[randommscience117]

Our high school concurrent credit chemistry II (organics) class recently began our esters experiment, and in a rush to choose an ester, I chose methyl benzoate. Methyl benzoate is kind of an odd ester with a smell “strongly reminiscent of the fruit of the feijoa tree.” Getting to the details, we used stoichiometry to find the amount of chemicals required. The reaction involved 2ml sulfuric acid as the catalyst, 8.9g benzoic acid for the carboxylic acid, and 2.3g Methanol for the primary alcohol. Since there was no where near enough liquid to wet the benzoic acid, we purposely overshot and used 40ml of methanol (31.6 g). I don’t remember exactly what the reasoning was, but it made sense. Anyway, we boiled our ester for a few hours and transferred it to a 400mL beaker, neutralized the acid with sodium bicarbonate and transferred it to a seperatory funnel. The strange thing is that there are 3 layers in the funnel. Liquid on top (water), solid middle layer, and a small bottom layer which we think is our ester. We are not sure what the middle layer is, and that’s the problem we would like to figure out.
Density of Water: 1g/cm^3
Density of Ester: 1.08g/cm^3
Density of Methanol: 0.79c/cm^3
Density of Benzoic Acid: 1.27g/cm^3
We think the problem may have possibly come from impure benzoic acid, lack of catalyst, not enough boiling time, sabotage? A student from the previous class made the same ester, and he says the benzoic acid used was the same for both of us, and he did not have the three layers. That class also used 3ml of sulfuric acid vs. our class’s 2ml. I don’t think the catalyst would be the problem since the rest of the class’s esters turned out fine (I think), and I hope it wasn’t sabotage because that would just be evil. Thanks!

[Dan]

The first thing that comes to mind is unreacted benzoic acid. Check the pH of your aqueous layer - you should aim for 8-9 to make sure all of the unreacted benzoic acid has been converted to (water soluble) sodium benzoate.

[Doc Oc]

Couple things:

1) The use of excess methanol is common in esterifications, so you didn't do anything wrong by adding that.  Brush up on LeChatelier's principle for the reasoning.
2) What you're seeing in your funnel is an emulsion.  It basically is a combination of stuff that can't decide if it wants to go into the organic or aqueous layer so it sits in the middle and makes a mess.  However, your emulsion doesn't sit quite cleanly in the middle for the next reason
3) Methanol and water mix very nicely, so you won't get a good extraction from those two.  If you have access to another solvent like ether or dichloromethane, add some of that to the funnel and you should get your product out.  Keep in mind that the density of ether and DCM are vastly different and therefore you will need to pay close attention to which layer you are saving.

[randommscience117]

So what you are saying is the ether demulsifies the solution right? Well, we tried it, it worked, and came out with a 76.8% yield. Thanks!

[Doc Oc]

Not exactly, the ether doesn't really remove the emulsion.  What's happening is you ran your reaction in methanol and then added water.  What you were looking for is a phase separation, the two very distinct layers of organic and aqueous solution.  But since water and methanol mix very well, you didn't get that.  Typically an emulsion will sit right at the line between the two solvents, but since your two solvents mixed really nicely the emulsion kind of got everywhere.  Ether is not soluble with either and is a great solvent for extractions, so it facilitated the process by giving you the two layers you wanted and pulling your product out of the aqueous phase.

[zaphraud]

The sulfuric acid increases the ionic character of the water in a manner analogous to the way the ether increases the organic character of the methanol; so using less sulfuric acid may indeed explain your different results - the sulfuric acid may have been playing a part other than just "catalyst"! Since its such a strong acid, however, its generally not used for this purpose.

If there's a water layer, and the product you want isn't going to be in the water layer, then you can also use a warm, saturated solution of table salt to do this, generally speaking. Fastest way there, heat some salt and water until it boils, then let it cool until its cold enough not to screw things up, and take some off the top - heck of a lot cheaper than ether too.