SN1 SN2 lab report PDF

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Alkyl Halides in Nucleophilic Substitution of SN2 and SN1 Reactions

Introduction Nucleophilic substitution is one of the most useful and well-studied classes of organic reactions1. Both SN2 and SN1 are two of the mechanisms used in nucleophilic substitution shown in Figures 1 and 2. The SN2 reaction is bimolecular, occurs in one step, and is favored by polar aprotic solvents2. In contrast, the SN1 reaction is unimolecular, occurs in two steps, and tends to proceed in polar protic solvents2.

Figure 1. The mechanism for a SN2 reaction

Figure 2. The mechanism for a SN1 reaction

The SN2 reaction is concerted2. That is, the SN2 occurs in one step, and both the nucleophile and substrate are involved in the rate-determining step2. The nucleophile enters as the leaving group, typically a halide ion and it departs from the opposite side2. Therefore, the rate is shown in Eq. 1 is dependent on both the concentration of the substrate and that of the nucleophile2. The big barrier in the SN2 reaction is steric hindrance2. Since the SN2 proceeds through a backside attack, the reaction will only proceed if the empty orbital is accessible2. The more groups that are present around the vicinity of the leaving group, the slower the reaction will be2. That is why the rate of reaction proceeds from fastest to slowest2. Eq. 1. (Rate = k [RX] [Nu]) The SN1 reaction is unimolecular and proceeds stepwise2. The leaving group first leaves, whereupon a carbocation forms that is attacked by the nucleophile2. The big barrier in the SN1 reaction is carbocation stability2. Since the first step of the SN1 reaction is the loss of a leaving group, it generates a carbocation intermediate1. The carbocation is captured by the nucleophile often referred to as solvolysis1. Carbocation stability increases with increasing substitution of the carbon as well as with resonance2. The rate only depends on the concentration of the alkyl halide shown in Eq. 21. Since carbocation stability increases from primary to secondary the rate of reaction for the SN1 goes from slowest to fastest.

Eq. 2. (Rate = k [RX])

Table 1. Table of reagents Compound

Molecular Weight (g/mol)

Melting Point (°C)

Boiling Point (°C)

Density (g/cm3)

Dielectric Constant (€)

2 – Bromo – 2 methylpropane

137.02

-16.20

73.30

1.271

10.10

2 – Bromobutane

137.02

-112.00

91.20

1.267

8.64

1 – Bromobutane

137.02

-112.40

101.30

1.272

7.16

1 – Chlorobutane

92.55

-123.10

78.50

0.874

9.60

Sodium Iodine NaI

149.89

660.00

1304.00

3.670

7.28

Silver Nitrate AgNO3

169.87

212.00

440.00

4.350

N/A

Methanol

32.04

-97.60

64.70

0.753

33.0

Ethanol

46.07

-114.10

78.20

0.789

25.3

Propanol

60.10

-126..10

97.20

0.786

20.1

Acetone

58.08

-94

56

0.773

20.7

Sodium Hydroxide

39.99

318

2530

2.130

57.5

Phenolphthalein

318.32

262

79

1.386

N/A

Experimental This experiment was isolated into three separate parts. Part A Structural Effects on SN2

To go about beginning the first part of the experiment, four clean, dry test tubes were placed in a rack. Around 0.5 mL of the following alkyl halides were pipetted into separate test tubes: 2‐ bromo‐ 2‐ methylpropane, 2‐ bromobutane, 1‐ bromobutane, and 1‐ chlorobutane. Each test tube was labeled based on its contents. Additionally, 0.5mL of 15% NaI solution in acetone were added to the test tubes. Afterward, the contents in each test tube were gently shaken with a stirring rod and observed for cloudiness. Time for cloudiness was recorded in the format (hh:mm: ss). To check for a reaction, the test tubes were shaken vigorously with a stirring rod and observed for the formation of a precipitate by looking at the sides and the bottom of the test tube. The exact time was recorded. If no reaction occurred after 5 minutes, the test tubes were placed in a warm water bath (50-60°C) and watched for an additional 5-6 minutes. Part B Structural Effects on SN1 During the second part of the experiment, the four test tubes from part A were rinsed with ethanol or acetone, not water. Around 0.5mL of the alkyl halide substrates in part A were added into each test tube. Also, 0.5mL of a 1% AGNO3 solution was obtained in ethanol to each test tube. Afterward, the contents in the test tubes were gently shaken and monitored for cloudiness, noting the exact time. After the cloudiness disappeared, the test tubes were observed for the formation of precipitates by looking at the sides and the bottom of the test tube and noting the exact time it was recorded. If no reaction occurred after 5 minutes, the test tubes were placed in a warm water bath (50-60°C) and watched for an additional 5-6 minutes. Part C Solvent Effects on the SN1 Reaction In the final part of the experiment, four clean, dry test tubes were placed on a rack. In consecutive order, a 1:1 mixture of methanol/water, ethanol/water, 1-propanol/water, and acetone/water was added to the test tubes and labeled. Additionally, 5 drops of 0.5 M NaOH and 3 drops of 1% phenolphthalein were added to each test tube, noting the pink appearance of the solution. Five drops of 2‐ bromo‐ 2‐ methylpropane were added to each of the test tubes for the disappearance of the pink color. It was shaken gently until the pink color completely disappeared and the exact time was noted and recorded to indicate the time of the reaction. Results Part A structural effects on SN2 is shown in Table 2. Part B structural effects on SN1 is shown in Table 3. Part C solvent effects on the SN1 reaction are shown in Table 4. Note that if no reaction occurred after 5 minutes, the test tubes were placed in a warm water bath (50-60°C) and watched for an additional 5-6 minutes. Table 2. Structural Effects on the SN2 Reaction

