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Kiss 發問於 科學及數學化學 · 10 年前

lab : Carbonyl Compound 20 點

Identification of a Carbonyl Compound by Preparing its Derivative

1. What soluble impurities may be present in your product before recrystallization? How can they be removed in the recrystallization process? 用methanol洗有咩用 ?

2. What factors should be considered in selecting a suitable solvent in the recrystallization step?

3. In the recrystallization procedure, why were the crystals dissolved in only the minimum amount of hot ethanol?

4. If the sample contains insoluble impurities such as pieces of filter paper, cork, etc., suggest how they can be removed.

5. If the melting point of the 2,4dinitrophenylhydrazone is 156C, suggest how you can confirm whether compound X is propanal or pentan-3-one.

1 個解答

  • 10 年前

    better to write down your experiment and produres next time, such that we can refer to it quickly and easily.

    1. the unreacted carbonyl compound.

    in recrystallization, the wanted compound will form pure crystals, excluding other impurities. the contaminants on the surface of crystals are washed and removed by a little methanol.

    2. for the compound to be crystallized, its solubility at high temperature should be high, and solubility be low at low temp..

    3. it is to ensure largest amount of solute to be precipitated when the solution is cooled.

    4. the sample is dissolved in minimal amount of hot solvent, then filtered with stemless funnel. the funnel without stem can reduce the cooling of solution, thus avoid precipitation of solute during filtration. the whole setup can be appropiately warmed.

    5. look up the literatures and check the melting points of derivatives of propanal and penta-3-one.

    But wait! does it says that both of these derivatives have melting point og 156 degree?

    in that case, another chemical test can be employed to distinguish the aldehyde (alkanal) and ketone (alkanone). you can use Tollen's reagent (silver mirror test) consists of silver oxide dissolved in excess ammonia sol., forms silver if aldehyde is present; OR Fehling's solution / Benedict's solution, consists of alkaline chelated copper(II), forms red copper(I) oxide solid when aldehyde is present.

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