cyclohex

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University of Texas, San Antonio *

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2612

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Chemistry

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Dec 6, 2023

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docx

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Synthesis Reaction of Cyclohexanol to Form Cyclohexene with Phosphoric Acid Using an E1 Mechanism Introduction In this synthesis reaction, cyclohexanol is treated with phosphoric acid to produce cyclohexane and water as a biproduct. This is an elimination reaction that results in two broken sigma bonds and the formation of a pi bond. The carbon hydroxide bond and the carbon hydrogen bond are broken on cyclohexanol and form the pi bond on cyclohexene. This reaction is an E1 mechanism because there is a secondary alcohol on cyclohexanol, and phosphoric acid is a polar protic solvent. An E1 mechanism is a unimolecular, stepwise reaction where the rate=k[cyclohexanol]. The reaction is only related to the alcohol. Therefore, if we double the concentration of the alcohol, it doubles the rate. If any of the other reagent’s concentration were doubled, nothing would occur. Since this is a stepwise reaction, the formation of a carbocation as the intermediate is the rate determining step. The preferred alkyl halides are the tertiary and secondary carbons due to hyperconjugation. The purpose of this experiment is to react cyclohexanol to form cyclohexene with phosphoric acid using an E1 mechanism. Cyclohexanol is dehydrated by a strong acid to form cyclohexene. The product will be characterized by calculating the percent yield, analyzing IR data taken of the product, and performing two separate confirmatory tests with KMnO4 and Br2 in DCM. Reaction and Mechanism

Reaction Step 1: Protonation of cyclohexanol Step 2: Formation of carbonation Step 3: Deprotonation of beta-hydrogen and formation of products and regeneration of phosphoric acid

Chemical Properties and Safety Chemical Molecular Formula Molecular Weight (g/mol) Boiling Point (°C) Density (g/mL) Structure Cyclohexanol C6H12O 100.16 161 0.96 Cyclohexene C6H10 82.12 83 0.811 Phosphoric acid H3PO4 98 158 1.88 Cyclohexanol – Combustible liquid and vapor. May be harmful if swallowed or absorbed through the skin. May cause eye and skin irritation. May cause respiratory tract irritation. May cause central nervous system depression. Hygroscopic. - If get in eyes or on the skin flush with water for 15 minutes. If ingested do not induce vomiting and give 2-4 cups of milk or water. Get medical aid. If inhaled remove from exposure and to fresh air immediately. Do not use mouth-to-mouth resuscitation.

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Cyclohexene – Extremely flammable liquid and vapor. Vapor may cause flash fire. Causes eye, skin, and respiratory tract irritation. May cause central nervous system depression. May formed explosive peroxides. - If get in eyes or on the skin flush with water for 15 minutes. If ingested give 2-4 cups of milk or water and get medical aid immediately. If inhaled remove from exposure and move to fresh air immediately. Get medical aid. Phosphoric acid – Corrosive. Causes burns by all exposure routes. Hygroscopic. - If get in eyes or on the skin flush with water for 15 minutes. If ingested, it causes gastrointestinal tract burns. Causes severe pain, nausea, diarrhea, and shock. May cause hemorrhages of the digestive tract. May cause corrosion and permanent tissue destruction of the esophagus and digestive tract. If inhaled irritation may lead to chemical pneumonitis and pulmonary edema. Causes severe irritation of upper respiratory tract with coughing, burns, breathing difficulty, and possible coma. Causes chemical burns to respiratory tract. Procedure Prior to starting the procedure, a fractional distillation was set up without the column packed with beads. In a 50 mL round bottom flask, a stir bar, cyclohexanol (8 mL), and 85% phosphoric acid (4 mL) was added. The flask was placed into an aluminum well on a hot plate and attached to the modified fractional distillation. Gentle stirring was applied, and the starting temperature of the hot plate was 180 °C. The temperature of the hot plate was gradually increased by 10 °C every few minutes until around 300 °C. The product was collected into a

small beaker placed in an ice bath. The product, cyclohexene, co-distilled over with water between 70-75 °C, and collected until 85 °C. After collecting the product, the layers in the beaker were separating using a pipette. The organic layer was dried with a few scoops of sodium sulfate. The remaining product was decanted into a tarred vial and the weight was taken to calculate the percent yield. An IR was run to characterize the product. Finally, confirmatory tests were run using KMnO4 and bromine in DCM. In one test tube, KMnO4 (4 drops) was added with a pipetted into the test tube, followed by the product (2 drops). In another test tube, bromine in DCM (4 drops) was added with a pipette into the test tube, followed by the product (2 drops). Both tests should come back positive, and a precipitate should form. To clean up after the experiment, the 50 mL round bottom flask was allotted time to cool. Cold water (25-30 mL) was added into the flask. Then solid sodium carbonate was scooped in small portions into the flask. Take caution, foaming will occur. Once the foaming stops upon addition of sodium carbonate, the solution has been neutralized. The solution was poured down the cup sink in the fume hood with the water running. Data and Observations Amount of product recovered: Weight of the vial – 16.427 g Weight of the vial + product – 19.476 g 19.476 g – 16.427 g = 3.049 g IR data of the product:

Observations of the confirmatory tests: Testing Agent I: Bromine (Br2) in Dichloromethane – positive, colorless Testing Agent II: Potassium Permanganate (KMnO4) (aq) – positive, colorless and brown precipitate formed Calculations C6H11OH -> C6H10 + H20 Density of Cyclohexanol: 0.921 g/mL Density of Cyclohexene: 0.733 g/mL Moles of reactants: 8 mL x 0.96 g/mL = 7.7 g 7.7 g / 100 g/mole = 0.077 mol Moles of product:

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3.049 g / 82 g/mole = 0.0372 mol Percent yield = (moles of product / moles of limiting reagent) x 100 Percent yield = (0.0372 mol / 0.077 mol) x 100 = 0.4829 = 48.29%