Conversion of Alcohols to Alkyl Halides Lab Report PDF

Preview

Summary

Lab 8 Writing Assignment...

Description

Lab 9: Conversion of Alcohols to Alkyl Halides Author: Charles Schullo Date: 3 December 2019

Results The theoretical yield, actual yield, and percent yield values of 1-bromopropane produced by the reaction of 1-propanol with sodium bromide and sulfuric acid as well as the theoretical yield, actual yield, and percent yield values of 2- and 3-bromopentane produced by the reaction of 2-pentanol with sodium bromide and sulfuric acid are shown in Table 2. Table 2. Amount of product produced after reaction with 1-propanol and 2-pentanol respectively Actual Theoretical Product Yield/Product Percent Yield (%) Yield (grams) Mass (grams) 1-bromopropane 4.133 0.460 2-bromopentane and 3-bromopentane 3.478 0.982 The calculations for the theoretical yield to be used in the percent yield calculations of both sets of products obtained in Table 2 are shown below using any necessary scientific values from Table 1. g 1 mol 1-bromopropane 1 mol g ≈ * * *122.99 2.5 mL of 1-propanol*0.804 mol mL 60.10 g 1 mol 1-propanol ≈ 4.133 g 1-bromopropane Eq. 1 As shown in Equation 1, the theoretical yield of the reaction of 2.5 mL of 1-propanol with 3.50 grams of sodium bromide and 6 mL of sulfuric acid was calculated to be approximately 4.133 grams of 1-bromopropane. g 1 mol 2- or 3-bromopentane 1 mol g ≈ *151.04 * * mol mL 88.15 g 1 mol 2-pentanol ≈ 3.478 g 2- and 3-bromopentane Eq. 2

2.5 mL of 2-pentanol*0.812

2

As shown in Equation 2, the theoretical yield of the reaction of 2.5 mL of 2-pentanol with 3.50 grams of sodium bromide and 6 mL of sulfuric acid was calculated to be approximately 3.478 grams of 2- and 3-bromopentane. The calculations for the mass of product obtained to be used in the percent yield calculations of both sets of products obtained in Table 2 are shown below. Mass of product = Mass of vial with product - Mass of vial Eq. 3 Mass of 1-bromopropane = 16.977 g - 16.517 g Eq. 4 Mass of 1-bromopropane = 0.460 g Eq. 5 As shown in Equation 5, the amount of product produced, or actual yield, the reaction of 2.5 mL of 1-propanol with 3.50 grams of sodium bromide and 6 mL of sulfuric acid was calculated to be 0.460 grams of 1-bromopropane. Mass of 2- and 3-bromopentane = 43.562 g - 42.580 g Eq. 6 Mass of 2- and 3-bromopentane = 0.982 g Eq. 7 As shown in Equation 7, the amount of product produced, or actual yield, the reaction of 2.5 mL of 2-pentanol with 3.50 grams of sodium bromide and 6 mL of sulfuric acid was calculated to be 0.982 grams of 2- and 3-bromopentane. The percent yield for both reactions was calculated using the calculated value for theoretical yield and the experimental value for the mass of the product formed, also called the actual yield. Percent Yield of Product =

Actual Yield of Product *100% Theoretical Yield of Product Eq. 8

0.460 g Percent Yield of 1-bromopropane = *100% 4.133 g Eq. 9 Percent Yield of 1-bromopropane ≈ 11.1% Eq. 10 As shown in Equation 10, the percent yield of product, the 1-bromopropane, from the reaction of 2.5 mL of 1-propanol with 3.50 grams of sodium bromide and 6 mL of sulfuric acid was calculated to be approximately 11.1%.

Percent Yield of 2- and 3-bromopentane =

0.982 g *100% 3.478 g 3

Eq. 11 Percent Yield of 2- and 3-bromopentane ≈ 28.2% Eq. 12 As shown in Equation 12, the percent yield of product, the 2- and 3-bromopentane, from the reaction of 2.5 mL of 2-pentanol with 3.50 grams of sodium bromide and 6 mL of sulfuric acid was calculated to be approximately 28.2%. The IR signal produced by 1-propanol displayed a signal characteristic of a hydroxyl group through a peak stretched around approximately 3300 cm-1. The signal also displayed a set of triplet peaks around 2900 cm-1 which is characteristic of three sp3 hybridized carbons as shown in Figure 1.

