[organicstudent20]

Hi,

I'm doing an experiment where I need to recrystallize benzophenone in pure form (original sample has a small amount of impurity) and I'm having trouble choosing a good solvent. I know the guidelines are that the benzophenone should be practically insoluble in the solvent at room temp but soluble in the solvent once heated/boiled. Any good ideas for a choice of solvent? I've currently been thinking about hexane or water, but I'm not sure.

Any help is appreciated!

[jeffmoonchop]

If its insoluble in water you shouldn't use it. Its better if the solvent isnt boiling when the solute is dissolved. I would try and decrease the polarity of the solvent more gently by trying ethanol and longer chain alcohols before using hexane. For a highly pure product you want to be able to fully dissolve at a mild temperature say below 50C then hold the temperature at a point where the solution is gently supersaturated. The compound will be less likely to nucleate but will form larger crystals, increasing the reliability of the purification. If it's not crystallising for a long time, gently reduce the temperature.

[blackcat]

If its insoluble in water you shouldn't use it. Its better if the solvent isnt boiling when the solute is dissolved. I would try and decrease the polarity of the solvent more gently by trying ethanol and longer chain alcohols before using hexane. For a highly pure product you want to be able to fully dissolve at a mild temperature say below 50C then hold the temperature at a point where the solution is gently supersaturated. The compound will be less likely to nucleate but will form larger crystals, increasing the reliability of the purification. If it's not crystallising for a long time, gently reduce the temperature.

Agree with the alcohol choices for traditional recrystallization.

Indeed, for carbonyl compounds, one well-established way (but may not be well known nowadays) is the purification through derivation. You use aqueous sodium bisulfite solution heated with benzophenone. The adduct (i.e. a salt) between the two is soluble in hot water but crystallize when cooled down. Then you add carbonate solution (caution: not to use hydroxide solution) to turn the salt back to carbonyl (i.e. benzophenone).

If you wanna more detailed info, you can check out synthesis of phenanthrenequinone in Organic Synthesis.

[clarkstill]

My normal go-to solvent for recrystallization of relatively non-polar compounds is toluene - it has a high BP so there's a large difference in solubilities between RT and its BP.

In general you can look up purification conditions for a large number of reagents etc. in Armarego and Chai: https://www.elsevier.com/books/purification-of-laboratory-chemicals/armarego/978-1-85617-567-8

I've just checked my copy, which says:

Benzophenone [119-61-91 M 182.2, m 48.5-49O, pK -6.0 (aq HzSOq). Crystd from MeOH,
EtOH, cyclohexane, *benzene or pet ether, then dried in a current of warm air and stored over BaO or P2O5.
Also purified by zone melting and by sublimation [Itoh J Phys Chem 89 3949 1985; Naguib et al. J Am Chem
SOC 108 128 1986; Gorman and Rodgers J Am Chem Soc 108 5074 1986; Ohamoto and Teranishi J Am
Chem SOC 108 6378 1986; Naguib et al. J Phys Chem 91 3033 19871.