Paper Chromatography 45526k

PAPER CHROMATOGRAPHY J.V. DE GUZMAN DEPARTMENT OF CHEMICAL ENGINEERING, COLLEGE OF ENGINEERING UNIVERSITY OF THE PHILIPPINES, DILIMAN QUEZON CITY, PHILIPPINES DATE PERFORMED: FEBRUARY 11, 2015 INSTRUCTOR’S NAME: ALLAN KENNETH REGUNTON

ANSWERS TO QUESTIONS 1. Why is the chromatogram developed in an essentially closed system? The chromatogram is kept in an essentially closed system in order to prevent it from acquiring contaminants and other substances that can hinder in the separation of the organic pigments. It also prevents the developing solvent used from evaporating since most solvents used in paper chromatography are highly volatile, and some can be flammable and toxic. 2. What are the considerations in choosing a chromatographic solvent? The chromatographic solvent holds large significance in separating the components of the sample. If the solvent is polar, the spots will all at the top of the plate only and if nonpolar, the spots will still be at their initially positions. In both cases, ΔR f will be zero. As such, polarity must be taken into consideration when choosing the appropriate solvent. Usually, the chromatographic solvent is a mixture of polar and nonpolar solvents combined in appropriate ratio. The properties of the compound to be separated must also be taken into since it will help identify the appropriate ratio of the polar and nonpolar components of the chromatographic solvent to be used. The nature of the solid is also of important consideration because it will help in providing the appropriate concentration of solvent without degrading the material. 3. Compare and contrast between normal phase and reverse phase chromatography. Normal phase and reverse phase chromatography both take advantage of the polarity of the constituents of the sample and make use of a stationary and mobile phase to separate them from each other. In normal phase chromatography, the stationary phase is the polar part while the mobile phase is less or nonpolar. However, in reverse phase chromatography, the reverse is true: the stationary phase is the less or nonpolar while the mobile phase is polar. 4. Explain how iodine crystals help to visualize the other pigments. Iodine crystals will form iodine vapor which in turn will form a

yellow or brown complex or stain with almost all organic compounds except saturated alkanes. When spots formed from the developing solvent faded away, iodine vapor forms iodine stains with these pigments that can be easily observe. Iodine is reversible and is nondestructive which makes it a convenient visualization technique. It is also useful in detecting carbohydrates since it forms black spots with iodine. 5. Suggest one method that can be used to visualize highly conjugated compounds but are not colored. Ultra-violet light detection is also a commonly practiced method that helps in visualizing compounds that are not very noticeable using ordinary solvents. It is a nondestructive technique that will visualize any compounds that absorb UV radiation. It makes use of phosphor as the absorbent that fluoresce in UV light. A compound, even if not colored, is detected if dark or black spots are observed on the plates since the compound blocks the glow or fluorescence of the absorbent. If the compound fluoresces too, it will appear as a bright spot on the plate when illuminated with UV light. 6. Given a mixture that contains the following compounds: 2chloropentane, pentanoic acid, cyclopentane and pentanal. Arrange the following compounds in increasing R f if the mixture is ran using paper chromatography with the same solvent used in the experiment. The polarity ranking of the functional groups in increasing order is: amide>carboxylic acid>alcohol>ketone~aldehyde>amine>ester>alkyl halides> ether>alkane. As such, if the given compounds are arranged in increasing polarity, it will become: pentanoic acid>pentanal>2chloropentane>cyclopentane. Note that the paper chromatography uses petroleum ether mixed with acetone at a ratio of 9:1 as the developing solvent. Since the developing solvent is mostly composts of the nonpolar petroleum ether than the polar acetone, it is expected that it will dissolve well the less polar compounds. As such, as the solvent ascends in the chromatographic paper, it will carry first the nonpolar compounds and their distance travelled together with Rf value will be greater than the more polar ones. Therefore, in order of increasing Rf: pentanoic acid <2chloropentane
Virtual ChemBook: Elmhurst College. [Online]. 2003. http://www.elmhurst.edu/~chm/vchembook/213organicfcgp.html (accessed February 15, 2015). University of Toronto Scarborough. Thin Layer Chromatography Theory. Chemistry Online UTSC. [Online]. 2014. http://webapps.utsc.utoronto.ca/chemistryonline/tlc.html (accessed February 15, 2015).