Sciencemadness Discussion Board

Product in attempt of acetylation of glucose?

Toluene - 23-11-2013 at 10:01

Hello everyone!

This evening, I tried to acetylate glucose by the Fischer's esterification method, I prepared an erelnmeyer flask and I added 1 part of sugar 5 of pure glacial acetic acid and a few drops of sulfuric acid just to catalyse the reaction.

I put it under reflux for an hour, and what I obtained, was a solid black product wich triples the volume of the sugar I added in the begging.

I've taken 2 test tubes and I filled to the half one with heptane and the other one with water and put a sample of this substance in each one, in water, the solution turns a little bit brown, but it has a very low solubility in water, In the heptane, when I shook the test tube the liquid stayed clear, nonetheless the "walls" (sorry about my english) of the tube, got brown.

The substance is jelly and when is pressed is easily deformed (Not like a foam but like gelatin)

Any thought? :D

Cheers!

[Edited on 23-11-2013 by Toluene]

kavu - 23-11-2013 at 10:03

With H2SO4 and refluxing you will no doubt get a side reaction extravaganza. A very reliable method is to use iodine as a lewis acid catalyst. This is cleaner and easier to reproduce than the common sodium acetate variant. I've run this experiment with highschool students as well as used it to make protected D-Glu for academic research. One reference to this procedure is http://depts.washington.edu/chemcrs/bulkdisk/chem347A_spr08/...

[Edited on 23-11-2013 by kavu]

plante1999 - 23-11-2013 at 10:12

I believe acetic anhydride is generally used for glucose acetylation. One could probably try acetyl chloride and an amine.

Nicodem - 23-11-2013 at 10:15

Quote: Originally posted by Toluene  
This evening, I tried to acetylate glucose by the Fischer's esterification method, I prepared an erelnmeyer flask and I added 1 part of sugar 5 of pure glacial acetic acid and a few drops of sulfuric acid just to catalyse the reaction.

What is the reference claiming that glucose can be acylated applying the Fischer esterification method? I find this totally unbelievable.

Please open threads without references only in the Beginnings section. See the forum guidelines for more information.

Random - 23-11-2013 at 10:16

Glucose probably got dehydrated to caramel or furan compounds.

Galinstan - 23-11-2013 at 10:18

ethylamine won't work as an acetylating agent,
Your best chance is to use acetic anhydride

Toluene - 23-11-2013 at 10:22

Quote: Originally posted by Nicodem  
Quote: Originally posted by Toluene  
This evening, I tried to acetylate glucose by the Fischer's esterification method, I prepared an erelnmeyer flask and I added 1 part of sugar 5 of pure glacial acetic acid and a few drops of sulfuric acid just to catalyse the reaction.

What is the reference claiming that glucose can be acylated applying the Fischer esterification method? I find this totally unbelievable.

Please open threads without references only in the Beginnings section. See the forum guidelines for more information.


Sorry about that, but I never claimed that Fischer esterification method will work for acetylation of glucose, I just said that I tried to acetylate glucose by that method (I'm maybe wrong, but this methods consist in an alcohol+carboxilic acid+catalyst =ester? )

Anyway, Thank you all for the help, but the acetic anhydre may be difficult to get and also to prepare :(

Hexavalent - 23-11-2013 at 10:39

Acetylation is not necessarily the same as esterification, and glucose is not a carboxylic acid.

[Edited on 23-11-2013 by Hexavalent]

plante1999 - 23-11-2013 at 10:44

Acetic acid is, beleive it or not, it is an ester hexavalent.

UnintentionalChaos - 23-11-2013 at 10:47

Quote: Originally posted by Hexavalent  
Acetylation is not necessarily the same as esterification, and glucose is not a carboxylic acid.

[Edited on 23-11-2013 by Hexavalent]


Except that it is an esterification. The glucose has alcohols all over the place (and a hemiacetal). However, because of the labile hemiacetal breaking down into a reactive aldehyde and the susceptibility of the alcohol groups to dehydration, traditional fisher esterification protocol is useless. To avoid the mess, you need a more potent acetylating agent and a milder lewis acid catalyst that will not promote dehydration, rearrangement, and condensation.

[Edited on 11-23-13 by UnintentionalChaos]

Nicodem - 23-11-2013 at 10:47

Quote: Originally posted by Toluene  
Sorry about that, but I never claimed that Fischer esterification method will work for acetylation of glucose, I just said that I tried to acetylate glucose by that method (I'm maybe wrong, but this methods consist in an alcohol+carboxilic acid+catalyst =ester? )

I know you said you tried, but wise people check the literature before they design an experiment (at least if the care about their safety and health). Anyway, glucose is a hemiacetal of an aldehyde. Thus, you can't just expect it to be compatible with the Fischer esterification conditions.
Quote: Originally posted by Hexavalent  
Acetylation is not necessarily the same as esterification, and glucose is not a carboxylic acid.

But acetic acid is a carboxylic acid. And glucose is an alcohol.

Acetylation in general is not necessarily an esterification, but the O-acetylation of alcohols is most necessarily an esterification.