Home > Community > Why is benzoic acid (pKa=4.20) more acidic than acetic acid (pKa=4.76)?
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Lee Ballentine

Why is benzoic acid (pKa=4.20) more acidic than acetic acid (pKa=4.76)?

Deuter Hew  Follow

Benzoate ion (conjugate base) which we get after removal of Hydrogen ion from Benzoic acid is more stabilized due to resonance.

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Arthur Macgregor  Follow

The answer lies in the stability of the conjugate bases formed by these two acids.

We only need to compare the point of difference among the two:

Benzoic acid : Ph-COO-

The phenyl ring has a minus I (inductive effect ) and also pulls the -ve charge density on oxygen atom through resonance. this leads to the formation of a stable conjugate base. so benzoic acid is quite acidic.

Acetic acid : CH3COO-

The methyl group has a plus I (inductive effect ) and pushes the electron cloud towards oxygen which already has a negative charge. This destabilizes the molecule and thus the acidity of the molecule and thus acetic acid is less acidic.

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