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SN1 - First-order Nucleophilic Substitution

Kinetics of SN1 Reactions

The rate of the reaction between 2-bromo-2-methylpropane and water actually depends solely on the alkyl halide's, and not the nucleophile's, concentration. In doubling the concentration of the alkyl halide, the reaction rate also increases two-fold. However, doubling the concentration of the nucleophile does not in any way alter the reaction rate. Thus, the reaction rate is proportional only to the alkyl halide's concentration. Therefore, the rate law of the reaction is:

Rate law
Reaction rate = k [2-bromo-2-methylpropane] mol/(l s)

Many organic reactions display a relatively complex mechanism with several subsequent reaction steps. The rates of the individual reaction steps are typically different. The slowest individual reaction step is known as the rate-determining step, or rate-limiting step.

Hint
A chemical reaction cannot proceed more rapidly than its own slowest individual step.

The individual step with the highest activation energy is the slowest one. The activation energy is the energy difference between the starting products and the transition state of the individual step. This is illustrated in the reaction energy diagrams below. In the first diagram (a)) the first individual step is the rate-determining step with the highest activation energy. In the second diagram (b)) the rate-determining step with the highest activation energy is the second step.

Fig.1
Fig.2
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