Sciencemadness Discussion Board

H2SO4 consentration for esters?

neo_90 - 29-6-2006 at 14:46

Hi!

I just wonder what concentration the H2SO4 has to be, to be able to catalyze esters? the esters are just for fun, and will have no specific use..

guy - 29-6-2006 at 18:45

Concentrated acid is used. All the other reactants are usually very pure without a lot of water. I think the reactants should not have a lot of water. Does anyone know if water slows down the reaction?

[Edited on 6/30/2006 by guy]

Darkblade48 - 29-6-2006 at 20:31

Well, you're most likely doing a Fischer esterification (alcohol + carboxylic acid), the reaction is actually an equilibrium, with water being produced along with the ester. If water was present, it would force the equilibrium back towards the alcohol + carboxylic acid side.

neo_90 - 30-6-2006 at 01:44

okey.

but, it's weird, because I get the feeling that concentrated acid, is not the same as a acid with a high percentage, if so is the case, then what dose change the concentration of something?
everyone says that it has to be concentrated. if it is a high percentage, how high could it be?

because I can get i think its around 30% H2SO4, will that work? I've done this in school a few years ago, and we wasn’t allowed to use high concentrated acids..

anyway, thanks for your replies!

enhzflep - 30-6-2006 at 02:28

Quote:
Originally posted by neo_90
but, it's weird, because I get the feeling that concentrated acid, is not the same as a acid with a high percentage, if so is the case, then what does change the concentration of something?
everyone says that it has to be concentrated. if it is a high percentage, how high could it be?

You may be confusing a strong acid with a concentrated one.
Hydrochloric = strong acid
Acetic Acid/Citric Acid = weak acid

The amount of water (or other dilutants) changes the concentration of something
The type of acid determines whether it is strong or weak.

Think about sweeteners.
1tsp sugar = 1 tiny pill Nutrasweet
Both 100%, but Nutrasweet is a stronger sweetener.

Sulfuric acid may be 100%, but 98% is more common.
Quote:

because I can get i think its around 30% H2SO4, will that work?

Not sure, but as Darkblade48 said - more water = less product

neutrino - 30-6-2006 at 11:03

>because I can get i think its around 30% H2SO4, will that work?

Boil it down to a higher concentration. This has been discussed around here, just search for it.

guy - 30-6-2006 at 11:44

Just wondering, how come 3% H2O2 works just as fine as concentrated H2O2 for AP synthesis? Water is a product for AP synthesis too and should shift equilibrium back.

Nicodem - 30-6-2006 at 12:02

That is a different reaction with a different equilibrium constant. Peroxide is a much better nucleophile than water and thus tends to compete much better in the reversible addition on the carbonyl group. Meanwhile, in the esterification, the alcohol is comparatively more or less equally nucleophilic as water, thus the water must be removed if the equilibrium is to be shifted in the favor of the ester.

PS: A Fischer Esterification is unlikely to give any noteworthy yield if anything bellow 60% H2SO4 is used.

[Edited on 30-6-2006 by Nicodem]

neo_90 - 30-6-2006 at 13:03

okey, then I will boil it down first.

thanks for all the anwsers!

this looks like the forum i've always wanted, but never found :)

pantone159 - 30-6-2006 at 20:50

For small scale esters, I've done:

c. 1 g alcohol, as dry as practical
c. 1 g acid, as dry as practical
c. 4 drops concentrated sulfuric acid

I think you really need concentrated acid, even though it is not that much. By its intense dehydrating action, sulfuric acid shifts the ester equilibrium towards the esters.

If you can't get concentrated sulfuric acid, 85% phosphoric acid might be a possible substitute, but I haven't tried it.

neo_90 - 1-7-2006 at 05:35

yea, if I get 1 liter of 30-40% sulfuric acid, then boil some down to a higer concentration, then lets say 100ml of 80-90% will be enough for me. as you said, I wont use much each time.

if the ester acid isn't that dry, should I add more sulfuric acid to dehydrate the water from the ester acid?

Darkblade48 - 1-7-2006 at 08:57

I'm not too sure what you mean by "ester acid", but if you are simply talking about the ester, I would not add sulfuric acid to dehydrate the water from it, as esters hydrolyze in strongly acidic conditions.

neo_90 - 1-7-2006 at 09:28

the acid I ment by "ester acid" is the acid you mix with the alcohol to form the ester, like Acetic acid and ethanol...

lets say that I have 50%acetic acid, then should I increes the amount of sulfuric acid to dehydrate the water from the acetic acid?

hope you understand now..

edit:
i've read your post again, and i suppose that I shouldn't..

[Edited on 1-7-2006 by neo_90]

neutrino - 1-7-2006 at 11:12

They also form in acidic conditions. I think that adding extra acid would just shift the equilibrium toward the product side.

neo_90 - 1-7-2006 at 12:29

well, then I guess that testing is the best way to answer that question :)
will try this later when I'm back from my vacation.

big thanks for all the answers and those who spent there time on answering my questions!

just one more fast question, has not anything with this to do but ! don't want to start a new thread just because of this..

H2SO4 + 2NaCl --> Na2SO4 + 2HCl will this work? I've searched but not found.. yes or no is enough :)

The_Davster - 1-7-2006 at 13:12

Yes, it will work.

neo_90 - 1-7-2006 at 13:21

thought so.. but some things in chemestry is really weird so I thought that it would be best to ask =)