Dehydration of 2-butanol Lab Report

Dehydration of 2-butanol Lab Report Objective: The objective of this experiment was to conduct a dehydration reaction of the secondary alcohol 2-butanol on a microscale level. Sulfuric acid was used as a catalyst to carry out this E1 reaction mechanism. A new technique of collecting gas was learned to hold the three butene isomeric products of the reaction using a collecting apparatus. This was done so that the sample of the gases would not become contaminated by atmospheric gas. Gas chromatography was used to analyze the final mixture of collected gases and determine the relative amount of each of the isomeric products of butene. Procedure: 1. The picture of the gas collecting apparatus in Zubrick was consulted and the apparatus was built. A rubber septum was substituted for the septum cap shown on gas reservoir and a boiling chip was substituted for the magnetic spin vane. The entire apparatus was assembled prior to mixing the reagents. 2. The gas reservoir was calibrated by sealing one end with the rubber septum and inverting it. 3. 3 mL of water were added and that level was marked with a Sharpie. Another 1 mL of water was added and the 4 mL level was marked as well. The external thermometer was set up on the apparatus. 4. The gas collection tube was positioned. This was done by filling the tube with water and placing a finger over the open end. It was inverted and lowered open end down into a 250 mL beaker filled with water. It was secured with a clamp. 5. 100 μL of 2-butanol was added to a clean, dry 1.0 mL conical vial fitted with a boiling chip via a pipet. 50 μL of concentrated sulfuric acid was also added using a calibrated 9” Pasteur pipet. 6. The conical vial was cap sealed to the gas delivery tube and the straight end was positioned under the water in the beaker so that the gas was delivered into the gas reservoir. The vial was clamped. 7. The reaction mixture was heated in the sand bath to a maximum of 140 °C. Observations: The apparatus was built with caution as many different components had to be clamped to maintain stability. The marking of the collection tube helped visualize when 3-4 cm³ of sample gas was collected. This step took extra caution because once it was lowered into the water, everything else had to be set up around it. Immediately after the 2-butanol and sulfuric acid solution began heating, the solution started to bubble intensely. Once 3-4 cm³ of gas was collected, heating was stopped but the reaction vessel was allowed to remain in the sand bath. The gas delivery tube was removed from the water bath while not allowing the reservoir to break the surface of the water. 8. A 0.5 cm³ sample of the isomeric products was taken using a gas-tight syringe for GC analysis. This was taken for a gas chromatography analysis. Diagram: The gas collected very quickly and more than 4 cm³ of gas was collected. The sample was collected using a syringe and taken to the GC for analysis. Overall Reaction and Reaction Mechanism: Stoichiometry Table: Compound Density (g/mL) .81 Volume (mL) 0.1 mmol 2-butanol Molar Mass (g/mol) 74.12 1.09 Molar Equivalence 1.16 Sulfuric acid 98.08 1.84 0.05 .94 1.00 Limiting Reagent: 2-butanol Results/Data: Theoretical Yield: 1.09 mmol 2-butanol = 1.09 mmol isobutene Gas Chromatography: Peak Peak Number Identity 2 but-1-ene 3 cis-but-2ene 4 trans-but2-ene Retention Time (s) 3.150 3.950 Peak Area Mole Fraction 288.1920 .0469 2208.4260 .3592 Mole Classification % 4.69% Very Minor 35.92% Minor 4.683 3650.3920 .5938 59.38% Major Discussion: This experiment had us conduct a dehydration reaction that was also an E1 mechanism elimination reaction. This reaction was favored to occur because the elimination was more likely to occur than a substitution due to the strong acid, sulfuric acid. At the molecular level, the hydroxy group of the secondary alcohol 2-butanol was protonated by the sulfuric acid. The O-H bond’s electrons were moved to the O-S bond and this created a double bond and released a hydrogen atom. The hydroxide group on the 2-butanol had a electron pair that then moved to the hydrogen released by sulfuric acid and formed a new bond. This created a water molecule with a positive partial charge on the oxygen making it a great leaving group due to the weakness of its basicity. The carbocation intermediate of this E1 reaction mechanism was formed in the next step when the C-O bond between butane and the charged H2O was broken. This was an unstable intermediate due to the positive charge on the central carbon. The sulfuric acid acted as a nucleophile and attacked the hydrogen on the carbon next to the positively charged varbon on the carbocation intermediate. This caused the electrons on the C-H bond to create a new pi bond between the alpha and beta carbons in order to stabilize their charges. This means that a new double bond was formed, in this case, in three different isomeric products: cis-2-butene, trans-2butene, and isobutene. Only one of these products was considered the major product due to its high abundance in the GC. Zaitsev’s rule states that the major product of any reaction that yields two or more isomeric products is the most stable isomer. In this case, trans-but-2-ene was the most stable since the double bond is disubstituted compared to the monosubstituted bonds in isobutene. The disubstituted bond is favored due to the inductive effects around it. Cis-but-2-ene also had a disubstituted double bond, but it is less stable than trans-but-2-ene. This is because the trans configuration has the two alkyl groups on opposite sides, reducing steric hindrance. The cis configuration has the alkyl groups on the same side, causing steric hindrance, making the isomer less stable. The gas chromatography procedure helped separate and analyze the amounts of the isomeric products that were obtained in the experiment in the gas collection tube. By analyzing the gas chromatograph, it was possible to analyze the identities of each peak with Zaitsev’s rule. The largest peak corresponded to trans-but-2-ene, being the most stable isomer out of the three. The second highest peak corresponded to cis-but-2-ene because it was more stable then isobutene due to the disubstituted double bond. Finally, the smallest peak was isobutene, being the least stable out of the three.