Archenemy_6
Harmless
Posts: 32
Registered: 26-1-2017
Member Is Offline
Mood: No Mood
|
|
Heptane and Toluene
So long story short just starting to get into chemistry been trying to learn as much as I can. I recently purchased a 200mm Vigreux column and wanted
to practice with it and have some non chlorinated brake cleaner that contains heptane and toluene.
I looked up the boiling points for each individually heptane was 98.4 and toluene 110.6.
I figured they might be useful solvents by themselves and it would give me some practice using my column. What I wasn't able to find was whether they
formed an azeotrope and if my distillation would be more or less useless.
Also if anyone has any suggestions on anything to use with my column that would give me some practice with using it, it would be appreciated.
|
|
|
JJay
International Hazard
Posts: 3440
Registered: 15-10-2015
Member Is Offline
|
|
According to this database, there is no binary azeotrope between heptane and toluene: http://vle-calc.com/azeotrope.html
|
|
|
Archenemy_6
Harmless
Posts: 32
Registered: 26-1-2017
Member Is Offline
Mood: No Mood
|
|
So I attempted the fractional distillation and it didn't seem to work. When I first started I could see the vapors rise through the column, hit the
thermometer, and head towards the condenser at around 80°. There didn't seem to be a point where the temp would stay steady and then rise to the next
point. I tried minute adjustments that would translate to a few degrees at the top of the column but it would always want to change. At one point it
was coming over quite quickly at 94°. Is this a fault of mine? Should I practice some more?
|
|
|
JJay
International Hazard
Posts: 3440
Registered: 15-10-2015
Member Is Offline
|
|
Is there anything else in the brake cleaner?
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
I think you are expecting too much from that column. It's only equivalent to a few equilibrium stages. A bp difference of only 12 degrees is not
much.
But if only heptane and toluene are present then the head temp should have been at least 98.4, barring any azeotrope.
[Edited on 14-2-2017 by Magpie]
[Edited on 14-2-2017 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
Archenemy_6
Harmless
Posts: 32
Registered: 26-1-2017
Member Is Offline
Mood: No Mood
|
|
I have a print out of the MSDS somewhere I will look for it. On the can it says heptane, heptane, toluene and carbon dioxide. The 2 heptanes have
different CAS numbers and I want to say on the MSDS one of the heptanes was listed as n-heptane. I don't recall anything else being listed.
|
|
|
JJay
International Hazard
Posts: 3440
Registered: 15-10-2015
Member Is Offline
|
|
If you're dealing with iso-heptanes, this may be a considerably more complicated distillation than simply separating n-heptane and toluene. You can
probably get some toluene out of it, but it might take multiple runs to achieve a reasonably pure product.
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
The CO2 can make a big difference. Try refluxing the solution for a half hour or so to see if you can expel all of the CO2. Then try diistillation -
very slowly.
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
Geocachmaster
Hazard to Others
Posts: 146
Registered: 5-3-2016
Location: Maine, USA
Member Is Offline
Mood: Corroded, just like my spatulas
|
|
My first few fractional distillations we complete failures. Like you the temp slowly rose instead of stabilizing. I'm very sure that my problem was
the speed. Fractional distillation goes much slower than simple distillation. For example, several weeks ago I obtained some ethyl rubbing alcohol and
wished to separate its components. I wanted to distill off azeotropic ethanol and get rid of the acetone, methyl isobutyl ketone plus excess water.
The rate of distillation was probably four drops per second, and the separation wasn't better than if I had done simple distillation. Just this
weekend I decided to try again with the same stuff. This time I went much slower, about one drop every three seconds (I slowed down as time went on as
well). The acetone didn't come off in a super good fraction but I collected a bunch of 95.6% ethanol coming over between 78.0 and 78.3 C. It took me
about five hours to get ~200ml, Going slow is key.
