achem500
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HCl for Fischer Esterification?
Can aqueous HCl be used as a catalyst in a Fischer esterification instead of H2SO4? Any help is appreciated.
[Edited on 16-8-2012 by achem500]
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Dr.Bob
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Yes, it works fine, although it has a tendency to boil off with time. I make many methyl and ethyl esters by simply putting the acid in the alcohol,
adding some HCl and refluxing overnight. The HCl is best anhydrous, either via the gas, a solution in the alcohol, or by adding a small amount of
acetyl chloride to the alcohol to generate some HCl and methyl acetate, which boils at RT and goes away.
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Nicodem
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Thread Moved
16-8-2012 at 07:20 |
achem500
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Anything that donates a proton works in a Fischer esterification? Could I use "hardware store" HCl and just increase the amount? Are Fischer
esterifications water-sensitive?
Thanks for the help.
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Nicodem
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Obviously not, or else you would not need to use a catalyst to effectively reach the equilibrium point.
It has to be a substantially acidic acid. Strong acids like H2SO4, sulfonic acids, HCl or BF3 are most commonly
employed. I would guesstimate that it has to have the pKa of about 0 or less to have a noteworthy catalytic activity and less than -2 to be effective
at all in practice. Exceptions exist, though. For example, alpha-hydroxycarboxylic acids and malonic acid can be esterified with boric acid (pKa = 9)
catalysis under azeotropic removal of water, but the reaction mechanism is very different when compared to the Fischer esterification (e.g., see DOI:
10.1071/CH07231). In some instances even phosphoric acid (pKa1 = 2) can catalyze Fischer esterifications, "but it leads to rather slow
reactions," as Kirk-Othmer says.
The more electrophilic carboxylic acids tend to reach the esterification equilibrium point much more rapidly and thus require considerably lower
catalyst loading, or even none (e.g., formic acid).
| Quote: | | Could I use "hardware store" HCl and just increase the amount? Are Fischer esterifications water-sensitive? |
Esterifications are not water sensitive, because water is the reaction product. However, for this same reason, the concentration of the water in the
reaction mixture affects the equilibrium point. Thus, it is completely unreasonable to use hydrochloric acid as a catalyst for preparative reactions.
You should use a solution of hydrogen chloride in the alcohol that is being used for the esterification (for example, methanolic HCl). All of this has
already been extensively discussed several times on this forum, so UTFSE for more information.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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achem500
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Thanks
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thelonious
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Dehydrating agents other than H2SO4 for Fischer esterification
I'd like to do a fisher esterification of benzoic acid and methanol, but I don't have access to any sufluric acid. Could I use something like CaCl2?
If not, are there any other dehydrating agents I could use, or are there any other realistic ways of pushing the equilibrium to the right?
Thanks!
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thelonious
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Nevermind, I forgot to UTFSE
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UnintentionalChaos
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The fisher esterification requires an acid catalyst. It is not present to remove water, merely to allow for the equilibration of alcohol/acid and
ester/water. Removing the water by some other method (azeotropic distillation, for example) or by simply using a great excess of one of the reactants
will drive the equilibrium to favor products.
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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Hexavalent
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Looking at the mechanism, it seems that the acid serves to protonate the oxygen on the carbonyl for nucleophilic attack, thus it is obvious that many
catalysts can be used. For example, one common one other than sulfuric acid is p-toluenesulfonic acid, or Lewis acids such as scandium(III) triflate
(trifluoromethanesulfonate).
The sulfuric acid does not remove water as such from the reaction; this is usually done if necessary, by the methods UC describes or by use of, e.g. a
Dean-Stark apparatus.
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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Ozone
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I really like strong-acid cation (sulfonated styrene-divinylbenzene) resin in the hydrogen-form. You get (if you use enough) equivalent catalysis and
you can filter the resin out when you are done.
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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Nicodem
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Threads Merged
26-10-2012 at 23:18 |
GreenD
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Would a Dean-Stark apparatus work if you were esterifying with methanol?? Since methanol boils at a lower temp than water, and is miscible as well,
esterification with methanol would best be pushed forward by using huge molar amounts of methanol yes?
Thanks
ʃ Ψ*Ψ
Keepin' it real.
Check out my new collaborated site: MNMLimpact.com
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