Interpretation:Diagrams drawn for the propagation steps of radical bromination of benzene should be identified as either representative of early or late transition states.
Concept introduction:Hammond postulate states that exothermic reactions are characterized by early transition states that resemble the structure of substrate more than the product. Slow or endothermic processes are characterized by late transition states that resemble the almost formed bonds as in the products.
Analogous to hydrocarbons the benzene can also undergo initiation to generate bromine radicals; propagation of radicals formed and finally termination. This sequence can be outlined as follows:
Step1: Initiation via homolytic cleavage of
Step2: Propagation: In first of the propagation steps bromine radical from step 1 abstracts hydrogen radical from
In subsequent propagation step,phenyl radical abstracts
Step3: Termination: Bromine radicals generated in propagation steps get quenched upon combination with one another illustrated as follows:
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Organic Chemistry: Structure and Function
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