[Rotwang]

I have synthesized acetilsalicilic acid, and I'd like to know how would affect an aqueous hydrolisis, in order to a further structural analysis. Thanks very much!

[mike]

I presume you can hydrolise it in a basic solution (saponification). This is what happens in the body, acetylsalicylic acid convertid to salicylic acid. Check out this pdf:

http://www.personal.psu.edu/faculty/j/t/jtk1/aspirinlab%20S05.pdf

[Yggdrasil]

I wrote a procedure for the saponification of acetlysalicilic acid in another forum and it worked pretty well for the person who tried it.  Here's the link:

http://www.scienceforums.net/forums/showpost.php?p=176856&postcount=31

The person who did the experiment extracted the acetylsalicilic acid from Bayer aspirin using acetone and performed the saponification by heating the acetylsalicilic acid in a strong solution of aqeuous sodium hydroxide at a slight boil for about a minute.  Cooling the solution then adding dilute HCl cooled with ice caused the salicylic acid to precipitate out of solution.

[Rotwang]

Thanks very much for your help. This is exactly what I wanted to know. The pdf's been specially useful.

[mike]

Can anyone please tell me how many equivalents of NaOH are need to completely hydrolyse aspirin? I need to write the equation in order to do a back titration of aspirin tablets.

Following the forum rules here is my attempted answer: I think that it would be 1:1 under mild condtions and if the titration was performed quite quickly after the reaction. If the reaction were heated I think that it would be 2:1. What is the answer?

[Yggdrasil]

In theory, 2 equivalents are needed to hydrolyze aspirin.  The first equivalent will deprotonate the carboxylic acid portion of the molecule, while the next equivalent will hydrolyze the ester.  Under mild conditions, the hydrolysis will be unlikely to occur, so you would just observe a normal acid-base reaction.  The titration curve would look resemble the titration of a weak monoprotic acid.

In practice, you would use an excess of base to speed up the reaction, since lye is relatively cheap.

[mike]

Thankyou for the reply.

Ok so I did both titrations yesterday. First I did a simple titration of aspirin tablet with NaOH, this was performed quickly and at room temperature so I could make the assumption that it was a simple acid base reaction with little or no hydrolysis. Second, I heated the aspirin with excess NaOH then did a back titration with HCl.

In the first method there was a 1:1 relation and in the second there was a 2:1 relation