1/17/2015 1 synthesis of cyclohexene synthesis of an alkene by dehydration of an alcohol via e1...
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Synthesis of Cyclohexene
Synthesis of an Alkene by Dehydrationof an Alcohol via
E1 (Elimination) Mechanism
Soloman’s & Fryle: pp 297 – 302Slayden, et al: pp. 67 – 68 Pavia et al: pp. 179 – 183
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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
E1 Synthesis of Cyclohexene
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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).
E1 Synthesis of Cyclohexene
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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)
E1 Synthesis of Cyclohexene
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E1 Synthesis of Cyclohexene
Acid Catalyzed Dehydration of an Alcohol to Alkene
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E1 Synthesis of Cyclohexene
Protonation of Hydroxyl Group
Formation of Carbocation – a strong Electrophile(slow rate determining step)
Proton Abstraction (fast)
Nucleophile
Mechanism
Carbocation
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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
E1 Synthesis of Cyclohexene
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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
E1 Synthesis of Cyclohexene
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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
E1 Synthesis of Cyclohexene
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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
E1 Synthesis of Cyclohexene
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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
E1 Synthesis of Cyclohexene
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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
E1 Synthesis of Cyclohexene
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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
E1 Synthesis of Cyclohexene
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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
E1 Synthesis of Cyclohexene
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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
E1 Synthesis of Cyclohexene