Acidic Strength of Benzoic Acid and Propanoic Acid [duplicate]








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[Why is benzoic acid a stronger acid than acetic acid?](/questions/41830/why-is-benzoic-acid-a-stronger-acid-than-acetic-acid)

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I want to know that why Benzoic Acid is more acidic than Propanoic acid. It should have to be less acidic, if we will observe the stability of conjugate bases. Because if you will look to the stability of Benzoate ion, then it is highly unstable : (See the image attached below)
  

  

[![https://www.quora.com/Which-is-more-acidic-propanoic-acid-or-benzoic-acid-and-why#:~:text=Benzoic%20acid%20is%20more%20acidic,stability%20of%20its%20conjugate%20base.](https://i.stack.imgur.com/APRbR.png)](https://i.stack.imgur.com/APRbR.png)


  

  

Now, you can also see that Phenyl ring is increasing the Electron density and the Aromaticity of the ring is also hampered. So, it will now become highly unstable, and due to this the rate of formation of benzoate ion will be very less. And in case of Propanoic Acid, there is no +M effect. So, there will be relatively high stability, only the +H effect of Ethyl group will increase the electron density. 
  

Please check it, and tell me where I am neglecting any other factor. Because if we look at the pKa value then, we will observe that Benzoic Acid is stronger than Propanoic Acid. Can anyone explain me why ? Please !!
 

In the last canonical structure you show , the aromaticity is even though hampered , but it don't makes it highly unstable in any sense, as the ion is still non-aromatic instead of anti-aromatic.
By drawing all canonical forms of both acids, you will see that benzoate ion is more stable as it's a hybrid of more number of contributions than propionic acid.


Also , note that the inductive effect also destabilises the propanoate ion .


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