Kinetics of the SN1 Reaction

The reaction of (CH3) CBr with H20 looks analogous to the reaction of CH3Br with OH-, and we might therefore expect to observe second-order kinetics. In fact, we do not. We find instead that the reaction rate is dependent only on the alkyl halide concentration and is independent of the H/) concentration. In other words, the reaction is a first-order process Only 

Reaction energy diagrams for two hypothetical reactions. The rate-limiting step in each Is determined by the difference in height between a low point and a subsequent high point. In (a), the first step is rate-limiting in (b), the second step is rate-limiting. 

Reaction rate = Rate of disappearance of alkyl halide = k X [RX] 

How can this result be explained To answer this question, we must first learn more about kinetics measurements. 

Spontaneous dissociation of the alkyl bromide occurs in a slow, rate-limiting step to generate a carbocation intermediate plus bromide ion. 

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