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SolutionsSEAr Mechanism

1. The energy diagram

Fill in the energy diagram with the corresponding species, indicating whether they are reactive intermediates (RI) or transition states (TS).

Blank energy diagram of an SEAr: reactants, two transition states, an intermediate and products, with the activation energy Ea and the enthalpy ΔH°
Completed diagram: benzene and E+; first transition state; benzenonium ion (intermediate); second transition state with the loss of the proton; substituted product and H⁺

The first transition state, where the C–E bond forms, is the highest: this step is rate-determining. The benzenonium ion is a reactive intermediate, and the second transition state, the loss of the proton, lies much lower.

2. Slow step

Why is the first step of SEAr much slower than the second?

In the first step the aromaticity of the ring is lost to form the benzenonium ion; in the second, loss of the proton restores it. That is why the second step has a very small activation energy.

3. Energy balance

Calculate the ΔH° of the bromination of benzene from these bond enthalpies (kcal/mol): Br–Br, 46; H–C6H5, 111; H–Br, 87; Br–C6H5, 80.

ΔH° = (46 + 111) − (87 + 80) = −10 kcal/mol. The reaction is exothermic, though much less than chlorination (−30 kcal/mol).

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