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Ochem Two- Preparation of Cyclohexene
Organic Chemistry Lab weeks 3 and 4
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Ochem Two- Preparation of Cyclohexene
Preparation and Distillation of Cyclohexene
The goal of this lab is to produce cyclohexene via the elimination of water in cyclohexanol in an E2 elimination reaction. This reaction occurs via an acid catalyzed dehydration and is seen in the conversion of and alcohol to an alkene. This is considered a green lab because 85% phosphoric acid H3PO4 is used in this lab instead of the more hazardous acid H2SO4 sulfuric acid. In this lab H3PO4 is the acid catalyst that causes this reaction to undergo a 1,2-elimination
reaction when heated that generates the products alkene and water. The reactant is cyclohexanol and it is called dehydration, because it involves the removal of a molecule of water. The major product usually ends up being the alkene that follows Zaitsev's rule and is the higher substituted one as well. This lab was done via distillation of the cyclohexanol and the catalyst to produce cyclohexene and water (H2O). Cyclohexene is not good for long term storage when exposed to light and air, because it produces peroxides. The product cyclohexene is used mostly as a solvent, but is also used in other chemical reactions as a precursor to produce things like adipic acid, maleic acid, dicyclohexyladipate, and cyclohexeneoxide.
SAFETY PRECAUTIONS: Phosphoric acid, while safer than sulfuring acid, is corrosive. Avoid contact, and clean up any spills immediately. Cyclohexanol does not appear to present any unusual safety hazards. Cyclohexene is flammable and has a disagreeable odor.
1. To a 50 mL round-bottom flask containing a magnetic stir bar (or boiling stone), add 0.074 moles of cyclohexanol and 1.75 mL of 85% H3PO4. Use gentle swirling to mix the two layers.
2. Fit the flask with a fractionating column, a distillation adapter, a thermocouple (or thermometer), a condenser, and a vacuum adapter as for fractional distillation (see illustration). A rubber septum should be used to provide a seal between the thermocouple or thermometer and the glassware. Be sure that the seal is good--if it is not, cyclohexene will escape from your glassware, causing your experiment to fail, and those of your classmates who find the odor of cyclohexene objectionable will complain loudly! A drying tube as shown in the illustration, can help to control the disagreeable odor of cyclohexene.
3. Heat the reaction mixture first at a gentle reflux for about 5 minutes, then heat the flask more strongly in order to distill the mixture into the collection flask. Keep distilling until the volume remaining in the distillation flask has been reduced to approximately 1 mL.
4. Transfer the distillate to a separatory funnel and wash with approximately 5 mL of water. Carefully separate the layers and transfer the organic layer into a small, dry Erlenmeyer flask. If any water droplets are visible, remove them before adding the drying agent (sodium sulfate). Add a small amount of anhydrous sodium sulfate to the flask. Let the mixture stand for 5 minutes, occasionally swirling it gently. If the drying agent completely clumps together, its capacity to remove water has been exceeded and a little more sodium sulfate should be added. If you have successfully removed the water, the liquid should be clear, and at least a little of the drying agent should remain free flowing.
5. Decant or pipette the organic liquid away from the drying agent and place it in a clean,dry round-bottom flask. This will be the distillation flask for the next step. The appropriate size depends upon your yield. The flask should be about half full at the beginning of the distillation.
8. Transfer the distilled cyclohexene to a clean, dry, pre-weighed sample vial and determine the mass of the product. Record an infrared spectrum of the distilled product.
Strategies, Tools, and Laboratory Experiments
by K.M. Doxsee and J.E. Hutchison, Thempson Brooks/Cole, 2004; pp 129-134.
Table 1: Mass and Observations
Molecular Weight (g/mol)
clear liquid with noxious odor
Table 2: Heat % Recordings Before Distillation Began
Temperature (degrees Celsius)
50.0 to 29.0
Table 3: Lab Quest Distillation Data
Temperature (degree Celsius)
Molecular formula: C6H12O
Molecular formula: H3PO4
Molecular Formula: C6H10
Based on the IR Cyclohexene graph directly above no cyclohexene was distilled via this acid catalyst reaction in the lab and the IR shows the spectrum of water (H2O) as the resulting product.
Well, you've got an interesting result, that's for sure! Your distillation temps never got above 60....maybe your cyclohexene was stuck in the fractionating column.
The overall goal of this lab was not completed, because no measurable amount of cyclohexene was produced from the lab. This gave inaccurate data to be recorded based on the temperature provided.
I don't fully understand why you would call it "inaccurate."
From the percent temperature graph
and the distillation data graph from above the constant change in temperature varies
I need you to explain to me "constant change in tempurature varies."
. There was no product of cyclohexene that was weigh able, but when the water and cyclohexene were combined in the seperatory funnel when the pressure was released an unknown amount of pressure was removed. This means there must have been a small amount of cyclohexene in the seperatory funnel.
good thinking here.
The goal of weighing the final product was not achieved, but an unknown amount of cyclohexene might have been produced based on that data. The IR graph of the product shows that water was produced based on the spectrum of the graph and when comparing it to the control graph of cyclohexene it is not the desired product.
The first factor of error in this lab could have been from the fact that the thermometer was touching the inside of the distillation adapter throughout most of the experiment, but near the time distillation started to occur it was noticed and changed. Another factor of error was when the drying agent anhydrous sodium sulfate was added to the product of the reaction before the water and cyclohexene were separated via the seperatory funnel resulting in a loss of product before it was washed with 5ml of H2O. When the products were put into the seperatory funnel with the 5ml of water only a small amount the size of a dime of what was thought to be cyclohexene rose to the top of the surface of the water and when the water was released all of the cyclohexene that was collected via distillation was released with the water into the beaker. Another error factor might have been a product being produced besides the desired products of the reaction.
You suggest that you saw some liquid distill over into your receiving vial, and that this was transferred into the separatory funnel. Did you estimate the volume of this product? Where did it go? At what exact point did you lose track of it?
Post Lab Question:
for this reaction. Atom economy is defined on Wikipedia, but essentially is the mass of the desired product divided by the mass of all reactants (don’t include catalysts or solvents in our reaction).
% atom economy= (82.143 g/mol)/(100.158 g/mol)=0.820134188 x 100=82.013%
This lab earned the following scores for: format (2/2) style 1.5/2) data (3/3) quality of result (0/1) quality of reported data (0.5/1) conclusion (1/2) error (1/1) PLQ (2/2) for a total of 11/14. A better result in the lab would provide an opportunity for a better score, but there are also a few issues that have to do with the construction of the report. I hope you will read my comments carefully.
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