# Separating substances under lower atmos press

1. Oct 11, 2005

### chiefy

Can a cyclohexane-toluene mixture be separated if the external pressure is 350mm HG instead of 760 mmHG ?

My guess is yes, lowering the atmospheric pressure will only lower the boiling point, it won't affect whether separation will occur. By the way, is this what we call vacuum distillation? I need a little greater explanation. DOes anyone have any ideas???

Last edited: Oct 11, 2005
2. Oct 12, 2005

### GCT

yeah, but I think that for the most part vacuum distillation is used for compounds with high boiling points, which decompose under such temperatures.

3. Oct 12, 2005

### renerob

The boiling point of cyclohexane at atmospheric pressure is 81.4 and that of toluene 110.8 which means that they can be separated easily. At reduced pressure,the difference in boiling points will probably be large enough to allow for separation.

4. Oct 13, 2005

### GCT

if you're meaning through simple distillation probably not, I think what they usually do with vacuum distillation is to trap the gaseous organics in a "cryogenic chamber", that is if the organics are gases at that temperature. Fractional distillation may work even at normal pressures.

5. Oct 13, 2005

### renerob

I am not quite sure understand your post. The only reason for vacuum distillation as opposed to atmospheric pressure is that because the boiling points are lowered, it is possible to operate with less thermal decomposition.

6. Oct 13, 2005

### renerob

Mr.Chiefy:
According to my Handbook of Chemistry and Physics,the change in boiling point with pressure is given by:
$$\deltat=(273.1+t)(2.8808-logp)/phi+.15(2.8808-logp)$$
where t is temperature,p is pressure and phi can be obtained from a graph. I picked 4.7 for cyclohexane and 4.6 for toluene.

Assuming my arithmetic is fairly correct,this is what transpires:
Cyclohexane at 350mm boils at 58.5 centigrade
Toluene at 350mm boils at 85.5 centigrade.
Consequently distillation at 350mm ought to separate the compounds very well.

7. Oct 14, 2005

### chiefy

vacuum distillation is something I just through in there, I simply wanted to know if they separate under a lower pressure, and I think the consensus is that they will. Thanks for the replies.

8. Oct 14, 2005

### GCT

thermal decomposition of organics? cyclohexane and toulene?

9. Oct 14, 2005

### movies

Personally I have had a couple of compounds that I have tried to purify by distillation and I got better results by distilling at atmospheric pressure than at low temperature. Then again, the bp difference was a bit smaller in my case.

Also, you don't necessarily need cryogenic cooling when you do vacuum distillation. A simple, water-cooled distillation apparatus is often good enough when you are dealing with high boiling materials. There definitely are cases where you need to condense the distillate at very cold temps, but not always.

10. Oct 15, 2005

### renerob

GCT:
Good point. In this particular case one other good reason is that both compounds are very flamable.