Chemistry - acids

Lebkuchen

why is carboxylic acid a stronger acid than an alcohol?

Reply 1

Methene

I would guess it's just because of the delocalisation in the carboxylate anion, which affords it stability.

Reply 2

Kallisto

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Original post by Lebkuchen

why is carboxylic acid a stronger acid than an alcohol?

Carboxylic acid has the carboxyl group (-COOH) as functional group, an alcohol the hydroxyl group (-OH)

I am not sure, but it has maybe to do with the inductive effect: according to this effect, the electrons are stronger attracted by the carboxyl group than the hydroxyl group. Certainly because the elimination of H-atom in carboxyl group leads to mesomerism, so to a state in which the electrons are moving through the bonds (intramolecular electron motion).

But to get maybe a better explanation than mine, I call you @5hyl33n.

(edited 8 months ago)

Reply 3

charco

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Original post by Methene

I would guess it's just because of the delocalisation in the carboxylate anion, which affords it stability.

This is correct

Reply 4

Kallisto

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Original post by charco

This is correct

So it has to do with the mesomeric effect of this functional group that this one is stronger than the hydroxyl group?

Reply 5

charco

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Original post by Kallisto

So it has to do with the mesomeric effect of this functional group that this one is stronger than the hydroxyl group?

No.

The conjugate base is relatively stable due to delocalisation of the negative charge over the three atoms of the carboxylate group. Both C-O bonds have the same length and strength indicating that there is a delocalised molecular orbital stretching from oxygen to oxygen atom. Spreading out the negative charge lowers the energy of the system and stabilises the anion.

If you compare this to the ethoxide ion, there is nowhere for the negative charge to go and consequently ethanol has a very high pKa value (and is only very very weakly acidic)

pKa = 15.9 far too high to be considered an acid.
Ethanol does not undergo typical acid reactions and cannot liberate carbon dioxide from hydrogencarbonates.

Reply 6

Kallisto

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Original post by charco

No.
The conjugate base is relatively stable due to delocalisation of the negative charge over the three atoms of the carboxylate group. Both C-O bonds have the same length and strength indicating that there is a delocalised molecular orbital stretching from oxygen to oxygen atom. Spreading out the negative charge lowers the energy of the system and stabilises the anion.

If you compare this to the ethoxide ion, there is nowhere for the negative charge to go and consequently ethanol has a very high pKa value (and is only very very weakly acidic)

pKa = 15.9 far too high to be considered an acid.
Ethanol does not undergo typical acid reactions and cannot liberate carbon dioxide from hydrogencarbonates.

Thank you for your explanations and the pictures, that helped a lot and was very informative. I learnt that an electron motion within a molecule or delocated electron is not mesomerism automatocally.

(edited 8 months ago)

Reply 7

username5887583

Could it be the inductive effect of the alkyl group pushes electron density further into the oxygen atom, further increasing the bond strength of the O-H bond, also if the alcohol does lose a proton the alkoxide ion formed is unstable since the charge is localised on the oxygen atom, no resonace is possible, whereas carboxylate anion is resonance stabilised......just some random thoughts!!