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Dehydration

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Dehydration

  1. 1. Dehydration of 2-methylcyclohexanol
  2. 2. Dehydration Reactions Alcohol dehydration is an acid catalyzed reaction, which can be performed by strong, concentrated mineral acids such as sulfuric acid (H2SO4) and phosphoric acid (H3PO4).
  3. 3. Background Alcohol dehydration proceeds by an elimination mechanism called E1. + H OH H O-H + H carbocation CH3 CH3 CH3 H rearrangement + H-O-PO3H2 CH3 rate determining step secondary carbocation tertiary carbocation oxonium ion O-PO3H2 O-PO3H2 O-PO3H2 CH3 1st step - Protonation changes the very poor CH3 leaving group (-OH) into a good leaving group (H2O). major product 2nd step - Water departs, leaving behind a carbocation. This step is the rate determining minor product step (rds). 3rd step - The base H2PO4- removes a proton Saytzev’s rule: The more from the carbon adjacent to the positively charged carbon, forming an alkene and substituted alkene is the more regenerating the acid catalyst H3PO4. stable product (major product).
  4. 4. Dehydration of 2-methylcyclohexanol bp 165° C carbocation rearrangement minor products major product bp 110° C
  5. 5. Experimental Procedure Changes from the Manual procedures: Use heating mantel instead of sand bath. Don't forget boiling chips. Use 6 mL of 2-methylcyclohexanol and 5 mL of phosphoric acid. No sulfuric acid is used. Only the qualitative test with Br2 is performed. Test acetone as a blank, and the starting material, and the product. No refractive index will be measured. GC is performed on the product mixture.
  6. 6. Distillation Set Up Set thermometer bulb even with the joint of the distilling head (NOTE: this drawing shows it to be too low!) Heating mantel sitting on lab jack Do not allow temperature to rise above 100 ºC. Do not distill to dryness.
  7. 7. Transfer distillate to a clean centrifuge tube, then allow layers to separate. Organic layer containing all products (KEEP!!!!) Water layer (Use pipet to discard into waste bottle) Add 0.5-1.0 g CaCl2 to the centrifuge tube, shake, allow to stand for 10 min. (Tube must be stoppered). (NOTE: Even if you cannot see any water separated from the distillate, you need to add CaCl2, shake it up, and let it sit for 10 min.) While the product stands, dissolve the pot residue in water and transfer to the same waste bottle as the aqueous layer. Transfer the dried product to a preweighed sample vial. Do not transfer any CaCl2. Weigh the product, calculate % yield and then perform the unsaturation test.
  8. 8. Bromine test: To distinguish between saturated and unsaturated compounds, employ the observation of color changes. Label three small test tubes, one for the blank, one for the starting material (methylcyclohexanol), and one for products. Place 2 drops starting material and the product in each test tube. Add 20 drops of acetone into each test tube and shake. Add 2 drops of Br2 test reagent to each tube and shake. Observe any color change within five minutes.
  9. 9. GC analysis: Dissolve 2-3 drops of product in 1 mL of acetone, then inject 1 µL into GC. Identify peaks by comparing retention times with those of the pure alkenes (These will be posted). Report the ratio of the products (equal to the ratio of peak areas). Do not use the calculated areas on your chromatogram. Measure the peak height (h) and width (w) (at half the peak height). Area = h X w Cautions: Phosphoric acid is highly corrosive. Methylcyclohexenes are highly fammable. Br2 is toxic and an oxidizer. Grading is 50% for the written report and 50% for results and observations.

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