Experiment 6: Dehydration of 2-Methylcyclohexanol Dehydration of 2-Methylcyclohexanol This week's reaction: A B - dehydration of a 2 alcohol to give a mixture of alkene isomers - H 3 PO 4 is a catalyst (facilitates reaction, but is not consumed) - water is lost as reaction proceeds Reading: Mohrig, Hammond & Schatz Ch. 19 pgs 256-276 Carey & Guiliano Ch. 5 pgs 184-207 Ch. 6 pgs 250-253 Which product will be major? A Why? Zaitsef's rule - most highly substituted (most stable) product is preferred How does this reaction occur (what mechanism)? E1 Reaction Mechanism E1 elimination: Types of Chromatography 1. Thin Layer Chromatography (TLC) stationary phase: spread over glass or plastic sheet mobile phase: liquid; drawn up plate by capillary action 2. Column Chromatography stationary phase: contained in a column mobile phase: liquid; passes through column (gravity or pressure) Note: reaction is reversible - drive to completion by removing product from reaction mixture e.g. distill - products are volatile (boiling points?) 3. (GC) stationary phase: contained in a column mobile phase: gas; passes through column (pressure)
Useful analytical technique - follow reaction progress (requires a standard) - use for product identification (requires a standard) - determine product purity - determine ratio of product or isomers (quantitative measure) Extremely sensitive - can separate very complex mixtures (hundreds of components) - don't need much material (typically 10-6 - 10-15 g µl) Often used in environmental & forensic labs - identification of trace amounts of material Basic principles, same as other forms of chromatography, but: - stationary phase: non-volatile polymer on glass or solid support - mobile phase: inert gas - typically He - column length: 10-30 feet - temperature: 25-300 C How does it work? - long column packed w/stationary phase & placed in an oven - inert gas (mobile phase) passed through column at a controlled flow rate - liquid sample injected (µls), vaporized as passes through column, & detected as it emerges Compounds travel through column at different rates depending on: - volatility - strength of interaction with stationary phase lower boiling compounds tend to come off first Longer GC column leads to better separation Detector response plotted vs. time detector response is proportional to amount of compound passing through it can quantify amounts by comparing peak areas Peak Area = (Peak Height) x (Peak Width at 1 / 2 Height) 0.4 cm 0.5 cm Retention time: time it takes a compound to emerge length of time compound is retained in the column Retention time = d cs = distance from injection point to sample peak (cm) chart speed (cm/min) RT will remain constant for a given set of conditions (column type & temp) Peak Area X = (1.2 cm) x (0.4 cm) = 0.48 cm 2 Peak Area Y = (4.1 cm) x (0.5 cm) = 2.05 cm 2 4.1 cm 1.2 cm Ratio X : Y = 1 : 4.3
Sample GC Trace - cyclohexane:toluene (1:1) Next Week (October 18-22) Experiment 6: Dehydration of an Alcohol/Analysis by GC retention time peak areas A. Synthesis of a mixture of alkene isomers dehydration of 2-methylcyclohexanol isolate products by distillation B. Analysis bromine test for unsaturation TLC IR GC Analysis: determine ratio of two double bond isomers injection point DUE: Column Chromatography Lab Report (exp 5) Lab Reports are due at the beginning of your regular lab session note baseline separation of peaks Experimental Details 1. Combine reactants in long neck roundbottom flask carefully combine 2-methylcyclohexanol and 85% phosphoric acid wear gloves phosphoric acid can cause burns 2. Add a boiling chip & assemble a modified fractional distillation apparatus (a long neck flask with stainless steel sponge in the neck) will drive reaction forward by removing product alkenes as they form water co-distills with your product 4. Before you begin check to be sure all joints are sealed & clamped check the thermometer position 5. Proceed with distillation cool the collection vial in ice (products are volatile) do not allow temperature of distillate to exceed 96 C Apparatus thermometer adapter distillation head connector with support rod long-neck roundbottom flask packed with stainless sponge boiling chip collection vial keep cold ice bath sand bath CHANGE do not tent apparatus with foil do not distill the flask to dryness; 6. Stop distillation; Keep the collection vial COLD DO NOT TENT THE APPARATUS WITH FOIL DURING DISTILLATION
Experimental Details 7. Wash distillate with NaHCO 3 (you are doing a mini extraction) add solid NaCl to product vial to saturate the aqueous layer add 1 ml saturated NaHCO 3 mix well by drawing layers in & out of a disposable pipet check ph to be sure any acid has been neutralized 8. Draw off aqueous layer using a pipet which layer is the aqueous layer??? the bottom layer keep everything just to be sure Bromine Test for Unsaturation Qualitative Test - helps identify what functional groups are present in this case we are looking for the presence of a C=C can be detected by addition of bromine to a solution of an organic compound positive test is decolorization of the solution (colored colorless) How it Works: Halogenation of the Double Bond 9. Dry organics over anhydrous Na 2 SO 4 cap & keep vial cold while drying 10. Transfer the dried product to a clean, dry, preweighed (with cap) vial include the cap when you pre-weigh the vial 11. Weigh product in capped vial mass of product can be used to calculate the percent yield 12. Analyze the product bromine test, TLC, IR, GC can be done in any order + Br 2 (reddish-brown) Br Br (colorless) Thin Layer Chromatography Will visualize spots using a chemical stain - KMnO 4 dip plate, let dry on paper towel do this in the hood ONLY KMnO 4 is a strong oxididant This is a destructive method of visualization compounds undergo a chemical reaction on the TLC plate will see light spots on a purple background may take some time to develop Writing the Lab Report: Exp #5 Column Chromatography Purpose - technique experiment: what will you learn? - what conclusions will you reach? - a general discussion of theory/expected results is not a purpose Results & Discussion - Identify contents of each flask (flask #1 and flask #2) which is ferrocene? which is acetylferrocene clearly explain how you reached this conclusion what data can you use? TLC - correlate R f to structure color (what color should it be?) IR discuss success of separation are the fractions pure? how efficient was the separation (e.g. discuss your % recovery) be sure to justify your answers using the data/info available
Writing the Lab Report: Exp #5 Column Chromatography Conclusion - a brief recap of your findings - include a general statement about column chromatography (what did you learn?) - do not include a lot of theory Appendix A: Calculations - Rf - Percent Recovery Appendix B: Spectra - Page of IR spectra (provided)