Sciencemadness Discussion Board

Toluene boiling point reading 105C?

aromaticfanatic - 23-12-2020 at 10:51

I attempted an extraction of toluene from carburetor cleaner yesterday and had some interesting results.

I have done this before and it worked just fine. I don't recall having a thermometer in the distillation head for some reason so no data was taken from that distillation.

This go around I didn't wash the toluene with water after the initial separation from acetone and methanol using some water. That was definitely a mistake on my part and I paid for it. I set up distillation and was perpexled that half of my initial volume of liquid came over far below 110C. I should theoretically have only some methanol and acetone but I suppose the low amount of water that I used for the single initial extraction wash was not enough.

I didn't write it down but I think I had what should be methanol and acetone azeotrope come over at around 55C, then the temp went to around 60 something (I think, I didn't pay much attention and was mainly just waiting for that 110C temp), and this next part confuses me. The temp at which I began collecting what I assume to be toluene was 105C. The temp stayed steady until the distillation was finished.

My thermometer is pretty accurate, usually within 1-2 degrees of the theoretical boiling point.

After the fact I checked the atmospheric pressure as the distillation took place right before a pretty hefty wind storm. The pressure did drop to a low pressure but after checking with a calculator I found no significant boiling point temperature reduction.

I decided to add water to the stuff that came over below 100C as I was suspicious that the liquid was not what I thought it might have been. Sure enough two layers formed. I was discussing this with my friend and I had completely forgotten there was a methanol and toluene azeotrope. So quite a lot of methanol must have come over since there was around 50mL of toluene recovered.

I tested some of what I suspected to be toluene and it burned with a pretty sooty flame and smelled of toluene so I suspect it was toluene.

Is it safe to say my toluene that came over at 105C is pure or was there a different ternary azeotrope that isn't commonly known? I never had such a big difference in boiling point before and this toluene needs to be pretty pure.

I might combine the distilled toluene and the recovered toluene, wash with some water, and then redistill it but I'd rather not have to set all of that up again. Anyone know if this is relatively pure toluene or if there is something fishy going on?

I should also clarify the extracted liquid before drying and distillation was very murky but cleared up within seconds after drying with CaCl2. The liquid was left to dry for around an hour before distillation so I can't imagine there being enough water to suppress the boiling point of the toluene for the entire fraction.

[Edited on 23-12-2020 by aromaticfanatic]

DraconicAcid - 23-12-2020 at 11:20

There might have been a bit of water in it, which makes things come over at lower temperatures.

Tsjerk - 23-12-2020 at 12:09

You got three layers when you added water? Then there has to be an impurity. Water as DraconicAcid suggests should have boiled off at around 90 degrees as the azeotrope.

aromaticfanatic - 23-12-2020 at 13:13

Quote: Originally posted by DraconicAcid  
There might have been a bit of water in it, which makes things come over at lower temperatures.

That could be the case but I find it strange that the temperature was so steady at 105 for the entire fraction. I forgot to say this (I added it now) but I also dried the collected toluene (and apparently methanol and acetone too) over CaCl2 for roughly an hour before distillation.

I just realized methanol forms an adduct with CaCl2 and seeing how much methanol was apparently in my initial mix of fluids, I am beginning to wonder if that may have led to anything. I can't see how as the adduct should be solid from what I am seeing on the internet but I can't find much info on if there is a possibility that it might interfere with boiling points.

aromaticfanatic - 23-12-2020 at 13:17

Quote: Originally posted by Tsjerk  
You got three layers when you added water? Then there has to be an impurity. Water as DraconicAcid suggests should have boiled off at around 90 degrees as the azeotrope.

Sorry that was bad wording on my part. I fixed it now. Just two layers formed. It looked like there were three at first but there ended up just being 2. Upper which seemed to be toluene and lower which should be water, methanol and acetone.

I would assume the water boiled off too at the azeotrope since the temp was very stable at 105C and the liquid mix was dried with CaCl2 beforehand. That part I want to look into. I didn't expect much methanol there but there obviously was quite a bit. I am aware that methanol and CaCl2 form an adduct and I have no idea how that adduct could affect the boiling point of the toluene but perhaps it did.

Fyndium - 24-12-2020 at 00:53

It seems that sometimes distillations give all kind of fancy results that are off the charts. Stuff come over at much lower or higher temps because of various reasons like azeotropes. When I purified my toluene, the thinner stuff had something that steadily raised the temperature from 60 to 110 without much of a noticeable plateaus, but then about 50% of rest boiled practically to dryness within 110-113C.

When extracting toluene solvent from reaction wash, distillate came over from 86+ to 120C, boiler temp being from 100 to 140C. If heat was reduced even a little, head temp started plummeting immediately and vapor generation ceased so there was no option of lower temp distillation.