Discussion and Conclusion: Preparation of 1-Bromobutane

May 25, 2018 General Studies

The purpose of this experiment was to demonstrate the conversion of a primary alcohol, 1-butanol, to a primary bromoalkane, 1-bromobutane, a SN2 reaction. The conversion of 1-butanol to 1-bromobutane relies on sulfuric acid which plays two important roles. First, it protonates the alcohol of 1-butanol to form an oxonium ion which is a good leaving group. Secondly, it produces the hydrobromic acid, the nucleophile, which attacks 1-butanol causing the oxonium ion to leave and forming 1-bromobutane. However, using sulfuric acid in this experiment has several downsides.

First, it poses a huge safety hazard as it can cause severe burns. Secondly, it reacts exothermically, which was solved by using an ice water bath. Lastly, it produces several side products including dialkyl ether and alkyl hydrogen sulfate. After mixing all the reagents, they mixture was placed under a gentle heat reflux in a simple distillation. This allowed the reaction to occur, but also removed any excess sulfuric acid and hydrobromic acid. This should have left water, 1-bromobutane, and any side products in the remaining distillate.

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The 1-bromobutane was isolated by stirring the distillate in 250 mL of distilled water because the side products as well as the water are less dense than the 1-bromobutane. This caused the 1-bromobutane to sink to the bottom and thus allowing it to be separated. The total amount of 1-bromobutane formed was 2. 065 grams. In order to confirm that the reaction occurred correctly and that 1-bromobutane was formed, infrared spectroscopy as well as halide tests were performed on the product. Infrared spectroscopy works on the basis that different covalent bonds of molecules can stretch and bend.

These bonds can undergo vibrations which require energy that corresponds directly with the energy absorbed from infrared radiation. Consequently, each bond will absorb different spectrums of infrared radiation. The infrared spectroscopy machine outputs a graph with peaks that correspond to how many bonds of each type are present and at which particular wavelength they absorb at. The graph is split into a functional group region and a fingerprint region. The functional group region shows the peaks based on the types of bonds present.

The infrared spectroscopy of 1-bromobutane should have a large carbon-hydrogen peak which absorbs at approximately 3000 cm-1 and a medium carbon-bromine peak which absorbs at 500 cm-1. However, due to the limitations of the infrared spectroscopy machine, a carbon-bromine peak should not be detected because the machine is not accurate at infrared ranges below 600 cm-1. As a result, relying simply on a carbon-hydrogen peak is not a reliable way to determine if the product is actually 1-bromobutane.

This is where the fingerprint region comes in handy. The fingerprint region relies on the fact that the all the bonds affect each other’s absorbency. This means that no two molecules will have the same fingerprint region and any molecule can be compared to known infrared spectroscopies. The infrared spectroscopy produced by the product of this experiment fit the criteria of 1-bromobutane. There was a large peak at approximately 3000 cm-1 which indicated that there were many carbon-hydrogen bonds that 1-bromobutane has.

The fingerprint region of the graph also matched a known infrared spectroscopy of 1-bromobutane quite well too. There were peaks of approximately the same length and absorbencies that matched the known 1-bromobutane infrared spectroscopy. The other tests used were the halide tests, particularly, the silver nitrate test and the sodium iodide test. These tests indicate whether or not a halogen is present, bromine in this experiment, and what type of carbon the halogen is attached to.

This is done on the basis that when these particular reactants react with a halide, the silver halide or sodium halide are insoluble in the mixture while silver nitrate and sodium iodide are insoluble. A precipitate forming indicates that a halide is indeed present in the unknown. The difference between these two tests is that the silver nitrate test undergoes the SN1 mechanism while the sodium iodide test undergoes the SN2 mechanism. Using these differences, the type of carbon the halogen is attached to can be determined because of the nature of each mechanism.

The SN1 mechanism reacts fastest with tertiary carbons and slowest with primary carbons while the SN2 mechanism reacts fastest with primary carbons and slowest with tertiary carbons. Essentially, they react at rates opposite of each other. As a result, a molecule with a halogen attached to a primary carbon should react fastest with sodium iodide, a molecule with a halogen attached to a tertiary carbon should react fastest with silver nitrate, and a molecule with a halogen attached to a secondary carbon should react at equal rates between silver nitrate and sodium iodide.

Knowing the structure of 1-bromobutane, the sodium iodide test should occur the quickest since bromine is attached to a primary carbon. In this experiment, the two tests were conducted on primary, secondary, and tertiary carbon molecules to establish that the tests worked the way they were supposed to before actually conducting them on the presumed 1-bromobutane. The silver nitrate test had an order of tertiary, secondary, then primary and the sodium iodide test had an order of primary, secondary, then tertiary.

This confirmed that the tests did undergo the correct mechanisms and worked out the way they should have. When conducting the two tests on the presumed 1-bromobutane, precipitates formed during both tests. The silver nitrate test turned cloudy first, but the sodium iodide test had actual precipitate form first at five minutes while the silver nitrate test had precipitate form at the seven minute mark. This fits the criteria of 1-bromobutane because the sodium iodide reacted first indicating that the bromine was attached to a primary carbon.

Knowing that the product produced was indeed 1-bromobutane, the percent yield could then be determined. It came out to be 27. 816% which was quite low. Glancing at other classmate’s amount of 1-bromobutane left me quite happy though as most of them had less than half of what I had. An improvement that could be made to improve percent yield would be to minimize material loss during transfer. Overall, this experiment did not go as well as I hoped in terms of percent yield, but went phenomenally in terms of getting 1-bromobutane.


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