Identifyanunknowncarbonylcompound.docVIP

Identify an unknown carbonyl compound ? KCl????? /fatherofchemistry Objective?????????? To classify a carbonyl compound by a simple test and to identify it by the precipitation of a derivative. Principle?????????? In this experiment, the unknown compound is either an aldehyde or a ketone from Table 90. In the first part, on undergoing Tollens Fehlings tests, ketone will have no reaction. In the second part, by preparing a derivative of the compound with 2,4-dinitrophenylhydrazine, the sharp melting point can be used to identify the unknown carbonyl compound.?????????? Carbonyl compound can form a precipitate with 2,4-dinitrophenylhydrazine with high melting point:?????????????????????  ? KCl????? /fatherofchemistry Results???????????? My sample is:????????? ethanal?????????????????????? Reason: 1. It forms a silver mirror with Tollens reagent:????????????? CH3CHO?? +?? 2Ag(NH3)2OH?? →?? RCOO-NH4+?? +?? 2Ag(s)↓?? +?? H2O?? +?? 3NH32. It turns deep blue Fehlings solution to green:????????????? CH3CHO?? +?? 2Cu2+?? +?? 5OH-?? ─△→?? RCOO-?? +?? CuO(s)↓?? +?? 3H2O3. When reacted with 2,4-DNP, it forms an orange precipitate, with melting point 148℃:????????????? ? KCl????? /fatherofchemistry Discussion1. What factors decide the choice of solvent in the recrystallization procedure?[ANS] The solvent must not react with the solid. The solubility of the solid in the solvent should be high at high temperature low at room temperature. If possible, the impurities should be insoluble in the solvent. 2. How were soluble impurities removed from the derivative?[ANS] By soaking the impure derivative in 1 cm3 of methanol and resume suction. Repeat this for about 2 times.3. In the recrystallization procedure, why were the crystals dissolved in only the minimum amount of ethanol?[ANS] If too much ethanol is used, the crystal formed will be very fine (thus separation of the solid is more difficult) and the time for recrystallization may take too long.4. I