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CHEM 232 - Expt#9 Elimination with Alcoholic Potassium Hydroxide Post-Lab Questions Post-Lab Questions: Answer the post-lab questions on the back of...


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CHEM 232 – Expt#9 Elimination with Alcoholic Potassium Hydroxide Post-Lab Questions Post-Lab Questions: Answer the post-lab questions on the back of the report sheet neatly and concisely. Use a blank sheet of paper if more space is needed. Your answer will be graded based on the quality of the answer not the quantity. You are expected to complete these post-lab questions independently as you would any other take-home assignment. All responses must be written in ink. Any lab report written in pencil will not be eligible for re-grade or if a white out was used on the lab report. 1. The elimination experiment performed in lab this week called for the use of KOH as the base forming two possible products. Would the expected result be different if instead we used potassium tert -butoxide as the base? Explain your answer including relevant structures. (4 pts) 2. Predict the major elimination product of the following reaction, support your answer by showing the mechanism of the reaction. (8 pts) Ph Ph Cl H EtONa Ph Ph Cl H EtONa 3. The above reaction was researched extensively by a Nobel Prize chemist. Find the study and print the first two pages of the article. From the article give three reasons why this chemist chose this particular system to study. (4 pts). 4. BONUS : The chemist from the above paper was the inventor of a model that is instrumental today in asymmetric synthesis (making exclusively one enantiomer in synthesis). What is the name of the model and what particular reaction did his model pertain to? (4 pts)
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Experiment #9 – Elimination with Alcoholic Potassium Hydroxide Removal of HBr from 2-bromo-2-methylbutane using non-sterically hindering base, KOH; an E2 reaction. Use of an alkyl halide substrate, whose á -C is substituted and a small nucleophile eliminates need to consider competition from substitution reactions. Transition state generated is highly substituted and therefore is of lower free energy and greater stability, resulting in the formation of the 2-methyl-2-butene, rather than the 2-methyl-1-butene product Recommendation: Purchase gloves from the stockroom ($1 per pair) Bullet List Protocol: 1. Take out glassware for fractional distillation. Set up hot plate and heating block; position ring stands in appropriate places. 2. Glassware set-up: Use a fractional distillation column as the condenser in the glassware set-up depicted on pg 3 41 . During the reflux portion of the experiment, water will pass through this condenser In the set-up attach a thermometer adapter and condenser to use to fractionally distill the products from unreacted reagents after the reflux In initial set-up, attach a vacuum adapter to the distillation condenser using a short piece of rubber tubing to hold it in place. During the reflux, low boiling point distillates will be collected in it 3. Lubricate all joints on glassware pieces with stop-cock grease. The strong KOH used in the reflux reaction can cause the joints to anneal. If this happens and you cannot disconnect the pieces at the end of lab, take the annealed pieces to the stockroom. A glassware vendor may be able to disconnect them, saving you the purchase price of a new set-up. 4. Scale reaction reagents for a 25-mL round bottomed flask: Place 12-13 mL 4M KOH in 1-propanol in the 25-mL round-bottomed flask, containing a stir-bar. Attach a drying tube to the flask. 5. Add 1.3 mL 2-bromo-2-methylbutane to the flask. Place on the heating block 6. Immediately attach the fractionating column to the flask. Use a Keck clip to clamp the flask to the column. 7. Attach the distillation glassware (thermometer adapter, condenser, vacuum adapter) to the fractionating column, securing all joints with Keck clips and using a clamp attached to a ring-stand to stabilize the condenser. 8. Attach water hoses to the fractionating column and start the water flowing (water “in” at the bottoml; “out” at the top). 9. Heat the hot-plate on a setting of 2-3 to begin the reflux. Reflux the mixture for between an hour and an hour and fifteen minutes (the text says an hour and a half, but Tas who have done the experiment say the shorter time is acceptable).
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10. The TA will give the pre-lab lecture when everyone has reflux going 11. Once reflux is done, reaction flask must cool before beginning distillation of products. To do this: Turn off the hot-plate. Raise the round-bottomed flask. ...along with all the attached glassware (rather than risking breaking it all, ask for TA or neighbor help to raise the condensers and resecure on the supporting rind-stand). of the heating block ~5". Cooling should take 5-10 minutes this way. 12. Drain the water from the fractionating column and attach tubing to the distillation condenser. Dispose of any low bp condensates collected. 13. When temperature drops below the bp of the products—37 C (1-methylbutene), 39 C o o (2-methylbutene)–lower the round-bottomed flask onto the heating block. Fit the vacuum adapter with a 10-mL round-bottomed flask; immerse the flask in ice. 14. With hot plate set between 3-3.5, it should take no more than 2-3 minutes for the distillation to begin. Directions call for collecting all distillates with bp < 45 C. That doesn’t get you o much Given that the 1-propanol (in which the KOH was dissolved) and 2-methyl-2- butanol have much higher bp (97 C), collect everything that comes off up to ~60 C o o 15. While distillation is going on, pre-weigh a sample bottle with tight-fitting stopper or cap. 16. Weigh the distillate to calculate yield. 17. Keep the product on ice while testing for unsaturation: 4 Baeyer Test (KMnO ): pg 1 5 0 : wear gloves; reagent can stain hands 4 Place 1-2 drops of sample in 2 mL 95% ethanol; add 0.1M KMnO , drop-wise until the permanganate color persists 4 Do the same by adding drops of KMnO to 2 mL 95% ethanol A significant difference in the number of drops is a positive test for unsaturation Bromine in dichloromethane: pg 149 : Bromine adds across a double bond of alkenes to produce dibromoalkanes 2 When reaction occurs, Br is rapidly consumed; its characteristic red-brown color disappears nearly instantaneously. 18. GC Analysis will be performed with the sample in o-xylene.
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Ghislaineb elimination.docx

1. In E2 elimination reactions, a base removes a proton from the adjacent carbon to a halide. The
more substituted alkene is formed, because it is more stable (Zaitsev rule). The sterically bulky...

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