English Deutsch Français Italiano Español Português 繁體中文 Bahasa Indonesia Tiếng Việt ภาษาไทย
All categories

What are the reasons for the acidic nature of acetylsalicylic acid (aspirin)??

2007-01-27 05:01:42 · 3 answers · asked by Pooh 4 in Science & Mathematics Chemistry

It's formula is C6H4(OH)CO2H.

2007-01-27 05:41:08 · update #1

We all know about wikipedia, yeraluzer, but what I was looking for is not there. If it was there, I wouldn't be asking an idiot like you.

2007-01-27 06:30:56 · update #2

Thanks for your answer, Le Scientist. So the proton is what makes aspirin acidic?

2007-01-27 07:13:00 · update #3

3 answers

*** Edited. See Bottom. If you have any other questions, just ask :)
Acetylsalicyclic acid is a carboxylic acid. The carboxylic acid side chain is given by COOH (or CO2H) If you want to be more technical, acetylsalicylic acid is a substituted benzoic acid. in a carboxylic acid there is one oxygen that is doublebonded to that carbon and another oxygen that is single bonded to both that carbon and a hydrogen. That proton is relatively acidic. There are several reasons for this, including induction (the C=O takes electrons away from the O-H bond) and the stable resonance structure of COO-. The reason that COO- is so stable is because the negative charge is delocalized between the two oxygens.Because of the proximity of the carboxylic acid group to the aromatic ring, there are many other possible resonance structures that are used to stabilize the negative charge, however I will not include them. They are probably more than you need to know. There will also be a possible inductive effect from the aromatic ring as well, drawing the electrons towards it thereby making it even more acidic than, for example, acetic acid.
Another very interesting thing to note is the subsitution of the aromatic ring. If you look at it, it is ortha substituted. The oxygen of the ester is in close proximity to the carboxylic acid group. Because the oxygen is electronegative, and has the two pairs of lone electrons, it will somewhat pull the hydrogen away from the carboxylic acid, making it even more acidic. You have a sort of intramolecular hydrogen bonding that occurs. This may or may not also occur with the carbonyl of the ester group. I am unsure. Furthermore, that ester connected to the aromatic ring may contribute to a resonance form, or may have an inductive effect on the aromatic ring which in turn has an effect on the carboxylic acid group.
You can observe the increase of pKa between a lone carboxylic acid group such as that of acetic acid, and one near an aromatic ring such as benzoic acid. The difference between the two acidities can be explained by the phenomenon that I have included above.
Acetic acid : pKa 4.75
Benzoic acid : pKa 4.20
You can see the effect of that side group on the benzoic acid by comparing the pKa of benzoic acid to that of acetylsalicylic acid.
Benzoic acid : pKa 4.20
Acetylsalicylic acid: pKa 3.5
If you look at the picture I have made, you can see what I am talking about. The arrows indicate electron movement.
I really hope this answers your question.
http://i176.photobucket.com/albums/w173/lescientist/acetylsalicylicacid.gif
If it does not answer your question please add another comment to your question, and I will try to include the necessairy information/clarify any points that I did not make clear enough.

***
Yes, it is because that proton is easily lost that Acetylsalicylic acid is acidic. All protons have a pKa. The carboxylic acid protons are lost quite bit easier than those of say... a standard alkane. When put in water, acetylsalicylic acid will lose that proton, and therefore make the solution acidic.

2007-01-27 07:01:39 · answer #1 · answered by Le Scientist 2 · 1 0

Hmm... well, I don't know the chemical formula, but if it has hydronium (H3O), then that causes acidity.

2007-01-27 05:26:35 · answer #2 · answered by Chrissy 3 · 0 0

someone needs to tell the rest of the world abot wikpedia, I seem to be the only one who knows:

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

2007-01-27 05:58:50 · answer #3 · answered by yeraluzer 4 · 0 2

fedest.com, questions and answers