Arrheniusthegreat
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Balanced equation for ASA --> SA
Could someone please help me with how I would write the balanced equations for Acetylsalicylic acid dissolved in NaOH and heated. Then I separate the
Sodium acetate and pour the solution of salicylate into HCl for it to crash out the salicylic acid. I'm having trouble figuring this. and I just keep
getting stuck. Thanks.
The clearest way into the Universe is through a forest wilderness. - John Muir
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Prometheus23
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Edit: kavu's explanation was just better, and in the interest of keeping this thread clear an concise I have deleted my post.
[Edited on 2/11/2013 by Prometheus23]
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kavu
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Quote: Originally posted by Prometheus23  | | When acetylsalicylic acid is treated with hydroxide solution the base will first remove the most acidic hydrogen present, which in this case is the
carboxylic acid hydrogen. This will form the negatively charged acetylsalicylate ion as well as one molecule of water. So this first step consumes one
molecule of hydroxide. Then a second molecule of hydroxide will cleave the acetyl group to create one molecule of acetate ion and one molecule of
salicylate, the deprotonated form of your final product salicylic acid. |
Actually 3 equivalents is needed. First (a) as you stated the carboxylic acid will be converted to a salt. Secondly one OH- will attack acetyl side
chain cleaving sodium acetate (b). There will still be the acidic phenolic hydrogen left and it is cleaved by third hydroxide (c):
(a) CH3COO-C6H4-COOH + NaOH → CH3COO-C6H4-COONa + H2O
(b) CH3COO-C6H4-COONa + NaOH → HO-C6H4-COONa + CH3COONa
(c) HO-C6H4-COONa + NaOH → NaO-C6H4-COONa + H2O
So as a net reaction a+b+c:
CH3COO-C6H4-COOH + 3 NaOH → NaO-C6H4-COONa + CH3COONa + 2 H2O
And upon acidification (it should be noted that some of the leftover sodium acetate is likely to react as well so an excess of acid is needed):
NaO-C6H4-COONa + 2 HCl → HO-C6H4-COOH + 2 NaCl
[Edited on 11-2-2013 by kavu]
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Arrheniusthegreat
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You guys are awesome. This has helped me tremendously. It's actually clear to me now.
but I must get back, I'm late for an important date. Till next time, A.
The clearest way into the Universe is through a forest wilderness. - John Muir
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Arrheniusthegreat
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Oh and another before it gets even later, Would I then make a corresponding 3 M solution of NaOH and then a 2 M soln of HCl for these processes?
Edit:
or say I get my crystalline solid ASA, could I just add it directly to a 3M soln of NaOH? Or should I say for instance, dissolve 6.2mol ASA into say
1025ml IPA to make a 6M solution, then add a 9M soln of NaOH to it? or how would I do that? I apologize for my confusion
[Edited on 11-2-2013 by Arrheniusthegreat]
The clearest way into the Universe is through a forest wilderness. - John Muir
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kavu
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It dependes on how much aspirin you wish to hydrolyze. For each 1 gram of ASA (5.55 mmol) you need 0,67 grams of NaOH (3*5.55 mmol). The exact
concentration is not all that important. During the acidification I would just use conc. HCl and add it dropwise with stirring until no further
precipitation can be seen.
[Edited on 11-2-2013 by kavu]
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Arrheniusthegreat
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Alrighty, thanks. Could 35% be considered concentrated? I've tried gassing a water solution to saturation before and it didn't seem to be able to
achieve full saturation. Like it absorbed it for a while then it stopped absorbing as if it had reached saturation, but then when I regassed moments
later, it started absorbing again for what seemed like forever. I could never even get it to the putrid looking color of muriatic acid.
The clearest way into the Universe is through a forest wilderness. - John Muir
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barley81
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35% HCl is concentrated. You can use that.
Alternatively, you could use acid-catalyzed hydrolysis instead. You take some dilute sulfuric acid (15mL of concentrated acid in a half-liter of
water) and boil aspirin in it for a few hours. Then, you cool the mixture to 0C and collect your product via filtration. Salicylic acid is sparingly
soluble in water at 0C, while acetic acid is miscible with water. I've done this before, but it produces a stink of acetic acid, so work outside or in
a fume hood. It is much cheaper.
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