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    2,4-DNP Lab Questions Unknown Aldehydes/Ketones

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    We are doing an unknown aldehyde and ketones test. The instruction for the test is below.

    1) The melting point you determine for the derivatives you prepare may not match exactly with the reference tables. However, you can be sure that the experimental melting points will always be lower than the reference values. Why? How much of a differences usually between the experimental MP and the reference values?

    When compounds are impure, they always have lower melting points. The impurities interfere with the way that pure crystals pack together, so the melting point is lower. Differences between experimental and reference values are usually in the range of 2-3 degrees.

    2) Why are 2,4-dinitrophenylhydrazones better derivatives than phenylhydrazone? (Beside the fact that 2,4-dinitrophenylhydrazones better derivatives, because they are crystalline compounds with well-defined melting or decomposition points, and they increase the molecular weight by 180).
    The 2,4-DNP reagent is more stable and less prone to formation of side products and is easier to purify than the phenylhydrazone.

    3) A rule of thumb is that about 20 drops of liquid is equal to 1 mL. This depends on density and viscosity of the liquid and is fairly crude estimate. But assuming the approximation is true and the density of most organic compounds is 0.9 g/mL what mass of unknown is delivered in 2 drops?

    4) Draw the structure of a compound C5H8O, that gives a positive iodoform test and does not decolorize permanganate. Please include all the 3-D configuration, and its boat formation and fischer projection.

    I drew the Newman projection, too, just for good measure. The permanganate not being discolored indicates there is no double bond. The positive iodoform tests indicates a methyl ketone is present.

    5) What happen when you apply the Tollen's test to butyraldehyde and acetone? And, how exactly does the tollens test distinguishes aldehydes from ketones.
    Butyraldehyde is an aldehyde so it gives a positive Tollens test. The sign of a positive test is formation of a "silver mirror" on the test tube.
    Acetone is a ketone and shows no reaction with the Tollens reagent.

    CH3CH2CH2CHO + [Ag(NH3)2]OH + H2O ---> CH3CH2CH2COO- + Ag(s) + 2 NH4OH

    CH3COCH3 + [Ag(NH3)2]OH + H2O ---> No reaction

    6) Why should we not add water right away in the Iodoform test using water-insoluble compounds?
    The water will interfere with the test by precipitating the compound before it has a chance to react.

    7) How does 2,4-Dinitrophenylhydrazine test for aldehyde or ketone carbonyl form a derivative? Please explain.

    The compound formed is a 2,4-dinitrophenylhydrazone. This derivative is crystalline and has a high molecular mass and usually gives sharp melting points.

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    Solution Summary

    Several 2,4-DNP lab related questions are answered in detail.