Reaction Analysis of SN1 and SN2 Leaving Groups PDF

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Reaction Analysis of SN1 and SN2 Leaving Groups Lab Report for Dr. Casselman's organic chemistry I lab (CH 236)....

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Reaction Analysis of SN1 and SN2 Leaving Groups

Lead Author: Garrett Canfield Reviewer: Gracemarie Cepero-Lopez Editor: Nathan Sim

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Introduction SN1 and SN2 are both classified as nucleophilic substitution reactions, in which a nucleophile replaces a leaving group to form some type of product(s).1 Within an SN1 reaction, a leaving group exits the original molecule, and the substrate forms a carbocation intermediate. Afterwards, a nucleophile attacks the carbocation, which in turn, forms the product(s).1 Within an SN2 reaction, a nucleophile attacks the substrate while the leaving group exits the substrate, which causes the product to be formed in one step.1 This process causes a backside attack on the carbocation, which in turn, causes inversion on the final product.1 In terms of structural effects, SN1 reactions tend to favor tertiary alkyl halides, while SN2 reactions usually favor methyl and primary alkyl halides.2 Both SN1 and SN2 reactions can react with a secondary alkyl halide, however, neither reaction prefers to utilize it.2 SN1 reactions effectively react with tertiary alkyl halides because it allows for the carbocation intermediate to remain stable as the nucleophile attacks the carbocation.2 This process also works favorably with SN1 reactions because it allows the leaving group to exit the molecule quickly while simultaneously being crowded by steric strain, which forces the reaction to take place in two separate steps.2 SN2 reactions react effectively with methyl and primary alkyl halides because the electrophile remains unhindered by surrounding groups, which stabilizes the carbocation.2 In turn, this process minimizes the presence of steric strain on the alkyl halide, which allows the nucleophile to attack the carbocation at the same time the leaving group exits the molecule.3 In terms of leaving group ability, SN1 reactions favor good leaving groups, so that the carbon bond directly bonded to the leaving group can be broken down more quickly, which in turn, will allow the formation of the carbocation and the nucleophilic attack to occur at a faster rate, yielding a faster, more stable product.4 For an SN1 reaction, a good leaving group will typically be a weak nucleophile/base, so that it will be able to hold an electric charge from the accepted electrons it receives in order to leave the molecule.4 SN2 reactions favor good leaving groups as well, however, they react faster with stronger nucleophiles and stronger bases, which results from the nucleophile being directly involved in the rate-determining step for SN2 reaction processes.3 In terms of how solvents effect the rate of SN1 and SN2 reactions, SN1 favor polar protic compounds while SN2 favors compounds that are polar aprotic.2 Polar protic compounds react favorably with SN1 reactions because they help stabilize the transition state and carbocation intermediate within the reaction process, while polar aprotic compounds react favorable with SN2 reactions because they enhance the reactivity of the nucleophile, which allows the reaction to take place more quickly.2 Within an SN1 reaction, the polar protic compounds being used contain a hydrogen atom directly bonded to an electronegative atom, such as an anion.4 This bond allows the molecule to become highly polarized, which in turn, helps to speed up the rate of the reaction, due to the large dipole moment stabilizing the transition state within the molecule.4 Within an SN2 reaction, the polar aprotic compounds being used are capable of solvating cations, but cannot react with anions.3 Because of this, the nucleophiles are not highly solvated, which accounts for why SN2 reactions do not contain hydrogen bonding, and if they do, why they do not react with electronegative atoms, like anions.3 2

Figure 1: Mechanism for the unimolecular (two-step) SN1 reaction. (5)

Figure 2: Mechanism for the bimolecular (one-step) SN2 reaction. (5) Table 1: Table of Reagents (6) Compounds (6) 2-bromo-2methylpropane 2-bromobutane 1-bromobutane 1-chlorobutane Sodium iodide Acetone Ethanol Silver nitrate Methanol 1-propanol 0.5M NaOH 1% phenolphthalei n

Molecular Weight (g/mol) (7) 137.02

Boiling Point (℃) (7)

137.02 137.02 92.57 149.89 58.08 46.07 169.87 32.02 60.1 40.0 318.3

Density (g/cm3) (7)

Dielectric Constants (7)

