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This text provides important due dates and instructions for the Organic Chemistry Lab course in Spring 2017 taught by Dr. Pant. It includes information on recrystallization, thin-layer chromatography, grading, and more.
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Organic Chemistry Lab 315 Spring, 2017 (Dr. Pant’s section)
DUE DATES • Today Nothing • Next Week • Recrystallization Report • Thin-layer Chromatography Report (follow instructions from the Professor)
Grading of M.Pt.Report • A common mistake was in citing physical properties of knowns and identified unknowns. • Citing physical properties was the reading assignment for the first week. The “Lab Reports” page with instructions for the report you were to turn in made special mention of it and provided a link. • The Manual was very explicit about approved sources (CRC Handbook, Merck, and DOC). No others are allowed. • All compounds have unique identifying codes in the sources, and these must be given as part of the reference, as explained and illustrated in the Manual. • You were asked to turn in a capillary tube containing a sample of your unknown. One purpose for this request was for you to demonstrate you could fill the tube with no more than 2 mm of solid – this was emphasized during recitation. You were told rulers were available. • The written reports were graded somewhat leniently, but questions that could have been answered by reading the text/Manual were graded accordingly.
In Lab Today-Recrystallization, cont’d. • Finish crystallization experiment • Weigh container + dry product (and container if not done last week) to get final dry mass of fluorene and calculate a % recovery. • Put product in vial (see next slide). • Determine m.pt. of recrystallized product and a m.pt. of the impure fluorene from last week. • Be sure to comment on your results in your notebook. • Affix label (see Manual for information to include on label). Store in class drawer. • Turn in product with Report next week. • Get products ready TODAY – there will not be time next week for you to fix a label, etc.
Notes • Transferring a solid to a vial If funnel bottom is not available, make a funnel from a small piece of paper.
Percent Recovery • whole = all of that with which you began • part = that which has been isolated Remember, you cannot achieve (nor expect to) 100% recovery. Some of the original substance was soluble in the solvent at 0 deg., and some of the original material in the vial was an impurity. Review significant figures for use in Recrystallization Report calculation.
Chromatography • Chromatographyseparates components in a mixture based on the differences in their intermolecular interactions between a stationary phase and a moving phase. • Phases can be solid, liquid, or gas. • In this course we will learn TLC (solid stationary phase/liquid moving phase) and gas chromatography (GC) (solid stationary phase/gas moving phase).
Thin-layer Chromatography • In this TLC experiment, the stationary phase is solid silica, thinly coated on a plastic plate. • Silica is a polar substance with many –OH (hydroxyl) groups. • The moving phase is dichloromethane, a moderately polar solvent. • The components of a mixture will partition between the stationary and moving phases, depending on the extent of their interactions in the two phases.
Thin-layer Chromatography • The components of the mixture to be separated are fluorene, fluorenol, and fluorenone. • The names give an indication of the functional groups that are present in each compound. • The compounds are named above in order of increasing polarity. • The interactions among the stationary phase, the moving phase, and the components are complex and the elution order may surprise you!
Thin-layer Chromatography • The separation of components is measured by the “retention factor”, Rf. • I • If all TLC conditions remain the same, the Rf would be constant for each compound (not easily achievable).
Experiment Notes • Follow the procedure in the TLC experiment . • The amount of solvent in the developing chamber depends on where you draw the baseline and spot your samples. • Pay attention to your technique for spotting and placing the plate in the chamber – this is one experiment where you may need to start over!
Experiment Notes • Carefully place the plate in the developing chamber, straight down so the solvent starts ascending the plate evenly (not at an angle). • Remember to trace the solvent front with a pencil when you remove the chromatogram from the jar.
Experiment Notes Visualization of components • The visualization chamber consists of a mixture of iodine (I2) and silica gel. • Put the developed TLC in the visualization chamber & screw the lid tightly (2 plates in each jar is OK – back to back). • Slowly oscillate the jar, making sure to coat the chromatogram with the mixture. Continue until dark spots appear. • Take out the plate, shake off the silica-iodine from the surface & immediately outline the spots with a pencil.
Experiment Notes • Be sure to accurately draw a picture of your developed chromatogram in your notebook (see procedure instructions). • Give the developed chromatogram to me, with your name at the top.
Report for Next Week • Fill in the Report Form • Attach a photocopy of the drawing of the developed chromatogram from your notebook.
