Sciencemadness Discussion Board

HCl for Fischer Esterification?

achem500 - 15-8-2012 at 16:33

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]

Dr.Bob - 15-8-2012 at 17:02

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.

achem500 - 17-8-2012 at 12:34

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.

Nicodem - 19-8-2012 at 05:33

Quote: Originally posted by achem500  
Anything that donates a proton works in a Fischer esterification?

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.

achem500 - 20-8-2012 at 02:39

Thanks

Dehydrating agents other than H2SO4 for Fischer esterification

thelonious - 25-10-2012 at 19:30

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!

thelonious - 25-10-2012 at 19:55

Nevermind, I forgot to UTFSE

UnintentionalChaos - 25-10-2012 at 19:56

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.

Hexavalent - 26-10-2012 at 03:04

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.

Ozone - 26-10-2012 at 16:57

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

GreenD - 11-1-2013 at 14:18

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