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Topic: Purification of an Ester  (Read 27477 times)

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Offline constant thinker

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Purification of an Ester
« on: May 30, 2006, 07:45:11 PM »
Personally I've never study up on how to purify anything past standard distillation, never mind and ester after esterification.

The sulfuric acid and methanol that I ordered has finally come in. After accidentally destroying a wooden table (I told my dad to be careful) and dehydrating some sugar both with sulfuric acid ~98% concentrated, I was happy enough to move into esterification. Now, I haven't physically measured anything out and started the reaction because I though to myself, "How do you purify methyl acetate?"

My question is, how do I purify methyl acetate after the esterification? I'll be using a few drops of the sulfuric acid as a catalyst, so I want to try to make sure that doesn't wind up in my final product.

Also, does anyone have any suggestions about how to get the most methyl acetate as I can out of this reaction.
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Offline pantone159

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Re: Purification of an Ester
« Reply #1 on: May 30, 2006, 08:31:07 PM »
The procedure I have used for making small amounts of esters:

1 - Put about 1 g each of your alcohol and your acid in a test tube.  The reaction uses equal moles of each, but it helps drive the reaction forward if you have an excess of one of them.  (Whichever is cheaper / more convenient.)  You want as little water in there as possible.
2 - Add 3-5 drops of concentrated H2SO4.
3 - Stick the tubes (LOOSELY stoppered) in a hot water bath.
4 - Wait 5-10 minutes.  For bigger esters, the reaction is slower.
5 - Add a few drops of NaHCO3 solution to react with the sulfuric acid.  (Optional)
6 - Dump the contents of the tube into a beaker half full of water.  In general, the esters are less water soluble than the other stuff, and tend to float on top, while the other stuff (acid, alcohol, H2SO4) dissolves in the water.  This helps bring out the ester smell, while hiding that of the reactants.
7 - Carefully smell the beaker and observe the ester smell.

When I did step 6 with methyl acetate, I could smell the ester distinctly for a few seconds, but it faded quickly.  Perhaps it dissolved in the water (it is one of the most soluble esters), or evaporated away, I'm not sure.  The ethyl acetate smell stuck around for much longer, hours probably.

This procedure does not give you a vial of the ester, or anything like that.  To concentrate it, I think you can distill off the ester as the reaction proceeds.  You'd need an organic chem glassware setup for that.

Some links:
http://www.chemguide.co.uk/organicprops/esters/preparation.html#top
http://personal.ashland.edu/~bmohney/ket_scholars/experiment.pdf

P.S.  If you haven't discovered this yet, pure ("glacial") acetic acid is thoroughly obnoxious stuff.  Don't be fooled into thinking it is "just vinegar".

P.P.S.  I'll soon be joining you in making some more esters like this.  I have some n-butanol I haven't used yet, so I'll make butyl acetate, and I will make some salicylic acid from asprin, and then make methyl (ethyl, butyl) salicylate.


Offline constant thinker

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Re: Purification of an Ester
« Reply #2 on: May 30, 2006, 09:43:46 PM »
Thank you Mark Kness. I actually had considered dumping it into water, but I was worried that because it was a small ester that it would have a tendency to dissolve in the water.

Thanks for the heads up about glacial acetic acid. I'm not treating it like vinegar, but with you telling me that it does pose a definitive threat I'll be extra weary.

I'll move into bigger esters when I refine my process, and can get my hands on larger (or different) alcohols/carboxylic acids.

Maybe one day I'll try out transesterification and/or see what happens with phenols. I haven't read much about them, but I guess they are used in ester rearrangement or something like that.

Maybe an ester using tequila to celebrate your birthday? ;)
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Offline joeflsts

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Re: Purification of an Ester
« Reply #3 on: May 30, 2006, 10:01:08 PM »
My kick is esters... In fact that has been my focus since jumping back into the hobby

I recently synthesized Methyl Salicylate and got a wonderful wintergreen smell.  I didn't purify it but that brings up an experiment I did this weekend.  I synthesized Ethyl Acetate using Sulfuric Acid, Ethanol, and GAA.  I got an okay yield but did spend the time to purify it.  The purification process involved  Sodium Carbonate (removal of acids) and Calcium Chloride (drying) followed by distillation.

