Describing a Reaction Intermediates

How can we describe the carbocation formed in the first step of die leaction of ethvJene with HBr The carbocation is clearly different from the reactants, yet it isn t a transition state andit isn t a final product. 

We call the carbocation, which exists only transiently during the course of the multistep reaction, a reaction intermediate. As soon as the intermediate is formed in the first step by reaction of ethylene with H+, it reacts further with Br in a second step to give the final product, bromoethane. This second step has its own activation energy (AG ), its own transition state, and its own energy change (AG°). We can picture the second transition state as an activated complex between the electrophilic carbocation intermediate and the nucleophilic bromide anion, in which Br- donates a pair of electrons to the positively charged carbon atom as the new C-Br bond starts to form. 

Each step in a multistep process can always be considered separately, bach step has its own AG and its own AG°. The overall AG° of the reaction, however, is the energy difference between initial reactants and final products. 

Problem 5.13 Sketch an energy diagram for a two-step reaction with an endergonic first step and an exergonic second step. Label the parts of the diagram corresponding to reactant, product, and intermediate. 

Just as the carbocation formed in the first step is a reaction intermediate, the protonated alcohol formed in the second step is also an intermediate. Only after this second intermediate is deprotonated by an acid-base reaction with water is the final product formed. 

FIGURE 6.7 An overall energy diagram for the reaction of ethylene with water. Three steps are involved, each with its own transition state. The energy minimum between steps 1 and 2 represents the carbocation reaction intermediate, and the minimum between steps 2 and 3 represents the protonated alcohol intermediate. 

Practice Problem S3 Sketch a reaction energy diagram for a one-step reaction that is fast and 

Strategy A fast reaction has a small AG-, and a highly exergonic reaction has a large negative 

Sketch an energy diagram fi)r a one-step reaction tfi lt is fast and highly exergonic, 

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Describing a Reaction Intermediates 177 CHEMISTRY WORK Explosives 179... 

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