the Prelab as before including pertinent instructions from your column chromatog
ID: 565477 • Letter: T
Question
the Prelab as before including pertinent instructions from your column chromatography. Answer the following PRELABORATORY EXERCISE: Techniques manual on questions I. Why must the column never go dry? 2. Why do you start with the least polar solvent? 3. Based on the structures of lycopene and P-carotene.: a) Are they stereoisomers, constitutional isomers or not even isomers? b) Do you expect them to be relatively polar or nonpolar substances? c) How many double bonds are in lycopene? How many in -carotene? d) As lycopene is transformed into -carotene, how many pi electrons Why? from the double bonds are transformed into single bonds? How many new single bonds are being formed in -carotene? e) The first step of a possible mechanism for the ring closure in the transformation is outlined below (only part of the structure is shown for simplicity). Draw the product that results from this arrow pushing mechanism. Be sure to include any formal charges that arise from the carbon atoms. ATIONS: bort all observations in the lab notebook. Be sure to note any deviation from the erimental set up and procedure.Explanation / Answer
1) If you let the column run dry the silica will start to crack and you will get poor separation of your compounds.
As you run the column, never let the level of solvent go below the level of the silica gel or you will get poor results.
If it goes dry, it tends to create crevices and cracks in the packed column.This affects the performance of the column also If it goes dry, it tends to makes breaks in the packed column. Therefore column will be uneven.
2) In non-polar solvents like pentane and hexane, most polar compounds will not move, while non-polar compounds will travel some distance up the plate.
Depending on the polarity of the components of the mixture, different compounds will travel different distances up the plate. More polar compounds will "stick" to the polar silica gel and travel short distances on the plate, while non-polar substances will diffuse into the solvent and travel large distances on the plate. The measure of the distance a compound travels is called Rf. This number, between zero and one, is determined by measuring the distance the compound moved from the baseline (where it was originally spotted) divided by the distance the solvent moved from the baseline.
In overall Make the solvent system more polar if you want a larger Rf or make it less polar if you want to decrease the Rf
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