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LAB 9: SYNTHESIS OF 2-CHLORO-2METHYLBUTANE Carolyn J Straub

Lab Partner: Ruchita P

Carolyn J Straub Lab 9: Synthesis of 2-chloro-2-methylbutane Performed on November 12, 2015 Lab Partner: Ruchita P

Introduction In experiment 4.1, the mechanism and method by which an alcohol converts to an alkyl chloride was explored, implementing HCl and simple distillation in the process. The reaction occurs by SN1 mechanism. SN1 reactions are first-order, unimolecular reactions, meaning that only the concentration of the alkyl halide affects the rate of the reaction. If the amount of alkyl halide in the reaction doubles, the rate of the reaction also doubles. But if the amount of the nucleophile doubles, the rate of the reaction stays the same. This sets SN1 reactions apart from SN2 reactions which depend on the concentrations of both. The “S” in SN1 stands for substitution. Unlike E1/E2 reactions where the leaving group is simply removed by the nucleophile, SN1/ SN2 reactions involve adding a functional group from the nucleophile that replaces the leaving group that it loses. They also form two stereoisomers, one that is of the same configuration (due to front-side attack) and one that is inverted (because of back-side attack). SN2 forms only back-side attack products. SN1 reactions form carbocation intermediates because they are not concerted reactions like SN2 are. The goal of experiment 4.1 was to convert 2-methyl-2-butanol (an alcohol) to 2-chloro-2methylbutane (an alkyl chloride) with the help of the acid HCl. The balanced equation for the reaction is C5H12O + HCl  C5H11Cl + H2O. Figure 1 shows the overall reaction that was expected to take place:

Figure 1: overall reaction for the synthesis of 2-chloro-2-methylbutane from 2-methyl-2-butanol

In it, one can clearly see how the OH- leaving group on the 2-methyl-2-butanol molecule was substituted by the Cl- from the HCl molecule.

Carolyn J Straub Lab 9: Synthesis of 2-chloro-2-methylbutane Performed on November 12, 2015 Lab Partner: Ruchita P

As stated in the introduction, SN1 reactions do not occur all in one step. Instead, the mechanism involves the formation of a carbocation intermediate. This intermediate can be seen being attacked by a Cl- ion in step 3 of figure 2:

Figure 2: mechanism of the synthesis of 2-chloro-2-methylbutane from 2-methyl-2-butanol

H2O is a notable side product, resulting from the joining of the OH - from 2-methyl-2-butanol and H+ from HCl. After a reaction has occurred, scientists want to know if it has been a success. Have all the reactants reacted? Is the product I have what I expected it to be? To get a view of the purity of a compound, scientists can bombard it with infrared radiation and see how much of it is transmitted. This is called IR spectroscopy, which is the study of the interaction of matter and electromagnetic radiation. The bonds in a molecule are always vibrating, and in different ways. When a frequency of radiation hits a molecule with the same wavelength as its vibration, it absorbs that energy. A graph is then produced that shows a downward spike at each wavelength that the molecule absorbed. These wavelengths are characteristic of certain molecular structures, so they give scientists information about the makeup of the compound they are studying. For example, C=O bonds display bands at ~1700 cm-1, while O-H bonds show around 3400 cm-1. There is also what is called a “fingerprint region” from ~1400-600 cm-1 that doesn’t show any specific bonds, but gives a view of the compound as a whole. The fingerprint is particular to that molecule alone, much in the way a human fingerprint is unique to that person. Being able to accurately interpret IR spectrums is an important skill for students to learn and will be crucial to this lab.

Carolyn J Straub Lab 9: Synthesis of 2-chloro-2-methylbutane Performed on November 12, 2015 Lab Partner: Ruchita P

In Materials and Methodology, the literature values of physical properties for the reactants and products are listed. A discussion of the procedure can also be found in that segment. The Results and Observations section includes a table with the experimental data, a figure of the IR spectroscopy results, two figures of IR literature reference data, and a discussion of observations made during the experiment. Discussion and Conclusions contains explanations for any findings and observed values; a discussion about the percent yield, IR data results, and boiling point; and a note about what was learned by conducting the experiment.