Yield wasn't exactly great but considering it was my first time I was pleased. So now I have about 100ml of the best finger polish remover and bug killer around... And I made it!

Joe



Offline pantone159

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Re: Purification of an Ester
« Reply #4 on: May 30, 2006, 10:51:30 PM »
100 ml, that's quite a bit.  I made mine on the 1 g scale, so I probably got around 0.1 ml.   :)

I think distillation is the way to go for making any real quantities.  The ester has a lower boiling point than any of the other stuff, so it comes off first, removing it drives the reaction forward (I think yields are otherwise pretty low) plus the heating makes the reaction go faster.

BTW - I think phenols react poorly with this method.  One of the links I listed mentioned that.

Now I need to build a collection of higher alcohols/acids to try more combinations.  (I think the bigger ones have more interesting smells, too.)

Offline mike

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Re: Purification of an Ester
« Reply #5 on: May 30, 2006, 11:36:37 PM »
Here are some of the odours:

methyl butanoate    pineapple
methyl salicylate     oil of wintergreen
methyl benzoate    marzipan
ethyl methanoate    raspberry
ethyl butanoate    pineapple
ethyl salicylate    mint
pentyl ethanoate    banana
pentyl pentanoate    apple
pentyl butanoate    pear or apricot
octyl ethanoate    orange
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Offline constant thinker

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Re: Purification of an Ester
« Reply #6 on: June 01, 2006, 05:22:00 PM »
On the United Nuclear website I found some glass piping and a condenser type thing. I'm considering building a still with the glass piping and the condenser.
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Offline pantone159

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Re: Purification of an Ester
« Reply #7 on: June 11, 2006, 12:49:03 AM »
Solvent extraction is another way that you might be able to purify esters...

E.g., take your ester/alcohol/acid mixture and add an organic solvent (e.g. pentane) to this, put this into a separatory funnel, and add water to the sep funnel.  Shake the sep funnel well.  The pentane and water do not mix, and so form two separate layers (pentane would be on top of water).  The ester will dissolve better in pentane then in water (especially bigger ones), but the alcohol/acid will have more tendency to dissolve in the water.  (Especially the smaller ones.)  Sep funnels have a valve at the bottom that lets you withdraw each layer separately.  The pentane layer would have most of the ester in it, so evaporate the pentane to leave the ester.  The water layer will tend to have the stuff you don't want.

Using NaOH instead of water is probably better, as even big organic acids will dissolve in that.

For very small amounts (e.g. 1 mL or so quantities), you can probably use a little beaker, stir up the organic/aqueous mixture, and separate the layers with a dropper.  That way you don't need a sep funnel or so much solvent.  (Using a normal sized sep funnel (e.g. 250 mL) with 2 mL of a mixture won't be very efficient.)

No flames!! Pentane is volatile and very flammable.  The same warning applies if you try distillation, all the stuff is flammable, so, e.g. using a burner to heat the flask for distillation would be a definite no-no.


Offline pantone159

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Re: Purification of an Ester
« Reply #8 on: June 11, 2006, 01:13:30 AM »
Another list of some esters and their aromas, from
http://personal.ashland.edu/~bmohney/ket_scholars/esters.html

propyl acetate - pears
octyl acetate - oranges
isoamyl acetate - banana
ethyl butyrate - pineapple
butyl acetate - apple
methyl trans-cinnamate - strawberry



Offline constant thinker

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Re: Purification of an Ester
« Reply #9 on: June 11, 2006, 05:50:31 PM »
Good point Mark. Thanks.
"The nine most terrifying words in the English language are, 'I'm from the government and I'm here to help.' " -Ronald Reagan

"I'm for anything that gets you through the night, be it prayer, tranquilizers, or a bottle of Jack Daniels." -Frank Sinatra

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