Experiment- Kinetics

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Chemistry

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Jan 9, 2024

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REACTION KINETICS: THE SOLVOLYSIS OF TERT-BUTYL CHLORIDE Performed February 10, 2023 Michael McClory By: Kian Houman and Adam Nachate Submitted on February 27, 2023

Theory: In an SN1 reaction, the crucial step that determines the rate of the reaction is the departure of the leaving group, which results in the formation of a carbocation intermediate. The stability of the carbocation intermediate plays an important role in determining the energy required for the transition state during the formation of the carbocation. The more stable the carbocation, the lower the energy required for the transition state, and the faster the reaction proceeds. The substrate involved in this reaction contains the leaving group (Cl - ), which, upon departure, creates the carbocation that subsequently binds with the nucleophilic hydroxyl group, as illustrated in Figure 1. Figure 1: SN 1 reaction mechanism of the solvolysis of tert-butyl chloride Procedure: Refer to the Chemistry 202 BLD Lab Manual (1)

Data and Results: Table 1: Rate constant and rate of reaction depending on concentrations and temperature of reactions. Part A 70% H2O/30% acetone Part B 70% H2O/30% acetone Part C 80% H2O/20% acetone Part D 70% H2O/30% acetone Part E 70% H2O/30% acetone Temperature ( K) 295.5 295.0 295.0 283.1 303.0 Tert-butyl chloride concentration (M) 0.030 0.015 0.030 0.030 0.030 Sodium hydroxide concentration (M) 0.003 0.0015 0.003 0.003 0.003 Average time for 10% of hydrolysis (s) 32.84 15.98 5.97 77.08 13.27 Rate constant (s -1 ) 0.0030 0.0063 0.0168 0.0013 0.0754 Rate of reaction (Ms -1 ) 0.000090 0.000095 0.000504 0.000039 0.002262 Sample calculation Part A Initial molarity of tert-butyl chloride in reaction mixture

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Number of tert − butyl moles = 0.15 mol L × 0.002 L = 0.00030 mol Molarityof tert − butyl ∈ reaction mixture = 0.00030 mol÷ 0.010 L = 0.030 M Initial molarity of sodium hydroxide in reaction mixture Number of sodiumhydroxide moles = 0.15 mol L × 0.002 L = 0.00030 mol Molarityof sodiumhydroxide ∈ reaction mixture = 0.00030 mol÷ 0.010 L = 0.030 M Average time average time = 36.34 + 33.11 + 29.06 3 = 32.84 s Rate constant k = 0.10 32.84 = 0.0030 s − 1 Rate of reaction Sn1 rate = k × [ initialmolarity for tert − butyl ] = 0.0030 s − 1 × 0.030 M = 0.000090 M s The only variation made in part A, D and E is the temperature, so a graph was made to show its effect on the rate and to calculate the energy of activation using the slope. Table 1: The natural logarithm of the rate constant and the inverse of temperature for parts A, D, and E Part A Part D Part E Ln(k) -5.8 -6.6 -2.6 1/T (1/K) 0.003384 0.003532 0.003300

Graph 1: Natural logarithm of the rate constant in function of the inverse of temperature for parts A, D, and E Energy of activation from the graph E A =− slope× R E A = 15887 K× 8.314 J K ×mol = 1.32 × 10 5 J mol 3. A) Yes, changing the concentration of tert-butyl affected the rate constant. As the concentration decreased from 0.030M to 0.015M (from part A to part B), we see that the rate constant has increased, and consequently the rate of reaction had slightly increased (only by 0.000005 Ms -1 ). This is because the average reaction time was a lot smaller in part B than in part A. Therefore using k = 0.10 avgtime we understand why the rate constant of part B (0.0063 s -1 ) was bigger than that in part A (0.0030 s -1 ).