Materials and Methodology The literature values for physical properties of reactants and products are shown in table 1. Masses and moles used will be necessary to know later on when calculating theoretical and percent yield. The boiling points will have to be kept in mind when analyzing the boiling point at which the product was collected during distillation. The grams used were calculated using the density and volume used for 2-methyl-2-butanol and HCl. Moles used were calculated from grams used and molar weight for 2-methyl-2-butanol and HCl.

Table 1: Physical Properties from Literature Values of Reactants and Products Compound MW Vol used d (g/mL) mol used Grams BP (°C) (g/mol) (mL) 2-methyl-2-butanol 88.15 5.0 HCl 36.46 12.5 2-chloro-2- 106.59 -----------

0.805 1.490 0.866

used 0.0457 4.025 101-103 0.5108 18.625 -85.1 ------------ --------- 85-86

methylbutane

The experiment began by adding 5 mL of 2-methyl-2-butanol and 12.5 mL of 12.1 M HCl to a separatory funnel and shaking it for one minute, loosening the tap regularly to expel any gas. When it was allowed to sit in the ring stand for about a minute, two layers separated. The lower aqueous layer was removed and the remaining organic layer was washed with 5 mL of

Carolyn J Straub Lab 9: Synthesis of 2-chloro-2-methylbutane Performed on November 12, 2015 Lab Partner: Ruchita P

water. Shaking, venting, and removing the aqueous layer was repeated, after which the organic layer was washed with 5 mL of sodium bicarbonate. The process was repeated, being especially cautious to shake gently and vent frequently. The aqueous layer was removed again and the organic layer was washed with 5 mL of water for the final time. After removal of the aqueous layer, the organic layer was dried over a minimal amount of anhydrous sodium sulfate and the remaining liquid was decanted into a 10-mL round-bottom flask with magnetic stir bar. A small flask to collect the distillate was pre-weighed. Everything was prepared for the final distillation. The simple distillation apparatus was assembled. The temperature at which the distillation began was recorded and it was allowed to run until almost all of the solution was gone from the round-bottom flask. The weight of the distillate was also recorded and an IR spectroscopy was run.

Results and Observations Following the procedure outlined in the Materials and Methodology section, synthesis of 2-chloro-2-methylbutane from 2-methyl-2-butanol was carried out. The purity of the resulting alkyl chloride product can be determined by measuring its IR spectrum and comparing it to the reactant, as well as the literature IR spectra for both the reactant and product. Another measure of the relative success of a reaction is the percent yield calculation. To do this calculation, the final mass of the solid must be known. Analyzing the boiling point is the final way to judge the purity of the compound in this reaction. Two of these three pieces of information (final mass and boiling point) were collected and reported in table 2:

Table 2. Results Final Mass (g) BP (°C)

Carolyn J Straub Lab 9: Synthesis of 2 Performed on November 12, 2015 Lab Partner: Ruchita P

2.392

35

The final product was weighed at 2.392 g. In order to find the percent yield, the theoretical yield must first be found, and that requires knowing the limiting reactant. The balanced equation for the reaction in this experiment is shown below: C 5 H 12 O + HCl →C 5 H 11 Cl+ H 2 O

Since there is a 1:1 stoichiometric ratio between all the reactants and products, determining the limiting reactant is relatively straightforward. The experiment started with 5 mL 2-methyl-2-butanol (which comes out to 4.025 g, or 0.04566 mol of 2-methyl-2-butanol) and 12.5 mL HCl (which is 18.625 g, or 0.5108 mol of HCl). Consequently, 2-methyl-2-butanol is the limiting reactant because it allows for the smallest amount of product to be formed (only 0.04566 mol compared to 0.5108 mol. The theoretical yield for the experiment is therefore 0.04566 mol, since it says that theoretically, every mole you start with should be accounted for in your products. Converting from moles to grams, the theoretical yield of 2-chloro-2methylbutane is 4.867 g. Now, percent yield can be calculated: Percent yield=

