Question

The recrystallization technique discusses using a drying agent before recrystallizing out of an organic solvent, however, we omitted the drying step before evaporating off the ethyl acetate. Since we didn't dry this sample, what potential issues might this have caused for analysis of the benzoin sample? Be specific about the effects it would have (if any) on each of the three analysis techniques we used. (4 pts) Melting Point, thin layer chromotography, and gas chromotography

Answer 1

Recrystallization is a slower but surer method of purification of solid compounds. It is based on the principle that a solid to be purified is soluble in the hot solvent (in this case, ethyl acetate: a volatile solvent than water) and insoluble when the whole system is cooled. The solvent, in this case, is ethyl acetate which is evaporated at cooler temperatures to get benzoin (the solid) but if the hot solution was not dried before recrystallization, it is evident that some amount of water would have remained in the final product i.e. crystals of benzoin.

The effect on melting point:

The melting point of pure benzoin is 137⁰C (from chemistry data booklet). The impurity of water in it would reduce its melting point because its lattice would be weaker. By how much? It depends on the amount of water of crystallization.

The effect on results of Thin-layer Chromatography (TLC):

In TLC, the stationary phase is polar. If water is present in the sample, it will travel more slowly. Therefore, the R_f (retardation factor) will be lesser than that of pure benzoin.

The effect on the results of Gas Chromatography:

In the case of gas chromatography, the sample is introduced in the vapour state and the boiling point of pure benzoin is 344⁰C. If the crystals of benzoin containing water of crystallization are converted to vapour phase, water will evaporate earlier leaving behind pure benzoin vapours. So, there will be no effect on the results of gas chromatography. However, there may be an extra peak before the peak that matches with the pure benzoin.