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While studying organic chemistry, I came across two practice questions given by my professor:

Compare the acidic strength of:-

  • Maleic acid and Fumaric acid
  • o-nitrophenol and p-nitro phenol

And the answer and explanation were as follows- enter image description here

while for the maleic acid qustion (my query is only pertaining to the first dissociation so kindly ignore the rest):

enter image description here

to summarize the second attachment, what is meant is that maleic acid's conjugate base is stabilized by the hydrogen bonds formed between the carboxylate ion and carboxylic acid group.

If we were to approach the questions along the lines of the nitro phenol question, fumaric acid should've been more acidic as the intramolecular h-bonds ( with substantial bond energy of about 21 KJ/mol) in maleic acid would cause some hindrance in the disocciation the proton but that isn't the case, why is that so?

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1 Answer 1

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In the nitrophenol case, H bond stabilizes the o-nitrophenol acid that becomes less acidic.

In the dicarboxyethene case, the H bond stabilizes the carboxylate of maleinic acid, what makes it less basic conjugate base, therefore the acid is more acidic.

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