¿

g actual yield ×100 % g theoretical yield

2.392 g product × 100 %=49.1 % yield 4.867 g 2−chloro−2−methylbutane

2-methyl-2-butanol, 2-chloro-2-methylbutane, and the product obtained from the experiment were analyzed through IR spectroscopy. The resulting figure is attached on a separate sheet of paper (pg 1). Literature spectra for the reactant and expected product were also found and included:

Carolyn J Straub Lab 9: Synthesis of 2-chloro-2-methylbutane Performed on November 12, 2015 Lab Partner: Ruchita P

Figure 4: Literature spectra for reactant (2-methyl-2-butanol)

Figure 5: Literature spectra for expected product (2-chloro-2-methylbutane)

At the beginning of the experiment, the HCl was a clear liquid with a strong odor and the 2-methyl-2-butanol was also a clear liquid. The anhydrous sodium sulfate was a fine white powder. After completing the simple distillation, the final product was a clear liquid.

Discussion and Conclusions

Carolyn J Straub Lab 9: Synthesis of 2-chloro-2-methylbutane Performed on November 12, 2015 Lab Partner: Ruchita P

A percent yield of 49.1% is comparable to what was obtained in previous experiments this semester. About half of the mass of the starting compounds was maintained throughout the experiment and was accounted for in the final product, which isn’t too bad. This means that the reaction most likely went reasonably far to the right. If some of the reactants didn’t react with each other, the actual yield would have been much lower because a lot of the mass would have remained as 2-methyl-2-butanol, which would have been discarded along with the rest of the aqueous waste during the separatory process. During simple distillation, some liquid must remain in the round-bottom flask. Not all of the solution can be allowed to boil, so some of the product is lost in that step. This may have lowered the percent yield. The boiling point achieved in this experiment is very low, but it is certainly closer to that of the expected product (85-86°C) than that of the starting compound (101-103°C). The boiling point could be artificially low because the thermometer was placed incorrectly (although this is unlikely), or because cold air was drafting in from under the hood and cooling the system down. That could cause the temperature of the thermometer to drop. Since the temperature reading is closer to the expected product, however, it supports the claim that the product is 2chloro-2-methylbutane. The alcohol starting material clearly shows on both the literature and experimental spectra (figures 3 and 4) to have bands at 3000 and ~3500 cm-1. These are consistent with a compound that has an O-H and C-H bonds, both of which are present in 2-methyl-2-butanol. For the expected product, figures 3 and 5 only show a strong band at 3000 cm-1, so that compound would be expected to have C-H bonds, but not an O-H bond. This is true of 2-chloro-2methylbutane. The product IR spectra shown in figure 3 also only has one band at 3000 cm-1. The bands present in the literature spectra for both compounds are consistent with the results obtained in the experimental spectra. The disappearance of the 3500 cm-1 band from reactant (in red) to product (in black) supports the idea that 2-methyl-2-butanol (an alcohol) was converted to an alkyl chloride (2-chloro-2-methylbutane) throughout the experiment by the substitution of an O-H bond.

Carolyn J Straub Lab 9: Synthesis of 2-chloro-2-methylbutane Performed on November 12, 2015 Lab Partner: Ruchita P

The more I look at IR spectroscopy results, the more comfortable I’m getting with interpreting them. I feel confident in my ability to perform simple distillations and filtrations once they are set up. I still second guess myself during the setup process, however.

Carolyn J Straub Lab 9: Synthesis of 2-chloro-2-methylbutane Performed on November 12, 2015 Lab Partner: Ruchita P

References

Schoffstall, A. M., Gaddis, G.A., Druelinger. M.L., Microscale and Miniscale Organic Chemistry Laboratory Experiments. 1st ed., Boston, McGraw-Hill Companies, Inc., 2000. pg 221-4....