Textbook Question
Propose a mechanism for the reaction of benzyl bromide with ethanol to give benzyl ethyl ether.
7. Substitution Reactions
SN1 Reaction
2:28 minutesProblem 38b,c
Textbook Question
When tert-butyl bromide is heated with an equal amount of ethanol in an inert solvent, one of the products is ethyl tert-butyl ether.
b. What happens to the rate if the concentration of tert-butyl bromide is tripled and the concentration of ethanol is doubled?
c. What happens to the rate if the temperature is raised?
Verified step by step guidance1
Step 1: Recognize the reaction mechanism. The formation of ethyl tert-butyl ether from tert-butyl bromide and ethanol is an SN1 reaction. In an SN1 reaction, the rate-determining step is the formation of the carbocation intermediate, which depends only on the concentration of the alkyl halide (tert-butyl bromide).
Step 2: Analyze the effect of tripling the concentration of tert-butyl bromide. Since the rate of an SN1 reaction is directly proportional to the concentration of the alkyl halide, tripling the concentration of tert-butyl bromide will triple the reaction rate.
Step 3: Analyze the effect of doubling the concentration of ethanol. In an SN1 reaction, the concentration of the nucleophile (ethanol in this case) does not affect the rate of the reaction because the nucleophile participates in a step after the rate-determining step. Therefore, doubling the concentration of ethanol will not change the reaction rate.
Step 4: Consider the effect of raising the temperature. Increasing the temperature generally increases the reaction rate because it provides more energy for the molecules to overcome the activation energy barrier. This effect applies to all chemical reactions, including SN1 reactions.
Step 5: Summarize the findings. Tripling the concentration of tert-butyl bromide will triple the rate, doubling the concentration of ethanol will have no effect on the rate, and raising the temperature will increase the rate by accelerating the formation of the carbocation intermediate.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Reaction Rate
The reaction rate refers to the speed at which reactants are converted into products in a chemical reaction. It is influenced by factors such as concentration, temperature, and the presence of catalysts. In this context, understanding how changes in the concentrations of tert-butyl bromide and ethanol affect the rate of ether formation is crucial for predicting the outcome of the reaction.
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Concentration Effects
Concentration effects describe how the rate of a reaction can change with varying concentrations of reactants. According to the rate law, if the concentration of a reactant is increased, the rate of reaction typically increases, often in a proportional manner. In this case, tripling the concentration of tert-butyl bromide and doubling that of ethanol will likely lead to a significant increase in the reaction rate.
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Temperature Dependence
Temperature dependence in chemical reactions indicates that increasing the temperature generally increases the reaction rate. This is due to the increased kinetic energy of molecules, leading to more frequent and effective collisions. In the context of the question, raising the temperature would enhance the rate of ether formation by promoting faster molecular movement and higher energy interactions.
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