Acidity of carboxylic Acids

[5words]

When it comes to acidity, Resonance Trumps Inductive effect.. Now if you look TPR workbook OChem
Passage 19 Q1 which is : which of the following carboxylic acids would be the most acidic?

- Ethanoic Acid
-2 Chloro Ethanioc Acid
- 3 Chloro Ethanoic Acid
- Benzoic Acid

I chose D because of the resonance provided benzoic acid but the answer is actually B??? So why is that?? Why is inductive effect overcoming resonance here?

And also btw, it's not a mistake from TPR since the PKA of 2-Chloro Ethanoic Acid is indeed lower than that of Benzoic Acid.

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[aldol16]

It's not. Draw out the "resonance" forms for benzoic acid. You'll quickly see that the negative charge that exists after deprotonation cannot be delocalized into the benzene ring. It can only be delocalized onto the other oxygen atom. This is the same delocalization you would see with ethanoic acid or any other carboxylic acid. 2-chloroethanoic acid has that same kind of delocalization and in addition, also has the inductive pull of the chlorine.

 

[5words]

It's not. Draw out the "resonance" forms for benzoic acid. You'll quickly see that the negative charge that exists after deprotonation cannot be delocalized into the benzene ring. It can only be delocalized onto the other oxygen atom. This is the same delocalization you would see with ethanoic acid or any other carboxylic acid. 2-chloroethanoic acid has that same kind of delocalization and in addition, also has the inductive pull of the chlorine.

If it's not stabolized by resonance then why is it more acidic than acetic and propanoic acid? (pka = 4.75 and 4.87 respectively) vs benzoic acid (pka= 4.20). If the resonance , as you have said , is the same amongst the three compound, shouldnt it (benzoic acid) have the highest pka because of benzene size?

 

[aldol16]

If it's not stabolized by resonance then why is it more acidic than acetic and propanoic acid? (pka = 4.75 and 4.87 respectively) vs benzoic acid (pka= 4.20). If the resonance , as you have said , is the same amongst the three compound, shouldnt it (benzoic acid) have the highest pka because of benzene size?

First off, if it were resonance-stabilized, you would expect to see a larger effect on acidity (greater than half a pH unit).

Second, the phenyl group is electron-withdrawing by the inductive effect as opposed to alkyl groups, which are electron-donating. All of the carbons in the phenyl ring are sp2-hybridized, meaning that they are more electronegative. This is because electrons in s-orbitals are very tightly held (no node at nucleus) and as s-character increases, so does electronegativity. Therefore, a slightly electron-withdrawing phenyl group will better stabilize the conjugate base than an electron-donating alkyl group.

Third, size does not matter when talking about acidity. Size is a kinetic property and not a thermodynamic one. Acidity is a thermodynamic property. Size would only matter if you were talking about, say, the nucleophilicity of phenol. Since acidity is a thermodynamic property and not a kinetic one, it is not affected by kinetic phenomena.

 

[5words]

First off, if it were resonance-stabilized, you would expect to see a larger effect on acidity (greater than half a pH unit).

Second, the phenyl group is electron-withdrawing by the inductive effect as opposed to alkyl groups, which are electron-donating. All of the carbons in the phenyl ring are sp2-hybridized, meaning that they are more electronegative. This is because electrons in s-orbitals are very tightly held (no node at nucleus) and as s-character increases, so does electronegativity. Therefore, a slightly electron-withdrawing phenyl group will better stabilize the conjugate base than an electron-donating alkyl group.

Third, size does not matter when talking about acidity. Size is a kinetic property and not a thermodynamic one. Acidity is a thermodynamic property. Size would only matter if you were talking about, say, the nucleophilicity of phenol. Since acidity is a thermodynamic property and not a kinetic one, it is not affected by kinetic phenomena.

wow, thanks dude! The second point really hammered it for me.

Lastly, as a rule would you say that Phenyl are 95% of the time electron withdrawing? and that the same logic can be applied to benzyl?

 

[aldol16]

Lastly, as a rule would you say that Phenyl are 95% of the time electron withdrawing? and that the same logic can be applied to benzyl?

Phenyl is known to be electron-withdrawing. Whether aryl is electron-withdrawing depends on the nature of the substituents and where they are located. Benzyl is trickier because there is an alpha carbon and depending on what kind of chemistry you're trying to do, there could be delocalization. For instance, the protons attached to the benzylic carbon are acidic because of resonance delocalization.

 

[BigBrent25]

"ARIO" Priority A,R,I,O
A- atom
R- Resonance
I- inductive effects
O- orbital
*These pertain to MOST but not all cases.