[lukas.stib]

Hello chemists,

I did fractional distillation of methyl acetate, but I have no pure methyl acetate, so I want to know where I probably made a mistake in distillation.

I measured 60 g acetic acid (99%) and 34 g methanol (99.9) and a few ml H2SO4. I refluxed for 1 hour with mol. sieves for drying. Then I fractional distillation. The distillation was held at 53°C all the time, so I assumed it would be a methanol/methyl acetate azeotrope (I found the azeotrope on the Internet). After 40 ml, the drip collection slowed down so I increased the power a bit and changed the collection vessel. The temperature started to rise slowly, but instead of the boiling point of the product it was approaching 60°C and more, so I finished it. I should have had about 70 g of product by stoichiometry, but I only have 40 ml of the methyl acetate/methanol azeotrope, and I have the rest of the reactants in the boiling flask.

Can you advise me why not the product distilled me, but only the azeotrope? Or is it not an azeotrope, but a pure product, and was there a thermometer error, between the heat transfer between the sensor, the glass and the product?

Thank you very much.

[AWK]

In your example expected yield should be about 60 ml of the azeotrope. Given that you are a beginner chemist, the reaction yield is quite good.

[lukas.stib]

So I understand that I can't get the pure product at 57°C in this amount of reactants?

[AWK]

Esterification of the acid-alcohol mixture is an equilibrium reaction. With a molar ratio of 1:1 of reagents, the degree of conversion is about 2/3.

[lukas.stib]

Fine. And what about trying to distill everything to about 60°C next time, then put it in the freezer to cool the methanol/methyl acetate azeotrope and then rinse with water? Methanol is very soluble in water, but methyl acetate less, then dry and distill?

[AWK]

Too big losses.
But methanol can be removed from the distillate. Good textbooks usually give this method but do not explain it. Students think it's all about drying because anhydrous CaCl₂ is used (but dry azeotrope should be used). Anhydrous calcium chloride forms with the methanol the CaCl₂·2CH₃OH solvate and ordinary filtration suffices to obtain an almost pure ester. By distillation, it can be checked that the methanol has been completely removed.
Higher conversion of alcohol to ester can be obtained by excess acid.
Acetic anhydride omits equilibrium and gives practically quantitative conversion alcohol to ester.

[lukas.stib]

Super, CaCl2 anhydrous I have. Thank you very much for your answers. I will try next time.

[AWK]

Dry so-called anhydrous calcium chloride at 250°C for at least 1 hour before using.

[lukas.stib]

Yes, I will do it.

[lukas.stib]

Hello,

so, I performed the same fractional distillation as last time; I collected everything up to about 55°C. I had about 50 ml of methyl acetate / methanol azeotrope. I dried CaCl2 at 250°C and little put it in an azeotrope. I gave a small amount of CaCl2 and something strange happened; after 2 hours CaCl2 has swollen greatly, and I have no liquid there. It looks like CaCl2 drank everything. I do not understand it at all. Do you know why this happened?

Thank you.

[pgk]

1). Molecular sieves can trap up to 25% water at ambient temperature. But this occurs by a physical procedure that is reversible at higher temperatures. So and by refluxing, the trapped water is excreted from molecular sieves and re-entered in the reaction that is an equilibrium one. In other words, you have prepared methyl acetate with very low yields.
Try another esterification technique.
2). CaCl2 can also form hydrite-like crystals with other small polar molecules, e.g. methanol and which has happened here.
Besides polyhydrite and polyhydrite-like CaCl2 have quite low boiling points.
Calcium chloride, Wikipedia
https://en.wikipedia.org/wiki/Calcium_chloride

[AWK]

Water still was in the mixture. Don't panic. Carefully distill this mass. Around 60°C CaCl₂·4H₂O is stable. This way you may remove all water and at least part, if not all, methanol. Observe boiling point ~54 or ~57°C.

[lukas.stib]

I used molecular sieves, but probably the reflux time was small, I only did 1 hour. Maybe I should reflux for at least a few hours.
But so much water ...
OK, thank you very much for your answers, PGK and AWK.
A distill it next time.

[lukas.stib]

Hello chemists,

so today I just made a simple distillation of the mass with CaCl2 in a water bath. I heated the water bath to about 80°C and distilled everything from 52°C to 54°C. Then I heated the bath to 90°C but the boiling temperature remained the same or even dropped and it dripped a little, so I ended it.

I have about 48 ml of mixture. I think it will not be pure methyl acetate, but it will be again with methanol. It has a scent of methyl acetate. I am satisfied, I did just for fun, and crude methyl acetate is OK.

Do you think this is a good result?

Thanks.

[pgk]

Methyl acetate boils at 57 oC, methanol boils at 64.7 oC and methyl acetate/methanol azeotrope boils at 53.8 oC.
Consequently, if the temperature does not rise above 54 oC during distillation, you still obtain the methyl acetate/methanol azeotrope.