Question

1. (8 pts) A crude reaction product (50 mg) is to be purified by recrystallization from a single solvent. a) (5 pts) The crude solubility in ethanol at boiling temperature (BT) is 50 mg/mL and at O'C is 5 mg/mL. Read the statements and fill in True or False Describe in a few words the reason. True False Describe reason for true or false Statement The crude was dissolved in 5 mL of hot ethanol (BT) and after cooling down to O'C and filtered, 45 mg were recovered 2 It is always recommended to quickly cool down the solubilzed crude to O'C to obtain pure crystals. 3 Diethyl ether is an excellent recrystallization solvent and can be used instead of ethanol Solubility of the organic solid should not be too large or too small in the solvent. The minimum amount of hot solvent has to be used to dissolve the crude b) (3 pts) Why would cyclohexane likely be a poor choice as a recrystallization solvent for this particular compound? (Hint: cyclohexane and ethanol have similar boiling points, so solvent boiling point is NOT a factor)

please provide clear answers.
Thank you!

Answer 1

7. a)

Statement	True/False	Describe reason for true or false
1	False	The solubility at 00C is 5 mg/mL. Since we have a volume of 5 mL, the amount that remains soluble at 00C is (5 x 5) or 25 mg. As the crude mass is 50 mg, only (50-25) or 25 mg can be recovered assuming no loss.
2	False	The solution is cooled to the room temperature first, and then cooled on an ice bath. Slow cooling, more often than not, leads to purer crystals.
3	False	The alcohol functional group is both a hydrogen bond donor and acceptor whereas the ether functional group is only hydrogen bond acceptor. Therefore, ethanol is a better solvent than diethyl ether. However, the major limitation with diethyl ether is that it has a very low boiling point (350C).
4	False	Ideally, the compound must be soluble (high solubility) at the high temperature and insoluble at the low temperature (low solubility). So, high solubility or low solubility at a particular temperature may not matter.
5	True	The quantity of solvent used in recrystallization is usually kept to a minimum to support the goal of recovering the maximum amount of crystals. The quantity (volume in mL) of hot solvent needed to dissolve the sample can be calculated by dividing the mass of the sample by the compound's solubility in the hot solvent (say, x g/mL at the boiling temperature).

b) Cyclohexane is a relatively non-polar solvent. Since like dissolves like, the solubility of the given organic compound in cyclohexane may not be good enough especially if the compound is a polar one.