I have a solid that I am to purify by recrystallization using pentane/ether. Does this mean to perform a two solvent recrystalliztion; Heat a solution of pentane with the solid until it dissolves, then add a bit of ether until the solution becomes cloudy, then add some more pentane until the solution goes clear then cool the solution? Or would you use a mixture of both solvents and recrystallize that way?

Also, since pentane boils at a very low temperature, do you just heat it a very small amount so it dosn't boil?

The procedure did say hexane/ether was another option. I will go with that. Thank you for your *delete me*

I would say your mileage may vary. I have done many recrystallizations from ether/hexane. I really liked this mixture because if I had to boil something off, I preferred something not too high boiling. If you have to use too much solvent, it is time to change solvents. Mixed solvents work best if your sample is quite soluble in one and much less in the other. Start with a small amount of the solvent your sample is more soluble in. Then begin diluting with the less soluble and higher boiling solvent. You can evaporate some of the lower boiling solvent as you dilute the solution.

I've seen a lot of papers where recrystallizing from ether/pet ether (or pentane) doesn't really mean recrystallizing - it is just crashing the compound out. The solvents are low-boiling enough that it isn't easy to get a big difference in solubility of your compound between the boiling point of the solvents and 0C.

Well, that is just baloney. Not that that it isn't done. I would agree some people don't know what recrystallization is. Recrystallizing is recrystallizing and crashing out is crashing out. I have seen people do both. You can crash out a solid just by heating it up and rapidly cooling it down or stripping it down on a rotary evaporator. That is usually not as fast as just adding an insoluble solvent, but I would never call dumping in a load of hexane, "recrystallization". Crashing out a solid is a potential with any mixed solvent. I favor a single solvent only for samples that are very soluble or very insoluble.

I avoided going into detail about crystallization as I didn't think that was the point of the question, but if that is the issue, of what advantage does a higher boiling solvent give to a recrystallization from a mixed solvent? It wouldn't seem to be solubility as that is the role of the low boiling component, but there can be a difference. Do you get a higher recovery with a higher boiling solvent than with hexanes? How much more? Are the impurities more soluble with a higher boiling solvent?

I've done this so many times. I liked ether as it was easy to boil off. Many compounds had good ether solubility. The boiling points are reasonably far apart to boil off ether without boiling off all of the hexanes. The boiling point of hexanes are low enough (ca. 66°C) I could remove my flask without burning myself. (I am nerdy enough to monitor the temperature of the vapor to know when I was near the end of ether evaporation.) If compounds were lower in melting points, I didn't have to chase down the temperature with ether to avoid it oiling out until I was sufficient blow the melting point to get a sample to crystallize. I always had sufficiently good material recoveries that I never worried about lost material. I was far more concerned with purity than recovery.

I have done many many recrystallizations from a variety of solvents. I have filtered to remove impurities in the progress of recrystallization many many times, though I can't remember going past six. (If you use a small amount of silica gel to absorb that small polar impurity that is coming out of solution first, you won't have to filter as much as just HyFlo. This is the non-polar equivalent of charcoal in alcohol-water.) If I were to train someone in how to do a recrystallization, ether-hexane would be my first choice. I understand that academic labs may use alcohol-water mixtures as water is cheap, nonflammable, and low toxicity, but if time is a factor, I can recrystallize a sample from ether-hexane in about ten to fifteen minutes, from beginning to weighing on a balance.

I find that if I start with the polar solvent I tend to add too much and then end up having to add a very large volume of hexane.

Me too. That is why I like using ether. What you describe is general though. It is more difficult to redissolve a solid once it has crystallized. You have to overcome the heat of crystallization in dissolving it. For this reason, I liked using a fairly low boiling more polar solvent. Sometimes I just boiled more off before I started adding hexane or I changed to dichloromethane or acetone at the start to dissolve my solid so I didn't have to add too much.

I always get bumping if heating with solid present, so I avoid that.