-OH

sp3 C

Figure 1: The IR signal produced by the 1-propanol used to react with sodium bromide and sulfuric acid to produce 1-bromoproane with the indicated peaks labeled with their respective functional groups. The IR signal produced by 2-pentanol displayed a signal characteristic of a hydroxyl group through a peak stretched around approximately 3300 cm-1. The signal also displayed a set of triplet peaks around 2900 cm-1 which is characteristic of three sp3 hybridized carbons as shown in Figure 2. 4

-OH

sp3 C

Figure 2: The IR signal produced by the 2-pentanol used to react with sodium bromide and sulfuric acid to produce 2- and 3-bromopentane with the indicated peaks labeled with their respective functional groups.

sp3 C Range

Figure 3: The IR signal produced by the 1-bromopropane produced from the reaction with 1propanol, sodium bromide, and sulfuric acid with the indicated peaks labeled with their respective functional groups. The IR signal produced by the mixture of 2-bromopentane and 3-bromopentane displayed a set of three peaks around 2900 cm-1 which is characteristic of three sp3 hybridized carbons as shown in Figure 4.

sp3 C Range

Figure 4: The IR signal produced by the mixture of 2-bromopentane and 3-bromopentane produced from the reaction with 2-pentanol, sodium bromide, and sulfuric acid with the indicated peaks labeled with their respective functional groups.

6

Figure 5: NMR Structure of 1-bromopropane with color-coded markers that reference to Table 3 that describe the characteristics of each group of non-equivalent hydrogens regarding NMR data characteristics. Table 3. 1-bromopropane NMR data characteristics for each non-equivalent hydrogen group. Group

Splitting Pattern (n+1)

A B C

Triplet (3) Sextet (6) Triplet (3)

Integration (Number of Hydrogens) 3 2 2

ppm ~1 ~2 3~4

Figure 6: The NMR spectrum resulting from the product of 1-propanol, sulfuric acid, and sodium bromide. Each important peak is labeled with a color-coded letter to represent each group of nonequivalent hydrogens specified in Table 3.

7

Figure 5 illustrates the structure of 1-bromopropane that was formed with each nonequivalent hydrogen group labeled with a color-coded letter. The NMR data characteristics described for 1-bromopropane in Table 3 all correspond to their respective peaks in the corresponding NMR spectra for the product of the reaction of 1-propanol, sulfuric acid, and sodium bromide as shown in Figure 6.

Figure 7: NMR Structure of 2-bromopentane with color-coded markers that reference to Table 4 that describe the characteristics of each group of non-equivalent hydrogens regarding NMR data characteristics. Table 4. 2-bromopentane NMR data characteristics for each non-equivalent hydrogen group. Group Splitting Pattern (n+1) Integration (Number of ppm Hydrogens) A Doublet (2) 3 3~4 B Sextet (6) 1 3~4 C Quartet (4) 2 1~2 D Sextet (6) 2 1~2 E Triplet (3) 3 1~2

Figure 8: NMR Structure of 3-bromopentane with color-coded markers that reference to Table 5 that describe the characteristics of each group of non-equivalent hydrogens regarding NMR data characteristics. 8

Table 5. 3-bromopentane NMR data characteristics for each non-equivalent hydrogen group. Group Splitting Pattern (n+1) Integration (Number of ppm Hydrogens) F Triplet (3) 3 ~1 G Quintet (5) 2 ~2 H Triplet (3) 1 3~4

A

B

H C

F G

E

D

Figure 9: The NMR spectrum resulting from the product of 2-pentanol, sulfuric acid, and sodium bromide. Each important peak is labeled with a color-coded letter to represent each group of nonequivalent hydrogens specified in Table 4 and Table 5. Figure 7 and Figure 8 illustrate the structures of 2-bromopentane and 3-bromopentane, respectively, that were formed with each non-equivalent hydrogen group labeled with a colorcoded letter. The NMR data characteristics described in Table 3 for 2-bromopentane and in Table 4 for 3-bromopentane all correspond to their respective peaks in the corresponding NMR spectra for the product of the reaction of 2-pentanol, sulfuric acid, and sodium bromide as shown in Figure 9. 9

If multiple products are formed in a reaction with NMR data, the relative integration of two identical peaks can be compared to determine the mole fraction of a single product in mixture of multiple products. The mole ratio of a single product was calculated using its respective single peak integration and the sum of all the product’s equivalent peak integrations as shown in Equation 13.

