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Synthesis of Cyclohexene

Synthesis of Cyclohexene. Synthesis of an Alkene by Dehydration of an Alcohol via E1 (Elimination) Mechanism Soloman’s & Fryle: pp 297 – 302 Slayden, et al: pp. 51 – 52 Pavia et al: pp. 179 – 183. E1 Synthesis of Cyclohexene. Background

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Synthesis of Cyclohexene

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  1. Synthesis of Cyclohexene Synthesis of an Alkene by Dehydration of an Alcohol via E1 (Elimination) Mechanism Soloman’s & Fryle: pp 297 – 302 Slayden, et al: pp. 51 – 52 Pavia et al: pp. 179 – 183

  2. E1 Synthesis of Cyclohexene • Background • An Elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism • The one-step mechanism is known as the E2 reaction • Two-step mechanism is known as the E1 reaction • The numbers have nothing to do with the number of steps in the mechanism, but rather the kinetics of the reaction, bimolecular and unimolecular respectively • Unimolecular reaction: an elementary reaction in which one or more molecules of product are formed • In most organic elimination reactions, hydrogens are lost to form the unsaturated Alkene double bond

  3. E1 Synthesis of Cyclohexene • The E1 reaction is a two-step process of elimination: • Ionization: the carbon-hydroxyl bond breaks to give a Carbocation intermediate • Deprotonation of the carbocation to form alkene • E1 typically takes place with tertiary alkyl halides, but is possible with some secondary alkyl halides • Today’s experiment involves a secondary alcohol • The first step in the mechanism is protonation of the alcohol group by the acid (slightly exothermic).

  4. E1 Synthesis of Cyclohexene • The second step is the loss of water to form the carbocation (highly endothermic) • The reaction rate is influenced only by the concentration of the Alcohol because the carbocation formation is the slowest step called the rate determining step • The rate equation for the unimolecular carbocation sets up as first order kinetics • Reaction usually occurs in acidic conditions and at high temperature • The final step is removal of a beta hydrogen by base (water) to form the alkene (exothermic)

  5. E1 Synthesis of Cyclohexene Acid Catalyzed Dehydration of an Alcohol to Alkene

  6. E1 Synthesis of Cyclohexene Mechanism Protonation of Hydroxyl Group Carbocation Formation of Carbocation – a strong Electrophile (slow rate determining step) Nucleophile Proton Abstraction (fast)

  7. E1 Synthesis of Cyclohexene • Procedure overview • Synthesis Experiment • Determine Limiting Reagent and Theoretical Yield • Cylcohexanol is dehydrated by acid to form an alkene Mol Wgt – 100.158 Density – 0.962 g/mL • The acid used is 85.5% Phosphoric acid (H3PO4) MW – 96.995 g/mol; M 14.8 mol/L; Den - 1.71 g/mL • The Phosphoric Acid acts as catalyst to increase the rate of reaction and serves as a source of protons to protonate the hydroxyl group; thus it is not a reagent • Therefore, Cyclohexanol is the limiting reagent • From the balanced reaction 1 mole of alcohol produces 1 mole of alkene • The theoretical yield of alkene in moles is therefore equal to the number of moles of alcohol used

  8. E1 Synthesis of Cyclohexene • Equipment • Simple Distillation Apparatus • West Condenser with rubber tubing • Distillation Head • Thermometer/Thermometer Adaptor • Distillation Flask • Hot Plate with sand bath • Separatory Funnel • Small Beakers, Small Erlenmyer Flasks • Materials • Cyclohexanol • Phosphoric Acid • Sodium Carbonate • Anhydrous Sodium Sulfate • Potassium Permanganate

  9. E1 Synthesis of Cyclohexene • Procedure • Determine Mass of Cyclohexanol by weighing • Compute moles of Cyclohexanol • Setup balanced Stoichiometric Equation • Determine Molar Ratio • Setup reaction mechanism • Determine Limiting Reagent • Compute Theoretical Yield mass

  10. E1 Synthesis of Cyclohexene • Procedure • Assemble Simple Distillation apparatus • Place the contents of the vile containing the Cyclohexanol and ca 2.5 mL 85% Phosphoric Acid in a 50 mL distillation flask • Mix reagents mixture thoroughly • Add boiling chip • Place a 25 mL receiving flask into an ice/water bath • Turn on water circulation for condenser • Heat mixture until product begins to distill – about 95oC but no more than 100 oC

  11. E1 Synthesis of Cyclohexene • Procedure (cont’d) • Continue to collect distillate until bubbling action stops or the temperature rises rapidly to over 100oC (a few mL of residue will remain in the distilling flask Stop the Distillation • Saturate the distillate with solid Sodium Chloride • Add the salt, little by little, and swirl the flask gently • When no more salt will dissolve, add enough 10% aqueous Sodium Carbonate solution to make the aqueous layer basic to litmus (blue) Insert plastic pipet into bottom aqueous layer to obtain a few drop of sample for testing with litmus • Pour the neutralized mixture into a separatory funnel

  12. E1 Synthesis of Cyclohexene • Gently swirl the mixture, vent • Allow layers to separate • Drain the bottom aqueous layer into a waste beaker • Pour the upper organic layer (Cyclohexene) through the “neck” of the separatory funnel into a dry 50 mL Erlenmeyer flask • Add enough Anhydrous Sodium Sulfate to the flask, with occasional swirling, to dry the product – it will appear clear when it is dry (10-15 minutes may be required) • Stopper the flask • Dry and reassemble distillation apparatus using 25 mL distillation flask

  13. E1 Synthesis of Cyclohexene • Place a 25 mL receiving flask into an ice/water bath • Decant the dried Cyclohexene into the distilling flask and add a boiling chip • Distill the Cyclohexene (BP  83oC) and collect the material that boils over a range of 2-3 degrees before and after the boiling point of cyclohexene • Determine the mass of the product in a tared vial • Calculate % yield • Determine the Refractive Index (1.4465) • Adjust Refractive Index for Temperature • Obtain IR Spectrum

  14. E1 Synthesis of Cyclohexene • Test your reagent and product for presence of a double bond • Place 4-5 drops of Cyclohexanol into each of two small test tubes • Place 4-5 drops of Cyclohexene product into each of two small test tubes • Take one test tube from each group and add a solution of Bromine in Carbon Tetrachloride (or Methylene Chloride) drop by drop until the red color is no longer discharged, i.e., solution becomes colorless Note: One of the test tubes will not discharge the color Note: record the number of drops added

  15. E1 Synthesis of Cyclohexene • To the remaining two test tubes add about 0.3 mL of 1,2-Dimethyloxyethane Note: This solvent makes the Potassium Permanganate used in the next step miscible with the organic compounds • Add a solution of Potassium Permanganate drop by drop to the remaining two test tubes (containing the 1,2-Dimethyloxyethane) until the purple color is discharged and replaced with a brown precipitate of Manganese Dioxide (MnO2) Note: One of the test tubes will not discharge the purple color

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