Preparation+and+Distillation+of+Cyclohexene

//Merissa Honey & Heather Browning//

__**Introduction:**__

Substitution reactions always compete with elimination reactions. However, elimination reactions are favored by high temperatures and acids whose conjugate bases are poor nucleophiles, whereas substitution reactions are favored by lower temperatures and acid whose conjugate bases are good nucleophiles. This combination would be exceedingly hard to find, since nucleophiles are (by definition) electron rich. Therefore acid-catalyzed eliminations usually don't get much competition from substitution reactions.

Cyclohexene can be prepared from cyclohexanol through the acid catalyzed dehydration of alcohol and acid; In this lab, an E1 reaction occurs and thus the alcohol will be dehydrated. A strong acid, such as H3PO4 will be used so that the alochol will be protonated and a hydronium ion may be eliminated. A distillation technique will be incorporated so that the anticipated products will achieve a high level of purity and then will be able to be analyzed using an Infrared Spectrum. Yes, well said. The rate expression for an E1 reaction shows that the only determining step involves the electrophile. This step (which is when the protonated alcohol group leaves the cyclohexanol) is what is allowing the reaction to result as an E1 reaction. The primary reason that this experiment results an E1 reaction is the fact that a stable carbocation can be formed, as well as the use of a weak "patient" acid which does not attack the beta hydrogen prior to the loss of the leaving group. I am confused here. How does the acid attack the beta hydrogen? If it takes the beta hydrogen it is acting as a base rather than an acid. The E1 reaction is in competition with the SN1 reaction in that they are very similar. In this experiment the cyclohexanol is being “eliminated” of its alcohol and a double bond is formed. This demonstrated that an E1 reactions had occured over a SN1. Heat is also added to the reactants and a thus indicates an E1 reaction.

The atom economy, which is what portion of the reactant molecules makes the desired product and how wasteful the reaction might be. This is a sentence fragment. The role of boiling points comes into play because it is desired that the cyclcohexene will be distilled into another new vessel and leave behind the cyclohexanol. The fact that the cyclohexanol has a higher boiling point (161 degrees C) is a good thing in that it will not boil off before or at the same time as the cyclohexene which has a boiling point of 83 degrees C.

Please always show the reaction you are performing, drawn out. Always always always.

__**Procedure:**__




 * A distillation apparatus similar to the image above was assembled for the experiment.

A 50 mL round bottom flask was filled with 0.074 moles (which was weighed to 7.399 g) of cyclohexanol and 1.75 mL of 85% H3PO4. The flask was hooked up to a fractionating column (a disconnected condenser was used in place of this and filled with copper). A Distillation adapter channeled the vapor produced from the reactant substances in the 50 mL round bottom flask past a thermocouple and down a functioning condenser. The thermocouple allowed for tracking of the temperature. Cold water was passed through the condenser to cool the vapor and encourage condensation to occur. The condensation that was produced was then gathered in a collection flask at the end of the apparatus. The distillation flask was heated until the remaining mixture was approximately 1.00 mL.

The distillate was then moved to a separatory funnel and washed with approximately 5 ml of water. The organic layer that formed on top of the water layer was carefully moved to an Erlenmeyer flask. After removing any stray water droplets, a drying agent, anhydrous sodium sulfate, was then added until the agent no longer clumped together and left to stand for five minutes with occasional stirring.

Successful results will have a clear liquid remaining and some of the drying agent still free flowing. The liquid was then removed with a pipette and placed in a clean test tube. The condensed product, cyclohexene, was then weighed in a pre-weighed vial so that the mass could be determined and an IR spectrum could be determined. (This information is courtesy of the Lab hand out found at : http://greenchem.uoregon.edu/Pages/Overview.php?CategoryIDString=&FullTextSearchKeywords=cyclohexene&CategoriesToSearch=&NumberOfMainCategories=7&AnyAll=Any&ID=70) Procedure is best written in a tense that is directive or passive but not past tense. __**Data:**__


 * Initial Mass of Cyclohexanol || Initial mL of 85% H3PO4 || Heat time of Distillation to achieve approximately 1.0 mL of reactants || End mass of product, Cyclohexene ||
 * 7.399 grams || 1.75 mL || 1 hour 13 minutes || 4.446 grams ||

7.399 grams - 4.446 grams =2.953 grams product/cyclohexene I am confused by this. Isn't 4.446 g the mass of your product--cyclohexene? 2.953 grams cyclohexene distilled from cyclohexanol / 7.399 grams reactants/cyclohexanol = 0.399 (X 100) = **39.9% recovery**

The spectrum labeled “heathercyclohexene” displays this lab's experimental results. The peaks correspond to the the structure of the substance. The small amount of water is evident in the far left, small peak to the left of the 3000 mark. The alkene, Csp2-H (present at 3013) and alkane Csp3-H (present at 2923) are also evident in the spectrum leading to the conclusion of cyclohexene as the product. The lab's results indicate that only through an elimination of alcohol on cyclohexanol could result in cyclohexene.



__**Conclusion:**__ Our initial hypothesis was that in the presence of a strong acid, an alcohol can be dehydrated to form an alkene. The acid used in this experiment is 85% phosphoric acid and the alcohol is cyclohexanol. The phosphoric acid is a catalyst and as such increases the rate of reaction but does not affect the overall stoichiometry.

An elimination, E1 reaction was anticipated due to the heat and strong acid (weak base). The dehydration of cyclohexanol to cyclohexene was confirmed by Infrared spectrosopy and is seen in the data section chart. The alkene peak was clear on the spectrum (cyclohexene) and the alcohol (cyclohexanol) was not evident on the spectrum also leading to the conclusion that an elimination reaction had occured. The mechanism of action below describes the reaction.



The atom economy calculated in the post lab question is close to the percent recovery displayed in the data section above. Atom economy and percent recovery are two different things. Can you imagine a situation where you would have 100% recovery and a terrible atom economy? This is possible. This demonstrates that little error occured in the distillation process and the elimination reaction was successful. What error could have occurred? I don't need you to tell me whether there was a lot of error or a little, as much as I need you to analyze the situation and tell me what might possibly have interfered with the experiment. Reflux occured during the lab as the reactantss started to boil but was reduced with a small increase in temperature and an additonal layer of aluminum foil wrapped around the fractionating column to reduce heat loss. __**Post lab question:**__ Atom Ecomony - conversion efficiency of a chemical process in terms of atoms involved. //Calculate % Atom Economy// catalysts are not factored in to atom economy. 82.14 g/mol cyclohexene / (100.15 g/mol cyclohexanol + 98.00 g/mol phosphoric acid) X 100 = **41.45%**

This lab earned the following scores for: format (2/2) style (1.5/2) data (2/3) quality of the result (1/1) quality of the reported data (0.5/1) conclusion (1/2) error (0.5/1) PLQ (1.5/2) for a total of 10/14.