Mole Fraction of One Product =

Single Peak Integration of One Product Sum of all Product's Equivalent Peak Integrations Eq. 13

The peaks of “B” from 2-bromopentane and “H” from 3-bromopentane on Figure 9 were found to be equivalent given its location to the bromide in Figure 7 and Figure 8. The single peak integration of “B” from 2-bromopentane, according to Figure 9, is 2.0 and the single peak integration of “H” from 3-bromopentane, according to Figure 9, is 0.8028. Mole Fraction of 2-bromopentane =

2.0 2.0 + 0.8028 Eq. 14

2.0 Mole Fraction of 2-bromopentane = 2.8028 Eq. 15 Mole Fraction of 2-bromopentane ≈ 0.714

Eq. 16 The mole fraction of 2-bromopentane was calculated using the respective single peak integration values and Equation 13 and the mole fraction of 2-bromopentane was calculated to be approximately 0.714 as shown in Equation 16. Mole Fraction of 3-bromopentane =

0.8028 2.0 + 0.8028 Eq. 17

0.8028 Mole Fraction of 3-bromopentane = 2.8028 Eq. 18 Mole Fraction of 3-bromopentane ≈ 0.286 Eq. 19 The mole fraction of 3-bromopentane was calculated using the respective single peak integration values and Equation 13 and the mole fraction of 3-bromopentane was calculated to be approximately 0.286 as shown in Equation 19.

10

Discussion The synthesis of alkyl halides from alcohols was done through the uses of several techniques. To initiate the reaction a technique called reflux was used to create an acid-catalyzed dehydration. Reflux is important to use in that it reduces product loss by only using a small amount of solvent. This technique was implemented in the experiment by sulfuric acid, sodium bromide, and the alcohol being placed in a round bottom flask and was heated until condensation was produced in the apparatus. This procedure allowed the product that formed to re-condense several times without evaporating out. The next technique used was distillation. Distillation separated the compounds that were produced based on their boiling points. This was executed by the alkyl halide products being boiled and then re-condensed into the receiving flask. The technique of distillation provided pure products in the distillate that were collected and boiled off any impurities. Due to the high product yield of distillation, if done correctly, the temperature range of the produced distillate was like the products. Due to the technique of reflux being used first in the experiment, the reaction was given time to produce products before being separate by distillation. Also due to the technique of reflux being used first in the experiment, the temperature, therefore, coincided with the correct range because the products were already formed. In the implementation of the next technique used, the product produced from the reflux and distillation was then separated and dried. The next technique implemented in the experiment was the technique of separation. This was done with a separatory funnel in order that the product can be separated and filtered out. The product was collected easily due to the products being denser than water which caused the water to rise to the top of the separatory funnel and the organic layer sunk to the bottom. The organic layer, or the product, was dispensed into a glass bottle. Lastly, the product was then dried with anhydrous Na2SO4 which removed any excess water. This step was important because due to water being polar and like dissolves like, it absorbed and removed any remaining impurities from the product. Due to reflux being used before simple distillation, the reaction could run to completion. On the contrary, if simple distillation were to be performed first, only the reactants would have been distilled. The technique of reflux uses a closed system to heat the products and does not allow the product to evaporate out to allow for more product to be collected when condensed. Therefore, this technique contributed to the efficiency of the reaction. The products produced from both substrates were produced in a small amount. Through analysis of IR and NMR, the products that were formed and their respective yields were approximately 11.1% of 1-bromopropane as shown in Equation 10 and approximately 28.2% of a mixture of 2-bromopentane and 3-bromopentane as shown in Equation 12. There are many possible reasons to account for the source of error in the percent yield of the experiment. There is a possibility for many errors to be considered when it came to the set up of the apparatuses used for reflux and simple distillation. One vital part of the experiment was the temperature. Heating slowly could make the reaction occur slower than preferred if at all in the case of the temperature being too low. More possible sources of error could pertain to the techniques that were used in the separation of the alkyl halide products being impure. When it comes to drying, if the 11

