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. 67 – 68 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
Formation of Carbocation – a strong Electrophile
(slow rate determining step)
Proton Abstraction (fast)
Nucleophile
Carbocation

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 – Density – g/mL
The acid used is 85.5% Phosphoric acid (H3PO4)
MW – g/mol; M 14.8 mol/L; Den 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
Using a glass funnel, 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 pre-weighed or 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