LAB Elimination Reactions Tested

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Purpose/Introduction:
In this experiment, four elimination reactions were compared and contrasted under acidic (H2SO4) and basic (KOC(CO3)3) conditions. The acid-catalyzed dehydration was done on 2-butanol and 1-butanol; a 2ᵒ and 1ᵒ alcohol, respectively. The base-induced dehydrobromination was performed on 2-bromobutane and 1-bromobutane; isomeric halides. The stereochemistry and regiochemistry of the four reactions were analyzed by gas chromatography (GC) to determine product distribution (assuming that the amount of each product in the gas mixture is proportional to the area under its complementary GC peak. The three butene products have been verified that they elute in the following order: 1-butene, trans-2-butene, and cis-2-butene.
Theory:
The dehydration of 2-butanol, a secondary alcohol, progresses readily in the presence of a strong acid like concentrated sulfuric acid (H2SO4). The reaction is completed via the E1 mechanism. Initially, the hydroxyl group is a poor leaving group, but that is remedied by its protonation by the acid catalyst (H2SO4) converting it to a better leaving group, H2O. The loss of this water molecule results in a secondary carbocation intermediate that continues to form an alkene in an E1 elimination. If the elimination happens with either protons on the terminal methyl group, the resultant product is 1-butene, a Hoffman product (monosubstituted alkene). In contrast, elimination of either β-hydrogens by the conjugate base of sulfuric acid (HSO4---) on the methylene group leads to an alkene that is disubstituted. Either the cis- or trans-2-butene can form depending which hydrogen is deprotonated. These are the Saytzeff products since they are the most substituted ones. In this case, the trans-alke...

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...bromebutane. Unfortunately, our group was only able to obtain the chromatograph for 2-bromobutane and the rest of the three chromatographs were provided by our T.A. Some possible reasons why the chromatographs for 2-butanol, 1-butanol, and 1-bromobutane were unable to be displayed properly is due to the malfunction of the syringes. If the syringe is not air-tight, the gaseous products can escape before being inserted into the injection port. In addition, the collection tube may have had a minor gas escape from the rubber septum, resulting in less concentrated gaseous products being inserted into the injection port. A possible solution is sealing the collection tube with parafilm. All in all, the provided data chromatographs and the rendered chromatograph by the 2-bromobutane in the lab session did match the expected results for the distribution of gaseous products.

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