Alkyl Halide

Time of Addition (hh:mm:ss)

Time of Cloudiness (hh:mm:ss)

Time of Solid Formation (hh:mm:ss)

Total Time Elapsed (hh:mm:ss)

2 – bromo -2 methylpropane

00:00:00

00:05:54

00:07:31

00:13:25

2 – bromobutane

00:00:00

0:05:57

00:07:00

00:12:57

1 - bromobutane

00:00:00

00:00:11

00:00:27

00:0038

1 - chlorobutane

00:00:00

00:09:03

00:09:27

00:18:30

Table 2. Shows the time of addition, cloudiness, the formation of a solid, and the total time elapsed. The precipitate formed when the alkyl halides were mixed with a 1:1 ratio of 5mL of a 15% NaI solution in acetone. 1-bromobutane is the only alkyl halide that did not have to undergo a warm water bath. Table 3. Structural Effects on the SN2 Reaction Alkyl Halide

Time of Addition (hh:mm:ss)

Time of Cloudiness (hh:mm:ss)

Time of Solid Formation (hh:mm:ss)

Total Time Elapsed (hh:mm:ss)

2 – bromo -2 methylpropane

00:00:00

00:00:05

00:00:10

00:13:25

2 – bromobutane

00:00:00

00:00:15

00:00:17

00:00:32

1 - bromobutane

00:00:00

00:00:05

00:00:32

00:00:37

1 - chlorobutane

00:00:00

00:09:27

00:10:05

00:19:32

Table 3. Shows the time of addition, cloudiness, the formation of a solid, and the total time elapsed. The precipitate formed when the alkyl halides were mixed with a 1:1 ratio of 5mL of a 1% AgNO3 solution in ethanol. 1-chlorobutane is the only alkyl halide that underwent a warm water bath. Table 4. Solvent Effects on the SN2 Reaction Solvent Mixture

Time of Addition of Alkyl Halide (hh:mm:ss)

Time of Disappearance of pink color (hh:mm:ss)

Total Time Elapsed (hh:mm:ss)

1:1 methanol / water

00:00:00

00:05:27

00:05:27

1:1 ethanol / water

00:00:00

00:02:27

00:02:27

1:1 propanol / water

00:00:00

00:01:46

00:01:46

1:1 acetone / water

00:00:00

00:01:14

00:01:14

Table 4. Shows the solvent mixtures, the time of addition of the alkyl halide, time of disappearance, and the total time elapsed. The color change was indicated by phenolphthalein which was observed to be a pink color. The 5 drops of the 2-bromo-2-methyl propane were mixed with the alkyl bromide for the disappearance of the pink color. The 1:1 mixture of methanol and water were the only solvents that had to be placed in a warm water bath. Discussion The objective of this lab was to determine the effect of alkyl halide, substitution, leaving group ability, and solvent polarity on the relative rates of the SN1 and the SN2 reaction1. In part, A of the experiment, 2-bromo-2-methylpropane, 2-bromobutane, 1-bromobutane, and 1chlorobutane were all reacted with sodium iodide in acetone. When iodine is mixed with a nonpolar solvent such as acetone an SN2 reaction which is favored by polar aprotic should occur3. Acetone is polar enough to dissolve the substrate and nucleophile but does not participate in hydrogen bonding with the nucleophile2. The trends noted in part A can be seen in Table 2, 1bromobutane cloudiness appeared the fastest while 2-bromo-2-methylpropane, 2-bromobutane and 1 chlorobutane took a much longer time to appear. For the time it took to form a precipitate, the same trend was noted except, 2-bromobutane reacted faster than 2-bromo-2-methylpropane. 1-bromobutane had the shortest elapsed time while 1-chlorobutane had the longest time recorded. Since 1 -bromobutane has a primary carbon, contains a strong nucleophile, and has the strongest leaving group, it reacted the fastest out of all the other alkyl halides. In part B structural effects on SN1 shown in Table 3, the four alkyl halides in part A reacted with a 1:1 mixture of silver nitrate in ethanol. The trend shown for the appearance of cloudiness appeared with 2-bromo-2-methylpropane and 1-bromobutane being the shortest in time and 2-bromobutane and 1-chlorobutane following behind afterward. For the time of the formation of a precipitate, the reactions occurred in the following order with the fastest time first and the slowest time last: 2-bromo-2-methylpropane, 2-bromobutane, 1-bromobutane, and 1chlorobutane. The shortest elapsed time was 2-bromo-2-methyl while 1-chlorobutane had the longest elapsed time. However, 2-bromo-2-methylpropane was expected to be the fastest reaction in part B because the reagent is highly substituted. The objective in part C was to look at the effect of solvent polarity on the rate of the SN1 reaction1. Since the charge separation is very important to the SN1 reaction, the better a solvent or a mixture of solvents is at separating charges, the faster the SN1 reaction should occur1. The ratio of 1:1 methanol/water had the highest dielectric constant out of the other solvent mixtures. According to Table 4, methanol and water took the longest amount of time for the disappearance of the pink color after the phenolphthalein was added. The shortest time recorded was acetone

