Question

In: Chemistry

Describe the observed order of arene reactivity. What will be the order of the arene reactivity...

Describe the observed order of arene reactivity. What will be the order of the arene reactivity and how can i know?

Arene: Toluene, Acetanilide, Benzhydrol, Anisole, Diphenyl ether.

All five arenes were asked to be mixed with 35mL of acetic acid and 0.5mL of a 0.05M bromine solution.

what will be the fastest and slowest? what will be the order of reaction and why?

Solutions

Expert Solution

  1. Reactivity of arene means its ability to show electrophilic substitution reaction. The reactivity of arene depends on the stabilization of intermediate sigma complex. Let’s explain electrophilic substitution reaction in detail. (see Box 1 in image file)
  1. The attack of pi-electron density at ortho position forms firstly pi-complex which then gives sigma-complex. The stability of sigma complex is governed by the stability of +ve charge generated. We can see that in both ortho and para attack case the +ve charge is on C which carries substituent R or we can say that on C which is ortho or para to E. If any group R is having electron donating ability will provide extra stability to the carbocation i.e. to sigma complex and will leads to the product formation. The final step of conversion is just regain of aromaticity which proceeds through loss of H from the carbon bearing E.
  2. So its clear that any electron donating group either + Inductive or by + resonance effect will make the arene reactive towards electrophilic addition-elimination (i.e. overall substitution) reaction.
  3. Hence among the given arenes we can make them into two groups as activating and deactivating groups.
  1. Activators : Anisole ,toluene,
  2. Deactivators: Benzhydrol, diphenyl ether and acetanilide.
  1. Now according to +I, +R effect and –I, -R effect the decreasing trend of reactivity toward ESR is given as Anisole > Toluene > Benzhydrol > Diphenyl ether > Acetanilide.
  1. The treatment of these arenes with Br2 in Acetic acid is an example of electrophilic substitution reaction or it’s a electrophilic bromination reaction. And hence the same trend as above will be followed here also.


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