73.3

Melting Point (℃) (7) -16.2

1.21

10.1

91.2 101.3 78.5 1304.0 56.0 78.2 440.0 64.7 97.2 1388.0 557.8

-112.0 -112.4 -123.1 660.0 -95.0 -114.1 212.0 -97.6 -126.1 323.0 262.5

1.26 1.27 0.88 3.67 0.79 0.79 4.35 0.79 0.81 2.13 1.28

8.64 7.16 9.6 7.28 20.7 24.5 9.0 32.7 20.3 57.5 Unknown

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Experimental To begin the procedure in part A, 0.5 mL of 1-bromobutane was measured and placed it into a dry/clean test tube. Afterwards, equal amounts of sodium iodide/acetone were added to the test tube, making sure that the contents of the tube were gently shaken periodically. Following this step, a stopwatch was used to observe the amount of time it took for cloudiness to form in the tube. Upon shaking the tube further, a precipitate was formed, and time was recorded using the stopwatch at the exact time of solid formation. The same procedure was performed for three other compounds, which were 0.5 mL of 2-bromo-2-methylpropane, 2-bromobutane and 1chlorobutane, and a stopwatch was used to measure the time it took to reach cloudiness as well as the time it took to observe a precipitate forming within each test tube. For part B, after rinsing the test tubes with acetone and using air suction to dry them off, 0.5 mL of 2-bromobutane was measured and added into another clean/dry test tube. Afterwards, equal amounts of silver nitrate in ethanol was added to the tube, making sure to observe and record the amount of time it took to display cloudiness. After this step, the amount of time it took to form a precipitate was observed, in which all cloudiness within the tube dissipated and formed a clear and colorless precipitate. Time was recorded at the exact moment of precipitate formation for use in the data tables. The same procedure was performed on the other three alkyl halides: 0.5 mL of 2-bromo-2-methylpropane, 1-bromobutane and 1-chlorobutane, and a stopwatch was utilized to measure the amount of time it took for cloudiness to appear in each tube as well as the amount of time it took for a precipitate to form in each tube. For part C, we took a and equal ratio of 1:1 1-propanol/water and added it to a clean/dry test tube. Afterwards, five drops of 0.5M NaOH and three drops of 1% phenolphthalein were added into the tube, then the visible color change (pink) that had occurred after the addition of the phenolphthalein indicator was recorded. To continue, five drops of 2-bromo-2-methylpropane was added to the tube, and, with use of a stopwatch, the amount of time it took for the pink color of the solution to completely disappear was observed. While waiting for the color change to occur, the contents of the test tube were gently shaken periodically, ensuring that the alkyl halide solutions were thoroughly being mixed with the content of the tube. After the pink color disappeared, the time was recorded, indicating the completion of the reaction. The same procedure was performed on three other equal ratio solutions, which were 1:1 methanol/water, 1:1 ethanol/water and 1:1 acetone/water, and added five drops of 2-bromo-2-methylpropane to each tube. Afterwards, a stopwatch was used to observe the exact amount of time it took for the pink color to disappear within the solution and recorded our results in a data table. Results Within the data table below, the amount of time it took for each alkyl halide to display cloudiness as well as the amount of time it took for the formation of a precipitate were observed and recorded as follows.

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Table 2: Part A: Structural Effects on SN2 Compound

Time of Cloudiness (hh:mm:ss)

Time of Solid Formation (hh:mm:ss) 00:00:27 0:07:00 0:09:27 0:07:31

1-bromobutane 0:00:11 2-bromobutane 0:05:57 1-chlorobutane 0:09:03 2-bromo-20:05:54 methylpropane Table 2: 0.5 mL of each alkyl halide compound was mixed with an equal ratio (1:1) of sodium iodide/acetone. The following compounds were heated in a hot water bath after five minutes: 2bromobutane, 1-chlorobutane and 2-bromo-2-methylpropane. Within the data table below, the amount of time for each test tube to display cloudiness as well as the amount of time it took to form a precipitate were observed and recorded as follows. Table 3: Part B: Structural Effects on SN1 Compound

Time of Cloudiness (hh:mm:ss)

Time of Solid Formation (hh:mm:ss) 0:00:32 0:00:17 0:10:05 0:00:10

1-bromobutane 0:00:05 2-bromobutane 0:00:15 1-chlorobutane 0:09:27 2-bromo-20:00:05 methylpropane Table 3: 0.5 mL of each alkyl halide compound was mixed with an equal ratio (1:1) of silver nitrate in ethanol. The 1-cholorbutane test tube solution was heated in a hot water bath after five minutes. Within the data table below, the amount of time it took for the pink color within each test tube to disappear was observed and recorded as follows. Table 4: Part C: Solvent Effects on SN1 Solvent Mixture

