What makes anyhydrides acidic?

[reising1]

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What makes an anhydride acidic? Is it the alpha proton on either side of the carbonyls?

What if it's an acid anhydride where the two "R" groups on the two carbonyls are just protons? So two aldehydes bounded by an oxygen.

 

[navho77]

What makes an anhydride acidic? Is it the alpha proton on either side of the carbonyls?

What if it's an acid anhydride where the two "R" groups on the two carbonyls are just protons? So two aldehydes bounded by an oxygen.

Yes, it would be the alpha protons. What makes the molecule particularly acidic is, as you probably know, the resonance situation of the conjugate base. The anhydride with alpha R groups has an extensive pi system over which to dissipate the negative charge of its conjugate base. But, if you try to draw out resonance structures for the formic anhydride molecule you described, you'll see that there is not nearly as favorable an environment in which to push around excess negative charge, and so the protons on the carbonyls will not be particularly acidic

 

[reising1]

Perfect, that's what I thought, thanks!

 

[PlumbLine]

It is very reactive relative to most other carbonyls. When placed in water it easily acts as an electrophile, effectively forming carboxylic acid, which has a pKa ~5.

You can usually just treat it the same way you would an acid halide.