Magpie is right as well. I used a 300mm vigreux to separate two things that had a BP difference of 22C, a 200mm vigreux trying to separate two liquids
12C apart may not work within a reasonable amount of time.
|
|
|
Archenemy_6
Harmless
Posts: 32
Registered: 26-1-2017
Member Is Offline
Mood: No Mood
|
|
The CAS numbers on the bottle are Heptane 426260-76-6
Heptane 142-82-5
Toluene 108-88-3
And carbon dioxide 124-38-9
If that helps at all
It is in an aerosol can and I figured the carbon dioxide was the aerosol propellent
I did purchase the 200mm because it was fairly cheap. Although even when I turned the heat down to when it was maybe 1 drop/sec the temp was about 76
or 78 anything below that nothing would come over so even the minimum temp was far below what I believed to be the lowest of the components. Maybe
there is an azeotrope that isn't well documented?
[Edited on 14-2-2017 by Archenemy_6]
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
I' m almost positive that the CO2 is the problem. When distilling ether from starting fluid the bp is depressed due to the CO2 propellant.
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
Archenemy_6
Harmless
Posts: 32
Registered: 26-1-2017
Member Is Offline
Mood: No Mood
|
|
I can get a reasonable amount of these cans for free so I'm really only spending my time. And as far as refluxing goes I can just use my condenser in
a vertical position above the boiling flask and let it run for half an hour then try again?
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
Yes. Boil as vigorously as possible within the capacity of your condenser.
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
Amos
International Hazard
Posts: 1406
Registered: 25-3-2014
Location: Yes
Member Is Offline
Mood: No
|
|
Is there any reason that this heptane shouldn't be boiling that low? The heptane in commercial products is a mixture of isomers produced from crude
oil, several of which boil in the lower 90s and upper 80s. Am I missing something?
You may want to use boiling water as a heating bath to ensure no part of the flask reaches the boiling point of toluene; I do this a lot when
separating two liquids on either side of the boiling point of water.
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
Quote: Originally posted by Amos  | Is there any reason that this heptane shouldn't be boiling that low? The heptane in commercial products is a mixture of isomers produced from crude
oil, several of which boil in the lower 90s and upper 80s. Am I missing something?
|
Yes, this is very likely involved in the "low" boiling points obtained by the OP. The diluent in these automotive cleaners and starting fluids is
surely "commercial heptane" and not solely n-heptane.
I fractionated some by-product heptane obtained from starting fluid using a 600mm packed column. I found the bulk of it had a boiling point range of
91-92°C.
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
Amos
International Hazard
Posts: 1406
Registered: 25-3-2014
Location: Yes
Member Is Offline
Mood: No
|
|
Thanks, Magpie. That should help assuage the OP's worries.
|
|
|
Archenemy_6
Harmless
Posts: 32
Registered: 26-1-2017
Member Is Offline
Mood: No Mood
|
|
I was out of town for a few days but I just refluxed for about 45 minutes. I also found the data sheet it says Heptane, branched, cyclic, and linear
50%-70%. Toluene 30%-50%. And Heptane 1%-5%
So if I'm understanding right it's probably the Heptane coming over at the lower temp due to it not being just pure Heptane?
I will definitely try the water bath and go slow. So with the water bath I should get about all the Heptane over right before the water boils and I
should be left with almost all Toluene?
I was also looking at some longer Vigreux columns. There's a 400mm for a decent price. What would be a good length for distilling things like this? Is
there such a thing as too long of a column?
|
|
|
Amos
International Hazard
Posts: 1406
Registered: 25-3-2014
Location: Yes
Member Is Offline
Mood: No
|
|
| Quote: Originally posted by Archenemy_6 | So with the water bath I should get about all the Heptane over right before the water boils and I should be left with almost all Toluene?
I was also looking at some longer Vigreux columns. There's a 400mm for a decent price. What would be a good length for distilling things like this? Is
there such a thing as too long of a column? |
I think that yes, you can probably get an okay separation this way, maybe with a second distillation. Between the end of the heptane distillation and
the time that the thermometer temperature actually makes it up to the boiling point of toluene, I'd advise pulling out an intermediate cut; this will
likely contain decent quantities of both heptanes and toluene.
If you're really enthusiastic about doing fractional distillations, you might consider looking at the kind of columns you can pack with material to
increase surface area/efficiency; that's not an area I'm very familiar with, though.
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