B) Yes, increasing the solvent percentage leads to a greater reaction rate in the case of an SN1 reaction because of the polar nature of water. Since water is protic, it can solvate and stabilize the intermediate carbocation formed after the departure of the leaving group, making the reaction happen faster. When comparing part A and C, which both have identical everything except for water concentrations, we see that part C, with 80% of water, had an average reaction time of 5.97 seconds, while part B, with 70% water, had an average time of 32.84 seconds. Thus, having a higher solvent percentage increases the rate constant (0.0168 s -1 ) and the reaction rate (0.000504 Ms -1 ) C) Yes, increasing or decreasing the temperature will affect the rate constant and the rate of reaction. In reaction D, lowering the temperature to 283.1K made the average reaction time slower (77.08 s). As consequence, the rate constant decreased to (0.0013 s -1 ) and the rate of reaction decreased to (0.000039 Ms -1 ). If we look at reaction E where the temperature was increased to 303.0K, we see that the average reaction rate was accelerated to only 13.27 seconds. By consequence, the reaction constant was increased to (0.0754 s -1 ) and the reaction rate was increased to (0.002262 Ms -1 ). Thus the temperature definitely played its role in the reaction process. D) In question 3.A, we would have expected that lowering the concentration of the substrate would have slowed the reaction rate, but our experiment showed the opposite effect. In part B, a lower concentration of tert-butyl chloride was used, however the reaction process happened quicker which isn’t consistent with how the substrate concentration normally affects and SN1 mechanism. This inconsistency could have happened due to bad manipulation of material, contamination, or errors in the measurements of solutions. In question 3.B and 3.C, both the impacts of changing the solvent concentration and the temperature are consistent with what we would have expected in a SN1 mechanism. For instance, increasing the concentration of water is helped the reaction go faster since the polar aspect of the solvent would help stabilize the tert-butyl carbocation. This shows that our experiment succeeded in showing the connection between a higher concentration of water and the increase in reaction rate that should result from it. In the case of temperature, increasing it helped add energy to the molecules involved in the process, making it easier for it to achieve the activation energy to allow it to go through the reaction mechanism faster. Thus, our experiment successfully showed that rising the temperature made our reaction go quicker just like we would have expected. To conclude, our experiment failed to demonstrate consistency in an SN1 mechanism for the effect of change in concentration of tert-butyl. However, it succeeded in showing consistency for the effect of concentration of solvent and temperature in an SN1 mechanism. Answers to questions to think about: Question 3: How does the rate of an SN1 reaction vary with temperature? polarity of solvent? concentration of nucleophile? concentration of substrate?

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The rate of an SN1 reaction can be affected by several factors, including temperature, solvent polarity, concentration of nucleophile, and concentration of substrate. First of all, an increase in temperature generally leads to an increase in the rate of the reaction, as it provides more energy for the reactant molecules to overcome the activation energy barrier. However, excessively high temperatures can cause side reactions or decomposition of the reactants, leading to a decrease in the rate of the reaction (2). Second of all, the polarity of the solvent can significantly affect the rate of an SN1 reaction. A polar solvent can stabilize the intermediate carbocation by solvation, leading to a faster reaction rate. In contrast, a nonpolar solvent may hinder the solvation of the intermediate, leading to a slower reaction rate. Third of all, the concentration of the nucleophile can also impact the rate of the reaction. An increase in the concentration of the nucleophile leads to an increase in the rate of the reaction because there are more nucleophilic molecules available to react with the carbocation intermediate. Finally, the rate of an SN1 reaction can also be influenced by the concentration of the substrate. An increase in the concentration of the substrate generally leads to an increase in the rate of the reaction, as there are more substrate molecules available to undergo the reaction. However, at excessively high concentrations, the reaction may become saturated, and the rate of the reaction may plateau or even decrease due to product inhibition or other factors. Question 6: Do you think that can be easily solvolyzed? Why or why not? (Molecular models may be helpful here). In the case of a bridged bicyclic alkane, performing an SN1 reaction is impossible due to the steric demands of the carbocation. Upon its formation, the carbocation must adopt a trigonal planar configuration where the unoccupied orbital bears the highest energy. Unfortunately, the bridgehead carbon, to which the chloride is bound, cannot be stabilized in this manner due to the significant strain involved. In this case, other types of reactions, such as SN2, may be more favorable to allow solvolysis. Bibliography

1) “Reaction Kinetics: The Solvolysis of Tert-Butyl Chloride.” Lab report for Organic Chemistry II 202-BLD-05 sect. 00001, compiled by Michael McClory, Winter 2023, Champlain Regional College – Saint-Lambert 2) “Characteristics of the SN1 Reaction.” Chemistry LibreTexts, 2016, https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map %3A_Organic_Chemistry_(Wade)_Complete_and_Semesters_I_and_II/Map %3A_Organic_Chemistry_(Wade)/07%3A_Alkyl_Halides- _Nucleophilic_Substitution_and_Elimination/7.09%3A_Characteristics_of_the_S1_Reaction