[Bro-Mide]

I have an organic product dissolved in a solvent (toluene). The procedure says that the solution should crystallize when the solvent is evaporated. The solvent evaporated quite efficiently for a bit then it stopped. I turned the temp up to around 120 degrees C and no more liquid would evaporate. The solution started changing from light yellow to a dark reddish brown. I don't know if I am destroying my product or if it is supposed to turn this color as it becomes a solid. Is it common for a solution to get darker/black when you heat it to evaporate the liquid?) I don't have a lot of experience evaporating solvents so I don't know what sort of temperatures or vacuum pressures you should expect to use. How you know to use less versus more heat or less versus more vacuum pressure?

BP of toluene = 110 degrees C . I heated the solution over this temperature in an attempt to evaporate it off but the solution changed from a light yellow clear liquid to a increasingly dark and eventually gritty brown/black sludge

One final question, how much would a roto-vap help me with these sorts of problems?

[discodermolide]

I have an organic product dissolved in a solvent (toluene). The procedure says that the solution should crystallize when the solvent is evaporated. The solvent evaporated quite efficiently for a bit then it stopped. I turned the temp up to around 120 degrees C and no more liquid would evaporate. The solution started changing from light yellow to a dark reddish brown. I don't know if I am destroying my product or if it is supposed to turn this color as it becomes a solid. Is it common for a solution to get darker/black when you heat it to evaporate the liquid?) I don't have a lot of experience evaporating solvents so I don't know what sort of temperatures or vacuum pressures you should expect to use. How you know to use less versus more heat or less versus more vacuum pressure?

BP of toluene = 110 degrees C . I heated the solution over this temperature in an attempt to evaporate it off but the solution changed from a light yellow clear liquid to a increasingly dark and eventually gritty brown/black sludge

One final question, how much would a roto-vap help me with these sorts of problems?

Looks like you decomposed your product. A rota-vap would help a lot. With a water pump vac. you can remove toluene at around 35-45°C, depending on how fast you want to do it.

[Arkcon]

Very likely, the heat was too much.  I'm assuming you've prepared some sort of organic molecule.  That temperature is roughly 3x human body temperature -- if your product had been a biological molecule, surely it would have been destroyed.  This is one example of ways you can gauge heat, without knowing the specifics.  At 120 C, you would just be gently heating an oven -- perhaps making toast(I have to use that trick, because I'm in the US, and we cook with the Fahrenheit scale,) so you should expect some chemical change to be starting.

It is possible however, that the slight color change was normal as a solution concentrates, even if your product is supposed to be clear, there could be colored impurities.  Its also possible that the product was so impure, that when all the toluene was gone, it still left some liquid impurity, or the combined products and impurities trapped some toluene into an organic mixture as a "syrup".

[Bro-Mide]

Is a water pump vac the same as a water aspirator? Does it have more pressure than other types?

[discodermolide]

Is a water pump vac the same as a water aspirator? Does it have more pressure than other types?

It is the same.
A better vacuum can be obtained by using a vacuum pump but this is not necessary for the removal of toluene.

[orgopete]

I'm with disco on this one. It sounds like decomposition or oxidation occurring. Check the TLC.

My gut reaction is that was some serious heating to boil off toluene at atmospheric pressure. Not only is the temperature pretty high, but it had to take a long time because of the high heat. If you did not have the heat source much above the boiling point, then it would have taken longer. If you did have the temp much above the boiling point, then I can also imagine that as the last traces of toluene evaporated, the pot temp would also spike.

I realize advice is the advice given after the horse has left the barn. What to try the next time.

• 1) Check by the reaction mixture by TLC. Are there impurities present and are they more or less polar than your product?
• 2) How easily will the product crystallize? Do you really need to remove all of the toluene? I am guessing the answer is no. Put some on a watch glass and see how easily it crystallizes. Add hexane to the drop, will it crystallize or oil out? This will also depend on the TLC. The less pure the sample is, the harder it is to crystallize. Hexane will push out polar impurities. Filter your sample through a plug of silica may help. This is a trial and error method. A high melting point is probably easy while the lower the melting point, the harder it will be. Pay attention to what is happening, think, and repeat several times.
• 3) If the sample will crystallize, consider just reducing the volume. I would not reduce it to less than 50% of toluene. Add a non-polar solvent like hexane. I would target a solution of less than 10% of product. If the sample begins to oil out before that point, you can add some toluene (or dichloromethane preferably) to regain a solution. When the solution is saturated, scratch the sides or add a crystal from the watch glass.
• 4) If it won't crystallize, then use a rotary evaporator (and consider using a rotary evaporator to reduce the volume anyway). If it won't crystallize, this can be a safe step. It is will crystallize, then be careful that crystallization doesn't occur in the rotary evaporator. This can result in splattering your solid in the flask and up the evaporator tube. If you have a guard in place, that may stop bumping into the evaporator, and you will just have to clean your guard when you are done.

Obviously, your mileage can vary. My advice is just a description of what has happened to me, such as when evaporating chromatography fractions that crystallize. Oops, time to clean the rotary evaporator (again).