During recrystallization, an orange solution of a compound in hot alcohol was tr
ID: 480623 • Letter: D
Question
During recrystallization, an orange solution of a compound in hot alcohol was treated with activated carbon and then filtered through fluted paper. On cooling, the filtrate gave gray crystals, although the compound was reported to be colorless. Explain why the crystals were gray and describe steps that you would take to obtain a colorless product. The solubility of compound A in ethanol is 0.8 g per 100 mL at 0 degree C and 5.0 g per 100 mL at 78 degree C. What is the minimum amount of ethanol needed to recrystallize a 12.0-g sample of compound A? How much would be lost in the recrystallization, that is, would remain in the cold solvent? Describe how you would purify p-nitrobenzoic acid from a mixture of 9.0 g of p-nitrobenzoic acid and 2.0 g of o-nitrobenzoic acid using recrystallization (see Table 3.1). Tell what solvent you would use and how the components would be separated. Assume that, 3.0 g of aniline and 4.5 mL of acetic anhydride are used in the preparation of acetanilide (Eq. 3.1) What is the limiting reagent? What is the theoretical yield of acetanilide? What is the percent yield if 3.3 g of acetanilide is obtained? Compound B is quite soluble in toluene, but only slightly soluble in petroleum ether. How could these two solvents be used in combination in order to recrystallize Compound B?Explanation / Answer
Recrystallization works on the principle that the solute must be soluble in it at higher temperature but insoluble at room temperature ,so that it gets precipitated by cooling.Also ,for separation of components of a mixture by recrystallization they must have variable solubilities in the chosen solvent.Impurities or one component must be negigibly soluble in the solvent ,so that it could be removed by filtration.
The ratio of solubilities of ortho to para nitrobenzoic acid are-
1) in methanol 43/9.6=4.5
2)in ethanol=28/2.2=12.7
3) ether 21/0.9=23.3
Ether would be a preferable solvent as it has low solubility for ortho-form ,so that the solubility is high at elevated temperatures and para-form is negligibly soluble in it, can be separated by filtration.
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