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Identify an Unknown Carbonyl Compound

By siutingluv Nov 29, 2011 503 Words
Identify an unknown carbonyl compound
© KCl
           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 Tollen's & Fehling's 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:                       [pic]

© KCl
             My sample is:          ethanal          

|1.> It forms a silver mirror with Tollen's reagent: | |              CH3CHO   +   2Ag(NH3)2OH   →   RCOO-NH4+   +   2Ag(s)↓   +   H2O   +   3NH3 | | | |2.> It turns deep blue Fehling's solution to green: | |              CH3CHO   +   2Cu2+   +   5OH-   ─△→   RCOO-   +   CuO(s)↓   +   3H2O | | | |3.> When reacted with 2,4-DNP, it forms an orange precipitate, with melting point 148℃: | |              [pic] |

© KCl
1.> 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.> If your sample had contained insoluble impurities, such as pieces of filter paper, cork, etc., suggest how these might have been       removed.
[ANS] By soaking the impure sample in distilled water. Since filter paper and cork are less dense than water, the impurities will floats on water. After discarding (or filtering out) water, the wet sample is obtained and then resume suction. Water can be used since the sample, hydrazone, is insoluble in it.

5.> Why is it not satisfactory to identify aldehydes and ketones by measuring their boiling points? [ANS] It's because the boiling points of most aldehydes and ketones are very close (e.g. the boiling points of pentan-2-one &  pentan-3-one are both 102℃), it's difficult to identify the unknown sample. By measuring the melting points of their corresponding hydrazone derivatives would be useful since their difference of melting points are large. Other Comments

1.> Except methanol and fitrate, water is also acceptable to remove any soluble impurities since the hydrazone cannot dissolve in water.

2.> There may have some difficulties in performing Fehling's test. Do it 2 or 3 times can be more accurate.

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