Disappearance of Color (hh:mm:ss) Acetone: Water 0:01:14 Methanol: Water 0:05:27 Ethanol: Water 0:02:27 Propanol: Water 0:01:46 Table 4: An equal ratio (1:1) of each solvent mixture were mixed with five drops of 0.5M NaOH, three drops of 1% phenolphthalein and five drops of 2-bromo-2-methylpropane. The methanol: water mixture was heated in a hot water bath after five minutes. In part A, a flaky yellow/white precipitate formed in each tube as a result of each compound reacting with an equal ratio of sodium iodide in acetone. In part B, A clear and 5

colorless precipitate formed in each tube as a result of each compound reacting with an equal ratio of silver nitrate in ethanol. Finally, in part C, a pink color formed in each tube after the addition of five drops of 0.5M NaOH and three drops of 1% phenolphthalein into each tube, and after the addition of fives drops 2-bromo-2-methylpropane into each tube, the color of the solution changed from pink to clear. Discussion For part A, after each alkyl halide compound reacted with an equal ratio of sodium iodide in acetone, the amount of time it took for each test tube to display cloudiness and form a precipitate were observed and recorded. After compiling the data, it can be determined that the identify of the best alkyl halide for the SN2 reaction was 1-bromobutane, as its precipitate time was significantly smaller than the other three alkyl halide compounds. This means that the 1bromobutane is likely a methyl or primary alkyl halide group, since SN2 reactions favor this group.2 For part B, after each alkyl halide compound reacted with an equal ration of silver nitrate in ethanol, the amount of time it took for each test tube to display cloudiness and form a precipitate were observed and recorded. After compiling the data, it can be determined that the identify of the best alkyl halide for the SN1 reaction was 2-bromobutane, because it formed a precipitate at a faster rate of time than the other alkyl halide compounds. This means that the 2bromobutane is likely a tertiary alkyl halide, since SN1 reactions favor this alkyl halide group,2 or it could have been a secondary alkyl halide with various factors, such as nucleophile strength and solvent preference, causing it to favor that particular alkyl halide group.3 For part C, after each solvent mixture was mixed with five drops of 0.5M NaOH, three drops of 1% phenolphthalein and five drops of 2-bromo-2-methylpropane, the amount of time it took for the pink color in each tube to disappear was observed and recorded. After compiling the data, it can be determined that the identify of the best alkyl halide group for the SN1 reaction data was acetone: water, as it resulted in a faster disappearance time than the other solvent mixtures. This means that the acetone: water mixture is likely polar protic, since SN1 reactions tend to favor these solvent conditions.2 The data we collected for each section of the experiment were not consistent with the results that we expected to receive. A reason for why this may have occurred could have resulted from the alcohol groups within the test tubes evaporating out of solution, with the lighter alcohol groups evaporating more quickly than others. Another potential source of error could have resulted from the test tubes not being rinsed with acetone or ethanol and being completely dried before starting the next steps in the procedure. Conclusion After performing the experiment, it was concluded that the best alkyl halide group for part A was 1-bromobutane, the best alkyl halide group for part B was 2-bromobutane, and the best solvent mixture for part C was acetone: water. Also, several compounds from each trial had to be heated in a hot water bath after five minutes had passed, which may have affected their leaving group 6

ability. To further improve this experimental procedure for future lessons, parafilm could be used to cover the test tubes after the compounds are mixed each solution, potentially minimizing the possibility in producing inaccurate results due to the evaporation of alcohols in solution.

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References 1. SN1 and SN2 Reactions - Illinois Institute of Technology. https://web.iit.edu/sites/web/files/departments/academic-affairs/academic-resourcecenter/pdfs/SN1_SN2.pdf (accessed Apr 1, 2020). 2. Libretexts. 7.12: Comparison of SN1 and SN2 Reactions. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map:_Organic_Chemistry_( Wade)/07:_Alkyl_Halides:_Nucleophilic_Substitution_and_Elimination/7.12:_Comparis on_of_SN1_and_SN2_Reactions (accessed Apr 1, 2020). 3. http://iverson.cm.utexas.edu/courses/310N/ReactMoviesFl05 /SN2text.html (accessed Apr 1, 2020). 4. Libretexts. Effects of Solvent, Leaving Group, and Nucleophile on Unimolecular Substitution. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Or ganic_Chemistry)/Reactions/Substitution_Reactions/SN1/Effects_of_Solvent,_Leaving_ Group,_and_Nucleophile_on_Unimolecular_Substitution (accessed Apr 1, 2020). 5. Casselman, Brock. SN1, SN2 Background.pdf, (accessed Apr 1, 2020). 6. Casselman, Brock. SN1, SN2 Safefy.pdf, (accessed Apr 1, 2020). 7. PubChem. https://pubchem.ncbi.nlm.nih.gov/ (accessed Apr 1, 2020).

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