Explain clearly why changes in base concentration do not affect the observed reaction rate of an E1 elimination.
In an E1 elimination reaction, the observed reaction rate is determined by the rate of the slowest step of the reaction, which is the formation of the carbocation intermediate. The rate-determining step involves the breakage of a bond and the subsequent formation of the carbocation.
The concentration of the base, which acts as a nucleophile in E1 elimination, does not directly affect the rate of the carbocation formation and, therefore, does not impact the observed reaction rate. This is because the base is not involved in the rate-determining step; instead, it participates in a separate step known as the deprotonation step, which occurs after the rate-determining carbocation formation step.
To understand this concept more clearly, we need to consider the mechanism of an E1 elimination reaction. In an E1 reaction, the elimination of a leaving group (such as a halide ion) occurs simultaneously with the formation of a carbocation. The rate-determining step is the formation of the carbocation, which is typically slow compared to the subsequent steps. Once the carbocation is formed, the nucleophile (the base) can react with it in a fast deprotonation step to complete the elimination reaction.
Since the rate-determining step is the formation of the carbocation, changing the concentration of the base would not have a significant effect on this step. Therefore, altering the concentration of the base will not affect the observed reaction rate of an E1 elimination.
To calculate the observed reaction rate, one should generally focus on the rate-determining step. In this case, it would involve determining the rate constant for the formation of the carbocation. This can be done by measuring the reaction rate at different concentrations of the reactants and plotting a rate vs. concentration graph. The slope of the graph will provide the rate constant.
In summary, changes in the concentration of the base do not affect the observed reaction rate of an E1 elimination because the rate-determining step, which involves the formation of the carbocation, is not affected by the base concentration.