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Acidity is dependent on charge dispersal. Greater the charge dispersal in conjugate base greater the acidity (assuming that both acids being compared have similar stablities).

So in the case of 4-Hydroxybenzaldehyde, in the conjugate base the charge would delocalise over two oxygen atoms as well as the benzene ring.

Whereas in formic acid charge would only delocalise over two oxygen atoms and hence charge would be concentrated. So the predicted order of acidity according to this logic should be formic acid < 4-hydroxybenzaldehyde.

This is contrary to actual data:

$\mathrm{p}K_\mathrm{a}(\text{4-hydroxybenzaldehyde}) = 7.61$ (Source),

$\mathrm{p}K_\mathrm{a}(\text{formic acid}) = 3.7$

Please put forth the correct logic.

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    $\begingroup$ You might want to take a look at similar problems with answers here: chemistry.stackexchange.com/questions/6155/… $\endgroup$ – Buck Thorn Feb 24 at 9:56
  • $\begingroup$ 4-Hydroxybenzaldehyde has a much lower $pK_a$ than phenol. Why? $\endgroup$ – Buck Thorn Feb 24 at 10:33
  • $\begingroup$ Aldehyde group disperses - charge in conjugate base of said compound. $\endgroup$ – Ramesh Agarwal Feb 24 at 10:36
  • $\begingroup$ Formic acid is a carboxylic acid, which is clearly more acidic since the negative charge in the conjugate base is dispersed over two electronegative oxygen atoms. $\endgroup$ – Tan Yong Boon Feb 24 at 10:57
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    $\begingroup$ @TanYongBoon, but so is the negative charge in the conjugate base of 4-hydroxybenzaldehyde. OP: It's not the number of atoms something is delocalised over which determines acidity, but rather how effective that delocalisation is. This is a recurring problem on SE and is explained more thoroughly in Night Writer's first link. $\endgroup$ – orthocresol Feb 24 at 11:20

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