[Swedish Architect]

I presume that the oxygen (double bonded to carbon) in carboxylic acids acts to cause this. Does it pull charge away from carbon, and by the inductive effect, pull electron density from the oxygen-hydrogen bond??
If the oxygen (double bonded to carbon) was on another atom, presumably the molecule wouldn't be acidic?

[Archer]

Please can you define "acidic"

[AlphaScent]

There needs to be clarification here.  As archer said, what do you mean acidic? Arrhenius? Lewis?  ect..?

Start with pKa and pH definitions.

A proton is almost always labile when heteroatoms are around.

[orgopete]

I presume that the oxygen (double bonded to carbon) in carboxylic acids acts to cause this. Does it pull charge away from carbon, and by the inductive effect, pull electron density from the oxygen-hydrogen bond??
If the oxygen (double bonded to carbon) was on another atom, presumably the molecule wouldn't be acidic?

I think you will find a similar effect, but reduced because oxygen is more electron withdrawing than nitrogen or carbon (oxygen has a larger nuclear charge). An amide RCONH2 will be more acidic than an R'NH2 and an RCOCH3 will be more acidic than an R'CH3.

[Arkcon]

There is a notable alcohol that is pretty acidic, by the Arrhenius definition.  Comparable to carboxylic acids.  Do you know which alcohol I mean?  It will give yo a big hint as to what makes the difference.

[ewangib]

I presume that the oxygen (double bonded to carbon) in carboxylic acids acts to cause this. Does it pull charge away from carbon, and by the inductive effect, pull electron density from the oxygen-hydrogen bond??
If the oxygen (double bonded to carbon) was on another atom, presumably the molecule wouldn't be acidic?

carboxylic acids are more acidic than corresponding alcohols due to resonance stabilisation of the oxygens on the carbocation when the proton is lost

[discodermolide]

I presume that the oxygen (double bonded to carbon) in carboxylic acids acts to cause this. Does it pull charge away from carbon, and by the inductive effect, pull electron density from the oxygen-hydrogen bond??
If the oxygen (double bonded to carbon) was on another atom, presumably the molecule wouldn't be acidic?

carboxylic acids are more acidic than corresponding alcohols due to resonance stabilisation of the oxygens on the carbocation when the proton is lost

When a proton is lost from a carboxylic acid you get a carboxylate anion, RCOO^-. So there is no carbocation.

[Kate]

Yep, carboxylic acids are more acidic from an Arrhenius perspective, because the negative charge is delocalized between the 2 oxygen atoms.

There is a notable alcohol that is pretty acidic, by the Arrhenius definition.  Comparable to carboxylic acids.  Do you know which alcohol I mean?  It will give yo a big hint as to what makes the difference.

I know the question wasn't directed at me, but I'll bite anyway.

First I thought about something like tert-butyl alcohol, but thinking about it some more it's probably something like trichloromethanol, if it even exists. 

[Corribus]

First I thought about something like tert-butyl alcohol, but thinking about it some more it's probably something like trichloromethanol, if it even exists. 

I'm guessing he's referring to phenol, AKA carbolic acid, which is acidic (Arrhenius definition) for the same general reason that carboxylic acids are.

[Kate]

I'm guessing he's referring to phenol, AKA carbolic acid, which is acidic (Arrhenius definition) for the same general reason that carboxylic acids are.

Yeah, phenol has a pKa of 9 but I don't get why it's so acidic relative to other alcohols. Aren't the pi bonding MO's in benzene fully occupied and so the negative charge on the oxygen would delocalize to pi anti-bonding orbitals?

[Archer]

Yeah, phenol has a pKa of 9 but I don't get why it's so acidic relative to other alcohols. Aren't the pi bonding MO's in benzene fully occupied and so the negative charge on the oxygen would delocalize to pi anti-bonding orbitals?

The -ve charge of phenoxide is resonance stabillised by the aromatic ring.

Look at the pKa of substituted phenols with electron withdrawing and electron donating substituents.

[Kate]

The -ve charge of phenoxide is resonance stabillised by the aromatic ring.

Look at the pKa of substituted phenols with electron withdrawing and electron donating substituents.

Phenols with electron withdrawing substituents lower pKa's, because then the delocalization of negative charge would be greater by the aromatic ring?

Yes, I know the explanation usually given is the stabilization of the negative charge by the aromatic ring, so oxygen doesn't get a full negative charge. But why doesn't this disrupt the aromaticity of benzene? And what happens to the C-C bond length in the ring when phenoxide is formed?

Hope I'm making my point clearly, sorry.

[Corribus]

It's true the negative charge will likely go into a delocalized antibonding orbital, and thus decrease the average C-C bond order, but so what?  This is still more favorable than the electron going into a localized antibonding orbital, or being stuck in one high energy atomic orbital.  If you are really curious about how this impacts the C-C bond order, you can do a simple Huckel treatment probided you have a little skill at matrix math or have access to a good math program like Mathematica.

[Kate]

It's true the negative charge will likely go into a delocalized antibonding orbital, and thus decrease the average C-C bond order, but so what?  This is still more favorable than the electron going into a localized antibonding orbital, or being stuck in one high energy atomic orbital. 

Yeah, you're right. I hadn't thought about it like that.

If you are really curious about how this impacts the C-C bond order, you can do a simple Huckel treatment probided you have a little skill at matrix math or have access to a good math program like Mathematica.

I am. Thanks for the suggestion. 

[cbn]

Using the definition of an acid as a "substance which donates protons (hydrogen ions) to other things", the carboxylic acids are acidic because of the hydrogen in the -COOH group.
In addition, electronegativeee substituents near the carboxyl group actt to increaseee thhe accidity..