In this experiment an alkene cyclohexene will be prepared by dehydration of an alcohol cyclohexanol using an acid catalyst such as sulfuric acid or phosphoric acid. Transfer approximately 2ml of your product to a dry test tube. Record the boiling point, and the mass and volume of the pure product. That is, the reaction takes place over two steps, the first being the formation of a carbocation intermediate. The final step is removal of a beta hydrogen by base water to form the alkene exothermic. Use a collection flask or beaker that is no less than 50 mL in volume. It is used in common industrial processes, but is also considered somewhat unstable due to it's tendency to form peroxides when exposed to light.
Next, redistill by simple distillation. Thus, water a much better leaving group is the leaving group in this reaction step 2 and the product is a secondary carbocation. This is in fact what happens- the carbocation undergoes a 1,2 hydride shift. Far better would be to first protonate the alcohol. This is known as Saytcheff orientation.
This is because a 1º carbocation is too unstable to form. Carbocation Rearrangement After the protonated alcohol leaves the molecule you are left with a secondary carbocation. But this secondary carbocation could become the more stable tertiary carbocation if the hydrogen- bond and all- moved over from the adjacent tertiary carbon. Note the color of each solution after addition of the bromine solution. From the dropping bottle provided, transfer 5 drops of 5% Bromine in 1,2dichlorocyclohexane.
Be sure that the temperature never exceeds 85 degrees. There is also the possibility of a carbocation rearrangement. So how can you prevent the reverse reaction? For example, refluxing 2-methylcyclohexanol in the presence of phosphoric acid gives 1-methylcyclohexene as a major product, 3-methylcyclohexene as a minor product, while very little methylenecyclohexane is formed. Keep the temperature below 96 ºC. Phosphoric acid is present as a catalyst which promotes the reaction but is not consumed in it. Specifically, cyclohexanol is heated in the presence of concentrated phosphoric acid to cause an E1 elimination reaction.
In the following step step 3 , a molecule of water deprotonates the carbocation at either of the adjacent carbons. The distillate will be a mixture of water and cyclohexene. This week you will be preparing an alkene by dehydrating an alcohol. Transfer the contents of the organic phase to a dry round bottom flask. Write a complete mechanism for each case, showing all steps and using electron-pushing arrows. Show stereochemistry if it is specific. Cyclohexene is a liquid which has no color, yet has a very strong smell to it.
You may need to add a toluene chaser if your volume is small. You will collect about 4-6 mL of product, which will include some water. Place it into a round bottomed flask no less than 100mL volume. Thus, the amount of methylenecyclohexane depends on the relative stabilities of the secondary and tertiary carbocation intermediates, and the ease with which 1,2-hydride shift occurs. This product can form when the proton is abstracted from the tertiary carbocation.
The reaction would all happen all at once in one step- it would go through an E2 mechanism. This video provides plenty of practice problems and examples on how to predict the major product and how to propose a mechanism. This will make the reverse reaction less likely. The reaction mechanism for this experiment. A more complex model is required to explain the formation of methylenecyclohexene. But this is the explanation for why you get some side products.
Other products form include 1-methylcyclopentene, cyclohexene, cyclohexanone, 2-methylcyclohexanone, and Benzene. From this you will determine the density of the distillate. Examples include 2-pentanol to form 2-pentene, 3,4-dimethyl-2-pentanol to form 2,3-dimethyl-2-pentene. Transfer the mixture to a separatory funnel and remove the aqueous phase. This is one of the most common methods of preparing alkenes. This new hydrocarbon is called cyclohexene.
The product ratios in this elimination reaction are determined by several factors. The reaction takes place rapidly with the formation of the more substituted alkene. Procedure Measure out the cyclohexanol 30 mL and determine its mass. When the protonated alcohol dissociates from the molecule it leaves behind a carbocation. The overall reaction is slightly endothermic.
Dry the organic phase using anhydrous sodium sulfate. The phosphoric acid is a catalyst and as such increases the rate of reaction but does not affect the overall stoichiometry. The second step is the loss of water to form the carbocation highly endothermic. Phosphoric acid gets this job done. It can be seen from the balanced reaction that 1 mole of alcohol produces 1 mole of alkene. Fryhle, Organic Chemistry, Chapter 7. Under kinetic control, a significant yield of 3-methylcyclohexene is expected when 2-methylcyclohexanol is dehydrated.