Experiment 4: Kinetics Of Nucleophilic Substitutions

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Experiment 4: Kinetics of Nucleophilic Substitutions

1. Determine the effect of varying [OH-] on the rate of the reaction. To do this, you should complete the following table: Experiment [tBuCl]0 (M) [OH-]0 (M) Time (s) Reaction rate (M/s) Rate constant (s-1)
1 0.03 0.003 65 4.89 · 10-5 1.62 · 10-3
2 0.03 0.006 213 3.15 · 10-5 1.05 · 10-3
3 0.03 0.009 362 2.96 · 10-5 9.85 · 10-4 What is the dependence of [OH-] on the rate law?

The effect of the increased or decreased [OH-] on the rate law is very minimal. As the concentration of OH- is increasing, the rate of reaction is increasing at a very low rate. This is a plausible conclusion because a reaction involving a tertiary alkyl halide will follow SN1 mechanism.
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The effect of the increase or decrease of [tBuCl] is evident by observing the reaction rate for each trial. As the initial concentration of tBuCl is increasing, the rate of the reaction is also increasing. As explained above, since this is a SN1 mechanism, the reaction rate is only dependant of the increase or decrease of the tertiary alkyl halide, Hence, the rate law for this reaction will be Rate = k [tBuCl], where the change in concentration of OH- will play no role other than being a nucleophile in the reaction. 3. Based on the experimental rate law, what substitution mechanism(s) do you think are happening in the solvolysis reaction of tBuCl? What substitution product(s) do you predict for this reaction (draw the reaction mechanisms and product structures)?

The substitution mechanism that is happening in the solvolysis reaction of tBuCl is the SN1 mechanism, where only one type of molecule will affect the reaction rate. The data is the tables above is indicative of this conclusion, because the change in [tBuCl] has an effect on the reaction rate while the [OH-] does not have an effect on the reaction rate. This the reaction involves a tertiary alkyl halide, which is tert-butyl chloride, which further indicates that the reaction follows a SN1
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The effect of temperature on reaction rate can be seen in the data shown in question 4. As compared to the rate of reaction at room temperature; which is trial 1, the reaction rate increases as temperature increases and decreases as temperature decreases. The rate of reaction increases with temperature because increase in temperature increases the kinetic energy present in the molecules. This leads to an increase in the number of successful collisions leading to the formation of the product. The opposite is true for the decrease in temperature. With temperature decrease, the kinetic energy is lowered, which also lowers the chance of molecular collisions leading to the successful formation of product. However, increase or decrease in temperature cannot change the activation energy, therefore, the activation energy will remain the same. 5. Comment on how the reaction rate constant varied when you adjusted the solvent composition, making reference to the mechanism(s) you proposed in question

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