and water followed by propanol and water than ethanol and water. A solvent with a high dielectric constant will have a quicker reaction rate. Based on the data in Table 1, methanol had the highest dielectric constant, but acetone reacted the fastest during this experiment. Common errors that could have occurred during this experiment potentially could have been the failure of allowing the test tubes to completely dry after cleaning them with acetone or ethanol. Another error could have resulted from not recording accurate times for cloudiness and precipitation during each part of the experiment. Also, in part C of the experiment, the data turned out to be unusual1. It is possible the alcohols evaporated out of the solution which resulted in the lighter alcohols evaporating more quickly1. Conclusion In conclusion, SN1 and SN2 were experimented in various ways to determine reaction rates. The experiment was separated into three parts with part A comparing alkyl halides reaction rates with sodium iodide and acetone. Part B compared reaction rates of alkyl halides with silver nitrate in ethanol while part C tested the effects on the SN1 reaction. According to the results concluded in part A, SN1 the alkyl halide, 2-bromo-2-methylpropane formation of cloudiness was supposed to have the shortest recorded time but instead, it was 1-bromobutane. The 1bromobutane reacted the fastest when recording the time of the precipitate. In part B of the experiment, 1-chlorobutane is favored by SN1 in terms of cloudiness while 2-bromo-2methylpropane was expected to appear the fastest. Both 2-bromo-2-methylpropane and 1bromobutane proved otherwise and had the shortest time recorded for cloudiness. However, the experiment did prove that 2-bromo-2-methylpropane reacted the fastest out of the other alkyl halides. The results for part C the 1:1 methanol in water should have resulted in the fastest reaction but it resulted in being acetone instead. Lastly, this experiment could have been improved by repeating the trials for more accurate results.

References 1

Casselman, B. Hydrolysis of t-butyl chloride background and procedure. The University of Alabama at Birmingham. https://uab.instructure.com/courses/1527297/ (accessed July 28, 2020) 2

Ashenhurst, J. Substitution Reactions. Master Organic Chemistry Blog. https://www.masterorganicchemistry.com/2012/08/08/comparing-the-sn1-and-sn2-

reactions/#:~:text=The%20SN2%20reaction,N1%20reaction%20proceeds%20stepwise. (accessed July 28, 2020) 3

Ashenhurst, J. A Primer On Organic Reactions. What Makes A Good Nucleophile? Master Organic Chemistry Blog. https://www.masterorganicchemistry.com/2012/06/18/what-makes-agood-nucleophile/ (accessed July 28, 2020)

Questions 1. What is the reaction that is causing the precipitate to form in Part B? The reaction that is causing the precipitate to form in Part B is the addition of AgNO3 solution in ethanol. The contents in the test tubes were gently shaken and monitored for cloudiness. After the cloudiness disappeared, the test tubes were observed for the formation of precipitates. 2. What reaction is causing the pink color to disappear in Part C? The reaction that causes the pink color to change is the addition of 2-bromo-2methylpropane to the mixture. 3. If 1-bromo-3-chlorobutane were allowed to react with one equivalent of NaI in acetone, what would the major product be? Justify your choice. 4. What would be the major product if 1,4-dibromo-4-methylpentane were reacted with: a) One equivalent of NaI in acetone? Justify your choice. 1-iodo-4-bromo-4-methylpentane. b) One equivalent of AgNO3 in ethanol? Justify your choice. 2-bromo-2-methylpentane and silver bromide precipitate....