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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 Erlenmeyer 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

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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 ten (10)

drops of a solution of Bromine in Methylene Chloride. If the red color is discharged, i.e., solution becomes

colorless, the double bond in the Cyclohexene has been broken followed by the addition of a Bromine atom to each of the two carbon atoms constituting the original double bond

Note: One of the test tubes will not discharge the color

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 10 drops of a solution of Potassium Permanganate drop by drop to the remaining two test tubes.

If the purple color is replaced with a brown precipitate, the cyclohexene has been oxidized (double bond broken) and converted to an alcohol.The Permanganate MnO4

- has been reduced to Manganese Dioxide (MnO2)

Note: One of the test tubes will not discharge the purple color

E1 Synthesis of Cyclohexene