anhydrous Na2SO4 was not allowed to completely dry the product, water would be an impurity that remained in the products which would result in inaccurate NMR and IR data. The possible errors listed as well as any other possible errors are negative factors that affected the efficiency of the reaction that converted alcohols to alkyl halides. Errors are an important factor to consider with the calculation of the percent yield. Infrared Spectrums (IR) can easily show if and what products are formed from alcohols. In both reactions, the OH group of the alcohol was hydrated to form H2O then acted as a leaving group. This allowed for bromine to come in through a substitution mechanism. If this happened, it would show up in its respective IR graphs. When comparing the IR graphs of the reactants and products, the reactants’ IR graphs contained the indication of an OH functional group when the products did not, as shown in Figure 1 and Figure 2. In IR, OH shows up as a broad-like spectrum stretch which could wipe out the C-H stretch. Both IR graphs of the products shown in Figure 3 and Figure 4, thereby confirming the theory of the product being formed by the substation of the OH functional group. The IR graphs of the products displayed a stretch in the sp3 C range indicating the final product contained at least one sp3 hybridized carbon. IR is effective in determining the functional groups present in a compound however, it does not help at all in determining the structure of the compounds. It just shows what kind of product is in the compound, not what kind of bonds are how the bonds are connected and it does not show how many compounds are present. Nuclear magnetic resonance (NMR) determines the formation of products and their respective structures through the identification of non-equivalent hydrogens illustrated by differing signals on an NMR graph. Splitting pattern, integration, and where the signals are located on the x-axis according to their parts per million (ppm) value are also important factors in NMR. The closer a hydrogen is to a more electronegative atom, the more downfield the peak is located and the higher the ppm value. On the contrary, the further a hydrogen group is from an electronegative atom, the more upfield the peak is located and the lower the ppm value. The splitting pattern gives the number of peaks in the signal and is determined by the n+1 rule where n is the number of hydrogens on neighboring carbons.1 NMR helps identify the number of hydrogens on each carbon with a certain condition. The integration number is given by NMR through the addition of any hydrogens on symmetrical carbons and the number of hydrogen atoms per carbon. The integration number is the number of hydrogen or hydrogens each peak or peaks resemble. The NMR spectrums displayed in Figure 6 and Figure 9 are analyzed in Table 3, Table 4, and Table 5 according to the compounds present with their respective structures shown in Figure 5, Figure 7, and Figure 8. In the tables, each of the three compounds are divided up by letters with each letter representing a non-equivalent hydrogen group and analyzed according to NMR. The analysis done by the tables are matched to the NMR signals showing the products formed were 1-bromopropane, 2-bromopentane, and 3-bromopentane.

12

Figure 10: SN2 mechanism of the reaction between 1-propanol and the HBr product of the reaction between NaBr and H2SO4.

Figure 11: SN1 mechanism of the reaction between 2-pentanol and the HBr product of the reaction between NaBr and H2SO4.

When it comes to distinguishing between two compounds in a mixture, NMR is a useful tool that is available.2 The corresponding NMR spectrum of an unknown mixture shows the different types of ways hydrogen is bonded to the molecule or molecules in the mixture. In the NMR spectrum for the product or products of the reaction with 1-propanol, sulfuric acid, and sodium bromide, only one product made an appearance and that single product matched the characteristics of 1-bromopropane. Since SN1 mechanisms would have preferred the more substituted 2-bromopropane, this reaction must have occurred through a SN2 reaction with 1bromopropane being the major and only product of the reaction.3 After analysis of the NMR product or products of the reaction of 2-pentanol, sulfuric acid, sodium bromide, two different products have appeared to form. One explanation for such is that a 3,2-hydride shift occurred. For a 3,2-hydride shift to occur, a carbocation intermediate must have formed therefore a SN1 reaction must have occurred. Since a hydride shift requires more energy to happen in a substitution reaction, 2-bromopentane would be favored over 3-bromopentane. This is further confirmed by the mole fraction of 2-bromopentane produced in mixture being 71.4% and the mole fraction of 3-bromopentane produced in mixture being 28.6%. 13

Conclusion The techniques of reflux, distillation, separation, and drying were used to synthesize alkyl halides from alcohols. Despite taking all necessary precautions, only 11.1% of 1-bromopropane and 28.2% of a mixture of 2-bromopentane and 3-bromopentane were able to be recovered. After being given enough drying time, the product alkyl halides were analyzed using NMR and IR to identify what alkyl halides were produced as well as the purity of the products. 1-bromopropane and 2-bromopentane were determined to be the major products after the analysis of the NMR spectra. Also, after further analysis, it was determined the 1-bromopropane was formed via a SN1 reaction and the mixture of 2-bromopentane and 3-bromopentane was formed via a SN2 reaction. There could be a few improvements to the procedure of the lab. A faster reaction could have occurred if HBr, instead of the using NaBr and H2SO4 to form HBr, was used. The reflux condenser could have been kept at a cooler temperature in order that none of the compound could evaporate during the setup of the distillation apparatus to increase the amount of products formed. Parafilming the joints of both the reflux and distillation apparatuses would also make sure none of the products could escape through any gaps in joints to produce an increased yield of product. To allow for more distinguishable IR and NMR results, possibly adding another drying step could be helpful.

14

References 1. Del Bene, J. E.; Ajith Perera, S.; Bartlett, R. J. Hydrogen Bond Types, Binding Energies, and 1H NMR Chemical Shifts. J. Phys. Chem. A 1999, 103, 40, 8121-8124. https://pubsacs-org.ezproxy3.lhl.uab.edu/doi/10.1021/jp9920444. (accessed Nov 26, 2019). 2. Williamson, K. L. Macroscale and microscale orga...