[Big-Daddy]

Suggest the relative orders of the 4 pKa values between cis-butanedioic acid and trans-butanedioic acid. i.e. each of the isomers can dissociate twice; of the therefore 4 acidic protons, they must be ranked in terms of acidity.

How do I go about this? Which makes it easier to dissociate, cis or trans, for each of the protons?

[sjb]

Suggest the relative orders of the 4 pKa values between cis-butanedioic acid and trans-butanedioic acid. i.e. each of the isomers can dissociate twice; of the therefore 4 acidic protons, they must be ranked in terms of acidity.

How do I go about this? Which makes it easier to dissociate, cis or trans, for each of the protons?

Compate the stability of the conjugate bases.

[Big-Daddy]

Compate the stability of the conjugate bases.

Of course, but how? I can't tell in what way one would be more stable than the other ... they're just geometrical isomers...

[sjb]

Compate the stability of the conjugate bases.

Of course, but how? I can't tell in what way one would be more stable than the other ... they're just geometrical isomers...

If you lose a proton from either, can the anion be stabilised?

[Big-Daddy]

I mistyped in my original post. Hopefully you realized this: the question should refer to cis-butenedioic and trans-butenedioic acid. Hence the cis and trans.

If you lose a proton from either, can the anion be stabilised?

It will be stabilized by conjugation with the electrons bound to the other O atom in the carboxylate group.  (I don't know whether there would be delocalization over the whole molecule, including the electrons in the double bond?) Anything else? I still haven't begun to see the difference between the two molecules as far as this question is concerned.

[sjb]

I mistyped in my original post. Hopefully you realized this: the question should refer to cis-butenedioic and trans-butenedioic acid. Hence the cis and trans.

That's fine, I had realised

It will be stabilized by conjugation with the electrons bound to the other O atom in the carboxylate group.  (I don't know whether there would be delocalization over the whole molecule, including the electrons in the double bond?) Anything else? I still haven't begun to see the difference between the two molecules as far as this question is concerned.

See perhaps http://www.everyscience.com/Chemistry/Organic/Acids_and_Bases/h.1179.php ?

[Big-Daddy]

See perhaps http://www.everyscience.com/Chemistry/Organic/Acids_and_Bases/h.1179.php ?

I see, thank you! Really clever explanation.