known example - Amino Acids as a proof
i read somewhere p-amino benzoic acid doesn't form zwitter ion but p-amino benzenesulphonic acid form it
I believe there is something wrong with your explanation.
Zwitter ion is the reason that ortho effect is not working(otherwise it is 3214). And chelation occurs to depress the acidity of benzoic acid as the acidic H with NH3 + is trapped in it.
But it is surprising that m-amino benzoic acid is more acidic than p-amino benzoic acid since in zwitter ionic form, -COO - group will have an +I effect which decreases with distance. Also, if somehow(it usually isn't so) it has a -M effect then too p-amino benzoic acid looks like more acidic. So it should be 1423
Moreover, it feels like anthranilic acid is in zwitter ion form, m-Amino benzoic acid as it is, p-Amino Benzoic acid as it is (such that -I v/s +M explains the effect) but then Benzoic acid will be less acidic than m-amino benzoic acid. So it should be 2143.
And there is so much confusion. So, where is the data? Support your answer with data. I could not find it.
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@Aniket Sanghi – If the data is so, then it is definitely not important for JEE. JEE doesn't test knowledge(so close data). Not even chemists can guess that.
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@Kartik Sharma – I don't think so , and can't gurantee it either !
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@Aniket Sanghi – Well, if that is so, please care to tell what is wrong in my explanation(and which one).
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@Kartik Sharma – Ortho explained by zwitter effect . In other cases I don't think zwitter tend to dominate because of geometry . When we see amino + cooh we get a combined acid + basic nature ( net acidic) therefore benzoic acid ( which is alone ) do lead . Then m and p follow the trend of +M and -I effect considerations!
what's your jee advanced rank?
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Result is on 11th June !
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oh , sorry your marks
I see it this way. 3,4,1,2.
in the mechanism of reaction of the orthoamino. You need to release a hydrogen so that the molecule stabilizes and the imine is formed. for the 4-aminobenzoic compound is found in a pKa 3.5. I find more acid to the benzoic acid, because the m-aminobenzoic acid when deprotating does not interact the other substituent for the stabilization (it does not break with the aromaticity) so there is only delocalisation of charge in the aromatic ring.
http://sgpwe.izt.uam.mx/files/users/uami/jrvc/QA 1/apendice F pKa.doc http://www.merckmillipore.com/MX/es/product/4-Aminobenzoic-acid,MDA CHEM-100102?ReferrerURL=https%3A%2F%2Fwww.google.com.mx%2F
I found this data online:
pKa values (at 298K) :
Benzoic acid = 4.20 o-amino benzoic acid = 4.95 m-amino benzoic acid = 4.73 p-amino benzoic acid = 4.89
This order should be 1,2,4,3.
Source: http://www.chegg.com/homework-help/pka-values-ortho-meta-para-substituted-benzoic-acids-shown-r-chapter-18-problem-87p-solution-9780134066585-exc
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I guess the answer itself is 1243 !!!
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Actually, I was actually adding credibility to your answer and in these days I've come up with my explanation. This is it:
First of all, due to the basic nature, all the amine substituted benzoic acids are less acidic than benzoic acid.The basic nature causes the carboxylic group to readily donate a proton to the amine group forming a stable dipolar ion. Next, the ortho is put at the last due to the close proximity of these two groups, the hydrogen is easily donated to the amine group and thus the acidity of the ortho one is least. Next, for para and meta-substituted ones, I think due to the distance, the effect of formation of dipolar ions is less than the characteristic (+R) effect of amine at ortho and (-I) effect at meta positions. Thus meta takes the lead over para isomer.
Tell me if this is a valid explanation.
Hey I xplained it some way. I didnt knew this exception. Do you guys want me to share my theory? Its completely theoretical.
Is it possible that ortho one has H bonding that prevents easy release of H+
Determine who has the most ACIDIC hydrogen ....and how easily they can let it go ...!!! and then your result will be dependent on the effect by amino groups attached to the ring ...{and their postions } (ortho >meta >para..) *
This is a very important exception for JEE in my opinion !
We had learned to compare the acidity of substituted benzoic acid . Also, we had learned to compare bascicity of substituted Aniline .
But this one is a Combo pack !!
I Have tried to derive an explanation for this trend -
Due to NH2 group the basic nature is also seen , therefore 1st is benzoic acid .2nd and 3rd position follow normal trend.
But I have not yet found satisfactory explanation for Ortho group in last position . If someone gets a nice explanation , Please share !!!
Also, do reshare the question to let it reach to all!!