Lab Report - Synthesis of alkenes via acid catalyzed dehydration of 3,3-dimethyl-2-butanol PDF

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Lab Report – Synthesis of alkenes via acid catalyzed dehydration of 3,3-dimethyl-2-butanol Date (of performed experiment): 9/27/17 Laboratory Partners: Goals: The goal of this lab was to synthesize an alkene with two methyl substituents from 3,3,dimethyl2-butanol using acid catalyzed dehydration. Overall Chemical Equation:

+ + Reaction Mechanism: (I DO MEAN FULL MECHANISM!) Use ChemDraw; (arrows must be shown, but can be hand drawn); Mechanism shall include the formation of all 3 possible alkenes for the reaction. Step 1: Protonation

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Step 2: Water Departs forming a Carbocation

+ Step 3: Base Removes a proton from the beta carbon

+ Step 2b: Rearrangement

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(2.5/30 points)

(3,2 methyl shift) Step 3b: Base Removes a proton from the beta carbon

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Step 3c: Base Removes a proton from the beta carbon

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Procedure: 

10mL of 3,3-dimethyl-2-butanol and 10mL phosphoric acid was added to a 25 mL flask with a spin bar

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A condenser was set up and the solution was heated

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Distillate was collected in a graduated cylinder in an ice bath

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Organic layer was washed with 20mL of NaCl

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The lower aqueous layer was extracted via microscale extraction

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It was then dried with anhydrous magnesium sulfate

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It was then filtered

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product’s volume was measured

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GC, IR, and RI were collected

Observations: 

Distillate began to form after heating for ~1hr

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Re-distillation was not performed

Results: Calculations: Theoretical yield

(10mL 3,3-dimethyl-2-butanol)(.812g/mL)(1mol/102.17g)(1mol alkenes/1 mol 3,3-dimethyl-2-

Experimental

butanol)(84.16g/mol)= 6.69g alkene 4.9mL collected, predicted major product 2,3-dimethyl-2-butene: density= .708g/mL

yield

6.69g(1mL/.708g)= 9.45mL

Percent Error

(4.9/9.45)= 51.86% ((4.9-9.45)/9.45)100= -48.15%

GC analysis:

Figure 1: shows the GC for 2,3-dimethyl-1-butene

Figure 2: shows the GC for 2,3-dimethyl-2-butene

Figure 3: Shows the GC for 3,3-dimethyl-1-butene

Figure 3: Shows the GC for 3,3-dimethyl-2-butanol

Figure 4: Shows the GC for the product collected RI: 3,3-dimethyl-2-butanol 3,3-dimethyl-1-butene 2,3-dimethyl-2-butene 2,3-dimethyl-1-butene Product

RI @20C 1.415 1.376 1.412 1.389 1.412 @20 1.412+(22-20).00045=

1.4129 Figure 5: Shows the Refractive index f Spectroscopic Analysis: IR Spectroscopy:

Figure 6: Shows the IR for the product In this spectra, the peak in 1600-1680 indicates a C=C bond. The peaks after 3000 are the sp2C—H bonds. This indicates an alkene with hydrogens around the double bond.

Discussion:

In this reaction, the phosphoric acid acted to protonate the alcohol. It is more effective than HCl because the goal of this reaction was a dehydration. If HCl was used, the chlorine anion been substituted in the protonated alcohol. Because the alkene that was formed was volatile, it had iced—this prevented it from evaporating. Highly volatile compounds vaporize readily. Because of the compound’s volitivity, it could have evaporated. This would have affected the yield of the reaction because some of the product would have been lost. Additionally, the organic and aqueous phases must be immediately separated because alkenes in the presence of water form alcohols. Out of the three alkenes that can be formed, 2,3-dimethyl-2-butene is the most stable. This is because it has the fewest hydrogens around the double bond. The RI data collected is indicative of impure alkene fractions, a combination of the three isomers. The GC collected shows a variety of peaks—indicating that all the isomers were formed and present in the product. GC indicated that the highest concentration was 3,3-dimethyl-1-butene. The final product has a 84.38% concentration of a peak with t=1.448. In 3,3-dimethyl-1-1butene t=1.443. In the IR spectra, there is an absence of the strong broad 3650-3200 peak that is indicative of alcohols. There is also a peak at ~1600—this indicates a C=C bond. The peaks after 3200 are due to the C=C—H bonds, this indicates a substituted alkene. In this experiment, pure alkenes were not obtained. This was due to not distilling after extraction. Distilling after extraction would have separated the alkenes via boiling points. Conclusion: In this experiment, an alkene with two methyl substituents was synthesized from 3,3,dimethyl2-butanol using acid catalyzed dehydration. Using IR and GC, the product that was formed could be identified as a substituted alkene. Because a distillation was not performed, the product that was collected was a mixture of 3,3-dimethyl-1-butene, 2,3dimethyl-1-butene, and 2,3-dimethyl-2-butene.

References: Analytical, Biology, Chemistry & Materials Science products and services. http://www.sigmaaldrich.com/ (accessed Oct 9, 2017). Schoffstall, A. ; Gaddis,B.; Drueling, M.; Microscale and Miniscale Organic Chemistry Laboratory Experiments (2011); McGraw-Hill; 2nd Edition, 229-34....