Experiment 4 ATQ Chem 31.1

EXPERIMENT 4 PAPER CHROMATOGRAPHY  QUESTIONS:

1. Why is the chromatogra chromatogram m developed developed in an essentially essentially closed system? 2. What are are the considerations considerations in choosing a chromatographic chromatographic solvent? 3. Compare Compare and contras contrastt between between normal normal phase and revers reverse e phase chromatography. 4. Explain how iodine iodine crystals crystals help to visualie visualie the other pigments. pigments. !. "uggest "uggest one method method that can be used to visuali visualie e highly highly con#ugated con#ugated compounds but are not colored. ANSWERS:

1. $he chromatogram chromatogram is is developed developed in an essentially essentially closed closed system system because because it needs to be developed in an atmosphere saturated with solvent vapor. %& it is not closed' then evaporation o& solvent may occur &aster than it is supposed to be' which replaces the capillary action that must ta(e place. %n this case' separation o& components may not be achieved. 2. "ince the basic principle principle o& chromatography chromatography is that di)erent di)erent substances substances have di)erent partition coe*cients between stationary stationary and mobile phases' it is important to choose a solvent that is polar enough to have di)erent a*nities with the compound. $he compound that interacts wea(ly with the stationary phase will spend most o& its time in the mobile phase and move rapidly while compounds that interact strongly with the stationary phase will move very slowly. +s a conse,uence' each will move through a system with di)erent rates that leads to success&ul success&ul separations. 3. %n normal normal phase' the stationary stationary phase is is polar and selectively selectively retains retains the polar substances in the mixture and the mobile phase is non polar that should move non polar compounds in the system. -n the other hand' reverse phase chromatography has a non polar stationary phase and polar mobile phase. 4. $he use iodine vapour in visualiing the other pigments depends on the ability o& the mixture to have a chargetrans&er complexes with iodine. Compounds that do not contain

π 

 or /ewis basic &unctions do not &orm

charge trans&er complexes with iodine. $hrough wea( electronic attraction' iodine will collect on the spots and &orm a yellow or brown stain on most organic compounds. !. %& the compounds compounds are are not colored' colored' the paper paper is 0rst 0rst sprayed with a solution solution o& sul&uric acid &or better charring. luorescent materials can be visualied with ultraviolet light. $he developed plate is irradiated irradiated with a low density unltraviolet lamp' and then the uorescence o& the dye being ,uenched by the compounds on the plate produces a light blue bac(ground o& the uorescing dye. References:

. asto' C. 5ohnson' 6. 6iller. 1772. Experiments and $echni,ues in -rganic Chemistry. rentice8all %nternational Ed. -rganic Chemistry. 2914 Ed. %nstitute o& Chemistry :niversity o& the hilippines