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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Morhas on September 21, 2008, 07:45:01 PM

Title: Different acidity in geometric isomers
Post by: Morhas on September 21, 2008, 07:45:01 PM

I can't seem to figure out why they have different pH levels. Our class tested cis-butendioic acid and trans-butendioic acid and one had a pH of 4 while the other had a pH of 1.

I thought that since we made saturated solutions of both, and we were able to dissolve more cis than trans, would that be why cis had a higher pH than the trans?

Other things I've heard is that since the functional groups are closer to each other on the cis, it is more reactive and more acidic? Doesn't seem like a very good reason to me lol.

Any help would be appreciated. ???

Title: Re: Different acidity in geometric isomers
Post by: Morhas on September 21, 2008, 10:24:21 PM

I can't find the answer to this anywhere  :(

If someone could help me out that would be really appreciated. Need to know this for tommorow  :(

Title: Re: Different acidity in geometric isomers
Post by: Astrokel on September 22, 2008, 03:04:38 AM

Quote from: Morhas on September 21, 2008, 07:45:01 PM

I thought that since we made saturated solutions of both, and we were able to dissolve more cis than trans, would that be why cis had a higher pH than the trans?

Other things I've heard is that since the functional groups are closer to each other on the cis, it is more reactive and more acidic?

hello, your sentences seem contracdicting, cis more acidic but it has higher pH? but you are quite right in what you have heard, its due to the 2 functional group relative position. cis form also known as maleic acid is more acidic in its first dissociation proton than trans(fumaric), but the acidity of second proton is reversed. This is because of intramolecular H-bonding formed within the conjuagte anion of maleic after first dissociation of proton which is stabilised to a greater extent than fumaric. In the second dissociation, you notice for cis, a two negative anion species close to each other is not as favourable as fumaric, the trans formed(the negative species is further away), hence the trans formed is more acidic than cis in the 2nd dissociation.

hope this helps some..

Title: Re: Different acidity in geometric isomers
Post by: Borek on September 22, 2008, 03:27:42 AM

http://en.wikipedia.org/wiki/Maleic_acid

http://en.wikipedia.org/wiki/Fumaric_acid

Title: Re: Different acidity in geometric isomers
Post by: Morhas on September 22, 2008, 02:22:20 PM

Thanks for the post Astrokel. That did help clear it up.

@ Borek: I already did a thorough search before posting, and the wikipedia links were the first things I read. The difference in pH level weren't addressed on those pages as far as I saw. Thanks though.