Taking a Solvent Off at a Temperature Your Compound Survives
Vacuum is the synthetic chemist's way of cheating thermodynamics. A product that darkens above 60 °C cannot sit in a flask of DMF boiling at 153 °C, but drop the pressure far enough and that same solvent comes over gently at bath temperature. The controller on a rotary evaporator or a short-path rig usually wants that target as a number in millimetres of mercury, while the reduction itself is easiest to think about as a fraction of an atmosphere.
What Sets the Number You Aim For
Thermal ceiling of the product
What the pump can actually reach
How much a halving buys you
How fast you get there
Turning a Target Into a Controller Setpoint
Most planning happens in fractions — "a quarter of an atmosphere", "a tenth" — while the hardware wants millimetres of mercury.
Enter the fraction you are aiming for
Type 0.25 or 0.05 into the left field and the millimetre figure appears immediately — 190 and 38 respectively. Type 0,05 with a comma if that is your keyboard layout; the field reads it identically and ignores any spaces you leave behind.
Check it against the solvent
Compare the result with the solvent table further down. If your target lands well below the pressure that solvent needs for a 40 °C boil, the bath can come down or the run will simply go faster.
Copy the setpoint straight across
The copy button beside each field hands over the digits with nothing attached — no unit, no padding — so it drops cleanly into a controller entry box or a line in the experimental section. Ctrl + C inside the field does the same.
Reverse it to read a gauge
Press ↔ and the direction flips to mmHg → atm, dividing by 760 (multiplying by 0.0013157895). A gauge sitting at 45 mmHg is therefore holding 0.059 atm — under six per cent of the air you started with.
Solvent Boiling Points Under Reduced Pressure
Atmospheric boiling points beside the pressure at which each solvent comes over at a 40 °C bath — the setting most rotary-evaporator work is planned around. Figures are rounded from standard evaporator solvent charts; treat them as starting points, not specifications.
| Solvent | Boils at 760 mmHg (1 atm) | Pressure for a 40 °C boil (mmHg) | Same in atm |
|---|---|---|---|
| Dichloromethane | 40 °C | 638 mmHg | 0.839 atm |
| Acetone | 56 °C | 417 mmHg | 0.549 atm |
| Methanol | 65 °C | 253 mmHg | 0.333 atm |
| Ethyl acetate | 77 °C | 180 mmHg | 0.237 atm |
| Ethanol | 79 °C | 131 mmHg | 0.173 atm |
| Water | 100 °C | 54 mmHg | 0.071 atm |
| Toluene | 111 °C | 58 mmHg | 0.076 atm |
| Dimethylformamide | 153 °C | 8.3 mmHg | 0.011 atm |
Two rows are worth a second look. Water and toluene are eleven degrees apart at atmospheric pressure yet need almost the same vacuum, because water's unusually large heat of vaporisation makes its vapour pressure climb steeply — a reminder that boiling point alone does not predict how hard a solvent will be to strip. And DMF, only fifty degrees above toluene, needs an order of magnitude more vacuum, which is exactly where an aspirator stops being an option.
Details That Matter at the Bench
Either box takes the number you have
Nothing is read-only here, so a target fraction can go in on the left or a gauge reading on the right, and the opposite side updates while you are still typing.
Swap when the rig speaks first
The ↔ control turns the pair around, which is the direction you want when a gauge has already settled somewhere and you are working out what fraction of atmospheric that represents.
Torr and mbar are one dropdown away
Both unit lists are searchable across twenty-six pressure units, so a European controller reporting millibars and an American method written in Torr can be reconciled without a second tab.
Small fractions stay readable
Results carry up to eight decimals and drop into scientific notation once a value falls below a millionth, so deep-vacuum figures do not collapse into a row of zeroes.
Questions From the Vacuum Line
How far does a boiling point fall if I halve the pressure?
For most organic liquids, somewhere in the region of 15 to 20 °C per halving over the useful range, though it is a curve rather than a constant. Near atmospheric pressure the change is gentle — about half a degree for each 10 mmHg removed — and it becomes dramatic once you are down in the tens of mmHg, where a small absolute change is a large relative one. Ethanol shows the pattern: 79 °C at 760 mmHg, roughly 31 °C by 100 mmHg.
Will a water aspirator get me deep enough, or do I need a pump?
An aspirator can never pull below the vapour pressure of the water driving it — about 17.5 mmHg at 20 °C, worse on a warm day, better with chilled water — so it handles anything down to roughly the ethyl acetate and ethanol rows of the table. Below that you want a pump: a multi-stage diaphragm unit is chemically robust and reaches single-figure mmHg, while an oil rotary-vane pump goes far deeper but needs a good trap and regular oil changes to survive solvent exposure.
My flask foamed straight up the vapour duct — what went wrong?
Almost always the vacuum arrived faster than the liquid could adjust, leaving it superheated until the whole charge flashed at once. Bring the pressure down in stages, keep the flask no more than about half full, start with the bath a little cooler and raise it once the boil is steady, and give surfactant-heavy or protein-containing mixtures extra headroom. Faster rotation helps too, since it spreads a thin film instead of leaving a deep pool to erupt.
How do I read a pressure-temperature nomograph?
It is three parallel scales — the boiling point at 760 mmHg, the boiling point under vacuum, and the pressure itself. Lay a straight edge through the two values you know and read the third where the line crosses. It is a graphical solution of the Clausius–Clapeyron relation with the heat of vaporisation assumed constant, so it is an estimate: expect it to be close for ordinary hydrocarbons and to drift for strongly hydrogen-bonded liquids like water, alcohols and acids.
Why insist on a cold trap between the condenser and the pump?
Whatever the condenser misses travels onward, and in a rotary-vane pump it dissolves into the oil. That raises the oil's own vapour pressure, so the pump quietly stops reaching its rated figure and your setpoint becomes unattainable — corroding the internals along the way. A trap chilled well below the vapour temperature catches the carry-over before it gets there. It also keeps solvent out of the drain or the exhaust, which is a housekeeping and safety matter as much as a performance one.
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